- 1. Introduction to Transistors
- 2. Transistor Fundamentals
- 2.1. Classification
- 2.2. Construction
- 2.3. Transistor Theory
- 3. Transistor Specifications
- 4. Transistor Identification
- 5. Transistor Maintenance
- 6. Precautions
- 7. Lead Identification
- 8. Transistor Testing
- 9. Testing Transistors
- 10. Testing Semiconductors
1. Introduction to Transistors
The discovery of the first transistor in 1948 by a team of physicists at the Bell Telephone Laboratories sparked an interest in solid-state research that spread rapidly. The transistor, which began as a simple laboratory oddity, was rapidly developed into a semiconductor device of major importance. The transistor demonstrated for the first time in history that amplification in solids was possible. Before the transistor, amplification was achieved only with electron tubes. Transistors now perform numerous electronic tasks with new and improved transistor designs being continually put on the market. In many cases, transistors are more desirable than tubes because they are small, rugged, require no filament power, and operate at low voltages with comparatively high efficiency. The development of a family of transistors has even made possible the miniaturization of electronic circuits. Figure 1 shows a sample of the many different types of transistors you may encounter when working with electronic equipment.
Transistors have infiltrated virtually every area of science and industry, from the family car to satellites. Even the military depends heavily on transistors. The ever increasing uses for transistors have created an urgent need for sound and basic information regarding their operation.
From your study of the PN-junction diode in the preceding chapter, you now have the basic knowledge to grasp the principles of transistor operation. In this chapter you will first become acquainted with the basic types of transistors, their construction, and their theory of operation. You will also find out just how and why transistors amplify. Once this basic information is understood, transistor terminology, capabilities, limitations, and identification will be discussed. Last, we will talk about transistor maintenance, integrated circuits, circuit boards, and modular circuitry.
2. Transistor Fundamentals
The first solid-state device discussed was the two-element semiconductor diode. The next device on our list is even more unique. It not only has one more element than the diode but it can amplify as well. Semiconductor devices that have-three or more elements are called TRANSISTORS. The term transistor was derived from the words TRANSfer and resISTOR. This term was adopted because it best describes the operation of the transistor - the transfer of an input signal current from a low-resistance circuit to a high- resistance circuit. Basically, the transistor is a solid-state device that amplifies by controlling the flow of current carriers through its semiconductor materials.
There are many different types of transistors, but their basic theory of operation is all the same. As a matter of fact, the theory we will be using to explain the operation of a transistor is the same theory used earlier with the PN-junction diode except that now two such junctions are required to form the three elements of a transistor. The three elements of the two-junction transistor are (1) the EMITTER, which gives off, or emits," current carriers (electrons or holes); (2) the BASE, which controls the flow of current carriers; and (3) the COLLECTOR, which collects the current carriers.
Transistors are classified as either NPN or PNP according to the arrangement of their N and P materials. Their basic construction and chemical treatment is implied by their names, "NPN" or "PNP." That is, an NPN transistor is formed by introducing a thin region of P-type material between two regions of N-type material. On the other hand, a PNP transistor is formed by introducing a thin region of N-type material between two regions of P-type material. Transistors constructed in this manner have two PN junctions, as shown in Figure 2. One PN junction is between the emitter and the base; the other PN junction is between the collector and the base. The two junctions share one section of semiconductor material so that the transistor actually consists of three elements.
Since the majority and minority current carriers are different for N and P materials, it stands to reason that the internal operation of the NPN and PNP transistors will also be different. The theory of operation of the NPN and PNP transistors will be discussed separately in the next few paragraphs. Any additional information about the PN junction will be given as the theory of transistor operation is developed.
To prepare you for the forthcoming information, the two basic types of transistors along with their circuit symbols are shown in Figure 3 and in Figure 4. It should be noted that the two symbols are different. The horizontal line represents the base, the angular line with the arrow on it represents the emitter, and the other angular line represents the collector. The direction of the arrow on the emitter distinguishes the NPN from the PNP transistor. If the arrow points in, (Points iN) the transistor is a PNP. On the other hand if the arrow points out, the transistor is an NPN (Not Pointing iN).
Another point you should keep in mind is that the arrow always points in the direction of hole flow, or from the P to N sections, no matter whether the P section is the emitter or base. On the other hand, electron flow is always toward or against the arrow, just like in the junction diode.
The very first transistors were known as point-contact transistors. Their construction is similar to the construction of the point-contact diode covered in chapter 1. The difference, of course, is that the point-contact transistor has two P or N regions formed instead of one. Each of the two regions constitutes an electrode (element) of the transistor. One is named the emitter and the other is named the collector, as shown in Figure 5, view A.
Point-contact transistors are now practically obsolete. They have been replaced by junction transistors, which are superior to point-contact transistors in nearly all respects. The junction transistor generates less noise, handles more power, provides higher current and voltage gains, and can be mass-produced more cheaply than the point-contact transistor. Junction transistors are manufactured in much the same manner as the PN junction diode discussed earlier. However, when the PNP or NPN material is grown (view B), the impurity mixing process must be reversed twice to obtain the two junctions required in a transistor. Likewise, when the alloy-junction (view C) or the diffused-junction (view D) process is used, two junctions must also be created within the crystal.
Although there are numerous ways to manufacture transistors, one of the most important parts of any manufacturing process is quality control. Without good quality control, many transistors would prove unreliable because the construction and processing of a transistor govern its thermal ratings, stability, and electrical characteristics. Even though there are many variations in the transistor manufacturing processes, certain structural techniques, which yield good reliability and long life, are common to all processes: (1) Wire leads are connected to each semiconductor electrode; (2) the crystal is specially mounted to protect it against mechanical damage; and (3) the unit is sealed to prevent harmful contamination of the crystal.
What is the name given to the semiconductor device that has three or more elements?
What electronic function made the transistor famous?
In which direction does the arrow point on an NPN transistor?
What was the name of the very first transistor?
What is one of the most important parts of any transistor manufacturing process?
2.3. Transistor Theory
You should recall from an earlier discussion that a forward-biased PN junction is comparable to a low- resistance circuit element because it passes a high current for a given voltage. In turn, a reverse-biased PN junction is comparable to a high-resistance circuit element. By using the Ohm’s law formula for power (P = I2R) and assuming current is held constant, you can conclude that the power developed across a high resistance is greater than that developed across a low resistance. Thus, if a crystal were to contain two PN junctions (one forward-biased and the other reverse-biased), a low-power signal could be injected into the forward-biased junction and produce a high-power signal at the reverse-biased junction. In this manner, a power gain would be obtained across the crystal. This concept, which is merely an extension of the material covered in chapter 1, is the basic theory behind how the transistor amplifies. With this information fresh in your mind, let’s proceed directly to the NPN transistor.
2.3.1. NPN Transistor Operation
Just as in the case of the PN junction diode, the N material comprising the two end sections of the NPN transistor contains a number of free electrons, while the center P section contains an excess number of holes. The action at each junction between these sections is the same as that previously described for the diode; that is, depletion regions develop and the junction barrier appears. To use the transistor as an amplifier, each of these junctions must be modified by some external bias voltage. For the transistor to function in this capacity, the first PN junction (emitter-base junction) is biased in the forward, or low-resistance, direction. At the same time the second PN junction (base-collector junction) is biased in the reverse, or high- resistance, direction. A simple way to remember how to properly bias a transistor is to observe the NPN or PNP elements that make up the transistor. The letters of these elements indicate what polarity voltage to use for correct bias. For instance, notice the NPN transistor in Figure 6.
The emitter, which is the first letter in the NPN sequence, is connected to the negative side of the battery while the base, which is the second letter (NPN), is connected to the positive side.
However, since the second PN junction is required to be reverse biased for proper transistor operation, the collector must be connected to an opposite polarity voltage (positive) than that indicated by its letter designation(NPN). The voltage on the collector must also be more positive than the base, as shown in Figure 7.
We now have a properly biased NPN transistor.
In summary, the base of the NPN transistor must be positive with respect to the emitter, and the collector must be more positive than the base.
184.108.40.206. NPN Forward-Biased Junction
An important point to bring out at this time, which was not necessarily mentioned during the explanation of the diode, is the fact that the N material on one side of the forward-biased junction is more heavily doped than the P material. This results in more current being carried across the junction by the majority carrier electrons from the N material than the majority carrier holes from the P material. Therefore, conduction through the forward-biased junction, as shown in Figure 8, is mainly by majority carrier electrons from the N material (emitter).
With the emitter-to-base junction in the figure biased in the forward direction, electrons leave the negative terminal of the battery and enter the N material (emitter). Since electrons are majority current carriers in the N material, they pass easily through the emitter, cross over the junction, and combine with holes in the P material (base). For each electron that fills a hole in the P material, another electron will leave the P material (creating a new hole) and enter the positive terminal of the battery.
220.127.116.11. NPN Reverse-Biased Junction
The second PN junction (base-to-collector), or reverse- biased junction as it is called (Figure 9), blocks the majority current carriers from crossing the junction. However, there is a very small current, mentioned earlier, that does pass through this junction. This current is called minority current, or reverse current. As you recall, this current was produced by the electron-hole pairs. The minority carriers for the reverse-biased PN junction are the electrons in the P material and the holes in the N material. These minority carriers actually conduct the current for the reverse-biased junction when electrons from the P material enter the N material, and the holes from the N material enter the P material. However, the minority current electrons (as you will see later) play the most important part in the operation of the NPN transistor.
At this point you may wonder why the second PN junction (base-to-collector) is not forward biased like the first PN junction (emitter-to-base). If both junctions were forward biased, the electrons would have a tendency to flow from each end section of the N P N transistor (emitter and collector) to the center P section (base). In essence, we would have two junction diodes possessing a common base, thus eliminating any amplification and defeating the purpose of the transistor. A word of caution is in order at this time. If you should mistakenly bias the second PN junction in the forward direction, the excessive current could develop enough heat to destroy the junctions, making the transistor useless. Therefore, be sure your bias voltage polarities are correct before making any electrical connections.
18.104.22.168. NPN Junction Interaction
We are now ready to see what happens when we place the two junctions of the NPN transistor in operation at the same time. For a better understanding of just how the two junctions work together, refer to Figure 10 during the discussion.
The bias batteries in this figure have been labeled VCC for the collector voltage supply, and VBB for the base voltage supply. Also notice the base supply battery is quite small, as indicated by the number of cells in the battery, usually 1 volt or less. However, the collector supply is generally much higher than the base supply, normally around 6 volts. As you will see later, this difference in supply voltages is necessary to have current flow from the emitter to the collector.
As stated earlier, the current flow in the external circuit is always due to the movement of free electrons. Therefore, electrons flow from the negative terminals of the supply batteries to the N-type emitter. This combined movement of electrons is known as emitter current (IE). Since electrons are the majority carriers in the N material, they will move through the N material emitter to the emitter-base junction. With this junction forward biased, electrons continue on into the base region. Once the electrons are in the base, which is a P-type material, they become minority carriers. Some of the electrons that move into the base recombine with available holes. For each electron that recombines, another electron moves out through the base lead as base current IB (creating a new hole for eventual combination) and returns to the base supply battery VBB. The electrons that recombine are lost as far as the collector is concerned. Therefore, to make the transistor more efficient, the base region is made very thin and lightly doped. This reduces the opportunity for an electron to recombine with a hole and be lost. Thus, most of the electrons that move into the base region come under the influence of the large collector reverse bias. This bias acts as forward bias for the minority carriers (electrons) in the base and, as such, accelerates them through the base-collector junction and on into the collector region. Since the collector is made of an N-type material, the electrons that reach the collector again become majority current carriers. Once in the collector, the electrons move easily through the N material and return to the positive terminal of the collector supply battery VCC as collector current (IC).
To further improve on the efficiency of the transistor, the collector is made physically larger than the base for two reasons: (1) to increase the chance of collecting carriers that diffuse to the side as well as directly across the base region, and (2) to enable the collector to handle more heat without damage.
In summary, total current flow in the NPN transistor is through the emitter lead. Therefore, in terms of percentage, IE is 100 percent. On the other hand, since the base is very thin and lightly doped, a smaller percentage of the total current (emitter current) will flow in the base circuit than in the collector circuit. Usually no more than 2 to 5 percent of the total current is base current (IB) while the remaining 95 to 98 percent is collector current (IC). A very basic relationship exists between these two currents:
In simple terms this means that the emitter current is separated into base and collector current. Since the amount of current leaving the emitter is solely a function of the emitter-base bias, and because the collector receives most of this current, a small change in emitter-base bias will have a far greater effect on the magnitude of collector current than it will have on base current. In conclusion, the relatively small emitter- base bias controls the relatively large emitter-to-collector current.
To properly bias an NPN transistor, what polarity voltage is applied to the collector, and what is its relationship to the base voltage?
Why is conduction through the forward-biased junction of an NPN transistor primarily in one direction, namely from the emitter to base?
In the NPN transistor, what section is made very thin compared with the other two sections?
What percentage of current in an NPN transistor reaches the collector?
2.3.2. PNP Transistor Operation
The PNP transistor works essentially the same as the NPN transistor. However, since the emitter, base, and collector in the PNP transistor are made of materials that are different from those used in the NPN transistor, different current carriers flow in the PNP unit. The majority current carriers in the PNP transistor are holes. This is in contrast to the NPN transistor where the majority current carriers are electrons. To support this different type of current (hole flow), the bias batteries are reversed for the PNP transistor. A typical bias setup for the PNP transistor is shown in Figure 11. Notice that the procedure used earlier to properly bias the NPN transistor also applies here to the PNP transistor. The first letter (P) in the PNP sequence indicates the polarity of the voltage required for the emitter (positive), and the second letter (N) indicates the polarity of the base voltage (negative). Since the base-collector junction is always reverse biased, then the opposite polarity voltage (negative) must be used for the collector. Thus, the base of the PNP transistor must be negative with respect to the emitter, and the collector must be more negative than the base. Remember, just as in the case of the NPN transistor, this difference in supply voltage is necessary to have current flow (hole flow in the case of the PNP transistor) from the emitter to the collector. Although hole flow is the predominant type of current flow in the PNP transistor, hole flow only takes place within the transistor itself, while electrons flow in the external circuit. However, it is the internal hole flow that leads to electron flow in the external wires connected to the transistor.
22.214.171.124. PNP Forward-Biased Junction
Now let us consider what happens when the emitter-base junction in Figure 12 is forward biased. With the bias setup shown, the positive terminal of the battery repels the emitter holes toward the base, while the negative terminal drives the base electrons toward the emitter. When an emitter hole and a base electron meet, they combine. For each electron that combines with a hole, another electron leaves the negative terminal of the battery, and enters the base. At the same time, an electron leaves the emitter, creating a new hole, and enters the positive terminal of the battery. This movement of electrons into the base and out of the emitter constitutes base current flow (IB), and the path these electrons take is referred to as the emitter-base circuit.
126.96.36.199. PNP Reverse-Biased Junction
In the reverse-biased junction (Figure 13), the negative voltage on the collector and the positive voltage on the base block the majority current carriers from crossing the junction. However, this same negative collector voltage acts as forward bias for the minority current holes in the base, which cross the junction and enter the collector. The minority current electrons in the collector also sense forward bias-the positive base voltage-and move into the base. The holes in the collector are filled by electrons that flow from the negative terminal of the battery. At the same time the electrons leave the negative terminal of the battery, other electrons in the base break their covalent bonds and enter the positive terminal of the battery. Although there is only minority current flow in the reverse-biased junction, it is still very small because of the limited number of minority current carriers.
188.8.131.52. PNP Junction Interaction
The interaction between the forward- and reverse-biased junctions in a PNP transistor is very similar to that in an NPN transistor, except that in the PNP transistor, the majority current carriers are holes. In the PNP transistor shown in Figure 14, the positive voltage on the emitter repels the holes toward the base. Once in the base, the holes combine with base electrons. But again, remember that the base region is made very thin to prevent the recombination of holes with electrons. Therefore, well over 90 percent of the holes that enter the base become attracted to the large negative collector voltage and pass right through the base. However, for each electron and hole that combine in the base region, another electron leaves the negative terminal of the base battery (VBB) and enters the base as base current (IB). At the same time an electron leaves the negative terminal of the battery, another electron leaves the emitter as IE (creating a new hole) and enters the positive terminal of VBB. Meanwhile, in the collector circuit, electrons from the collector battery (VCC) enter the collector as Ic and combine with the excess holes from the base. For each hole that is neutralized in the collector by an electron, another electron leaves the emitter and starts its way back to the positive terminal of VCC.
Although current flow in the external circuit of the PNP transistor is opposite in direction to that of the NPN transistor, the majority carriers always flow from the emitter to the collector. This flow of majority carriers also results in the formation of two individual current loops within each transistor. One loop is the base-current path, and the other loop is the collector-current path. The combination of the current in both of these loops (IB + IC) results in total transistor current (IE). The most important thing to remember about the two different types of transistors is that the emitter-base voltage of the PNP transistor has the same controlling effect on collector current as that of the NPN transistor. In simple terms, increasing the forward- bias voltage of a transistor reduces the emitter-base junction barrier. This action allows more carriers to reach the collector, causing an increase in current flow from the emitter to the collector and through the external circuit. Conversely, a decrease in the forward-bias voltage reduces collector current.
What are the majority current carriers in a PNP transistor?
What is the relationship between the polarity of the voltage applied to the PNP transistor and that applied to the NPN transistor?
What is the letter designation for base current?
Name the two current loops in a transistor.
3. Transistor Specifications
Transistors are available in a large variety of shapes and sizes, each with its own unique characteristics. The characteristics for each of these transistors are usually presented on SPECIFICATION SHEETS or they may be included in transistor manuals. Although many properties of a transistor could be specified on these sheets, manufacturers list only some of them. The specifications listed vary with different manufacturers, the type of transistor, and the application of the transistor. The specifications usually cover the following items.
A general description of the transistor that includes the following information:
The kind of transistor. This covers the material used, such as germanium or silicon; the type of transistor (NPN or PNP); and the construction of the transistor(whether alloy-junction, grown, or diffused junction, etc.).
Some of the common applications for the transistor, such as audio amplifier, oscillator, rf amplifier, etc.
General sales features, such as size and packaging mechanical data).
The "Absolute Maximum Ratings" of the transistor are the direct voltage and current values that if exceeded in operation may result in transistor failure. Maximum ratings usually include collector-to-base voltage, emitter-to-base voltage, collector current, emitter current, and collector power dissipation.
The typical operating values of the transistor. These values are presented only as a guide. The values vary widely, are dependent upon operating voltages, and also upon which element is common in the circuit. The values listed may include collector-emitter voltage, collector current, input resistance, load resistance, current-transfer ratio (another name for alpha or beta), and collector cutoff current, which is leakage current from collector to base when no emitter current is applied. Transistor characteristic curves may also be included in this section. A transistor characteristic curve is a graph plotting the relationship between currents and voltages in a circuit. More than one curve on a graph is called a "family of curves."
Additional information for engineering-design purposes. So far, many letter symbols, abbreviations, and terms have been introduced, some frequently used and others only rarely used. For a complete list of all semiconductor letter symbols and terms, refer to EIMB series 000-0140, Section III. // todo:tk.
4. Transistor Identification
Transistors can be identified by a Joint Army-Navy (JAN) designation printed directly on the case of the transistor. The marking scheme explained earlier for diodes is also used for transistor identification. The first number indicates the number of junctions. The letter "N" following the first number tells us that the component is a semiconductor. And, the 2- or 3-digit number following the N is the manufacturer’s identification number. If the last number is followed by a letter, it indicates a later, improved version of the device. For example, a semiconductor designated as type 2N130A signifies a three-element transistor of semiconductor material that is an improved version of type 2N130. The fields of the type lable for the 2N230A is shown in Table 1.
NUMBER OF JUNCTIONS (TRANSISTOR)
You may also find other markings on transistors that do not relate to the JAN marking system. These markings are manufacturers' identifications and may not conform to a standardized system. If in doubt, always replace a transistor with one having identical markings. To ensure that an identical replacement or a correct substitute is used, consult an equipment or transistor manual for specifications on the transistor.
5. Transistor Maintenance
Transistors are very rugged and are expected to be relatively trouble free. Encapsulation and conformal coating techniques now in use promise extremely long life expectancies. In theory, a transistor should last indefinitely. However, if transistors are subjected to current overloads, the junctions will be damaged or even destroyed. In addition, the application of excessively high operating voltages can damage or destroy the junctions through arc-over or excessive reverse currents. One of the greatest dangers to the transistor is heat, which will cause excessive current flow and eventual destruction of the transistor.
To determine if a transistor is good or bad, you can check it with an ohmmeter or a transistor tester. In many cases, you can substitute a transistor known to be good for one that is questionable and thus determine the condition of a suspected transistor. This method of testing is highly accurate and sometimes the quickest, but it should be used only after you make certain that there are no circuit defects that might damage the replacement transistor. If more than one defective transistor is present in the equipment where the trouble has been localized, this testing method becomes cumbersome, as several transistors may have to be replaced before the trouble is corrected. To determine which stages failed and which transistors are not defective, all the removed transistors must be tested. This test can be made by using a standard Navy ohmmeter, transistor tester, or by observing whether the equipment operates correctly as each of the removed transistors is reinserted into the equipment. A word of caution-indiscriminate substitution of transistors in critical circuits should be avoided.
When transistors are soldered into equipment, substitution is not practicable; it is generally desirable to test these transistors in their circuits.
List three items of information normally included in the general description section of a specification sheet for a transistor.
What does the number "2" (before the letter "N") indicate in the JAN marking scheme?
What is the greatest danger to a transistor?
What method for checking transistors is cumbersome when more than one transistor is bad in a circuit?
Transistors, although generally more rugged mechanically than electron tubes, are susceptible to damage by electrical overloads, heat, humidity, and radiation. Damage of this nature often occurs during transistor servicing by applying the incorrect polarity voltage to the collector circuit or excessive voltage to the input circuit. Careless soldering techniques that overheat the transistor have also been known to cause considerable damage. One of the most frequent causes of damage to a transistor is the electrostatic discharge from the human body when the device is handled. You may avoid such damage before starting repairs by discharging the static electricity from your body to the chassis containing the transistor. You can do this by simply touching the chassis. Thus, the electricity will be transferred from your body to the chassis before you handle the transistor.
To prevent transistor damage and avoid electrical shock, you should observe the following precautions when you are working with transistorized equipment:
Test equipment and soldering irons should be checked to make certain there is no leakage current from the power source. If leakage current is detected, isolation transformers should be used.
Always connect a ground between test equipment and circuit before attempting to inject or monitor a signal.
Ensure test voltages do not exceed maximum allowable voltage for circuit components and transistors. Also, never connect test equipment outputs directly to a transistor circuit.
Ohmmeter ranges that require a current of more than one milliampere in the test circuit should not be used for testing transistors.
Battery eliminators should not be used to furnish power for transistor equipment because they have poor voltage regulation and, possibly, high-ripple voltage.
The heat applied to a transistor, when soldered connections are required, should be kept to a minimum by using a low-wattage soldering iron and heat shunts, such as long-nose pliers, on the transistor leads.
When it becomes necessary to replace transistors, never pry transistors to loosen them from printed circuit boards.
All circuits should be checked for defects before replacing a transistor.
The power must be removed from the equipment before replacing a transistor.
Using conventional test probes on equipment with closely spaced parts often causes accidental shorts between adjacent terminals. These shorts rarely cause damage to an electron tube but may ruin a transistor. To prevent these shorts, the probes can be covered with insulation, except for a very short length of the tips.
7. Lead Identification
Transistor lead identification plays an important part in transistor maintenance; because, before a transistor can be tested or replaced, its leads or terminals must be identified. Since there is no standard method of identifying transistor leads, it is quite possible to mistake one lead for another. Therefore, when you are replacing a transistor, you should pay close attention to how the transistor is mounted, particularly to those transistors that are soldered in, so that you do not make a mistake when you are installing the new transistor. When you are testing or replacing a transistor, if you have any doubts about which lead is which, consult the equipment manual or a transistor manual that shows the specifications for the transistor being used.
There are, however, some typical lead identification schemes that will be very helpful in transistor troubleshooting. These schemes are shown in Figure 15. In the case of the oval-shaped transistor shown in view A, the collector lead is identified by a wide space between it and the base lead. The lead farthest from the collector, in line, is the emitter lead. When the leads are evenly spaced and in line, as shown in view B, a colored dot, usually red, indicates the collector. If the transistor is round, as in view C, a red line indicates the collector, and the emitter lead is the shortest lead. In view D the leads are in a triangular arrangement that is offset from the center of the transistor. The lead opposite the blank quadrant in this scheme is the base lead. When viewed from the bottom, the collector is the first lead clockwise from the base. The leads in view E are arranged in the same manner as those is view D except that a tap is used to identify the leads. When viewed from the bottom in a clockwise direction, the first lead following the tab is the emitter, followed by the base and collector.
In a conventional power transistor as shown in views F and G, the collector lead is usually connected to the mounting base. For further identification, the base lead in view F is covered with green sleeving. While the leads in view G are identified by viewing the transistor from the bottom in a clockwise direction (with mounting holes occupying 3 o’clock and 9 o’clock positions), the emitter lead will be either at the 5 o’clock or 11 o’clock position. The other lead is the base lead.
8. Transistor Testing
There are several different ways of testing transistors. They can be tested while in the circuit, by the substitution method mentioned, or with a transistor tester or ohmmeter.
Transistor testers are nothing more than the solid-state equivalent of electron-tube testers (although they do not operate on the same principle). With most transistor testers, it is possible to test the transistor in or out of the circuit.
There are four basic tests required for transistors in practical troubleshooting: gain, leakage, breakdown, and switching time. For maintenance and repair, however, a check of two or three parameters is usually sufficient to determine whether a transistor needs to be replaced.
Since it is impractical to cover all the different types of transistor testers and since each tester comes with its own operator’s manual, we will move on to something you will use more frequently for testing transistors-the ohmmeter.
8.1. Testing Transistors with an Ohmmeter
Two tests that can be done with an ohmmeter are gain, and junction resistance. Tests of a transistor’s junction resistance will reveal leakage, shorts, and opens.
8.1.1. Transistor Gain Test
A basic transistor gain test can be made using an ohmmeter and a simple test circuit. The test circuit can be made with just a couple of resistors and a switch, as shown in figure Figure 16. The principle behind the test lies in the fact that little or no current will flow in a transistor between emitter and collector until the emitter-base junction is forward biased. The only precaution you should observe is with the ohmmeter. Any internal battery may be used in the meter provided that it does not exceed the maximum collector-emitter breakdown voltage.
With the switch in Figure 16 in the open position as shown, no voltage is applied to the PNP transistor’s base, and the emitter-base junction is not forward biased. Therefore, the ohmmeter should read a high resistance, as indicated on the meter. When the switch is closed, the emitter-base circuit is forward biased by the voltage across R1 and R2. Current now flows in the emitter-collector circuit, which causes a lower resistance reading on the ohmmeter. A 10-to-1 resistance ratio in this test between meter readings indicates a normal gain for an audio-frequency transistor.
To test an NPN transistor using this circuit, simply reverse the ohmmeter leads and carry out the procedure described earlier.
8.1.2. Transistor Junction Resistance Test
An ohmmeter can be used to test a transistor for leakage (an undesirable flow of current) by measuring the base-emitter, base-collector, and collector- emitter forward and reverse resistances.
For simplicity, consider the transistor under test in Figure 17, Figure 18, and Figure 19 as two diodes connected back to back. Therefore, each diode will have a low forward resistance and a high reverse resistance. By measuring these resistances with an ohmmeter as shown in the figure, you can determine if the transistor is leaking current through its junctions. When making these measurements, avoid using the R1 scale on the meter or a meter with a high internal battery voltage. Either of these conditions can damage a low-power transistor.
The transistor is:
LOW (NOT SHORTED)
LOW (NOT SHORTED)
*Except collector-to-emitter test.
By now, you should recognize that the transistor used in figure 2-19 (view A, view B and view C) is a PNP transistor. If you wish to test an NPN transistor for leakage, the procedure is identical to that used for testing the PNP except the readings obtained are reversed.
When testing transistors (PNP or NPN), you should remember that the actual resistance values depend on the ohmmeter scale and the battery voltage. Typical forward and reverse resistances are insignificant. The best indicator for showing whether a transistor is good or bad is the ratio of forward-to-reverse resistance. If the transistor you are testing shows a ratio of at least 30 to 1, it is probably good. Many transistors show ratios of 100 to 1 or greater.
What safety precaution must be taken before replacing a transistor?
How is the collector lead identified on an oval-shaped transistor?
What are two transistor tests that can be done with an ohmmeter?
When you are testing the gain of an audio-frequency transistor with an ohmmeter, what isindicated by a 10-to-1 resistance ratio?
When you are using an ohmmeter to test a transistor for leakage, what is indicated by a low, butnot shorted, reverse resistance reading?
9. Testing Transistors
Most transistorized equipments use printed circuit boards on which components are neatly arranged. This arrangement makes the transistors and other components easy to reach while you are troubleshooting and servicing the equipment. While investigating with test probes, however, you must be careful to prevent damage to the printed wiring.
One of the outstanding advantages of transistors is their reliability. Tube failures account for over 90 percent of the failures in electron-tube equipments. Transistors, however, are long lived. This factor, among others, decreases maintenance required to keep transistorized equipment operating. The techniques used in testing transistorized equipment are similar to those for maintaining electron-tube circuits. Basically, these techniques include several checks and inspections.
9.1. Power Supply Checks
When using test equipment to localize a trouble, you should check the power supply to see that its output voltages are present and of the correct values. Improper power supply voltages can cause odd effects. You will prevent many headaches by checking the power supply first.
9.2. Visual Inspection
Visual inspection is a good maintenance technique. Occasionally, you will find loose wires or faulty connections, making extensive voltage checks unnecessary.
9.3. Transistor Checks
Transistors can be checked by substitution. Transistors, however, have a characteristic known as leakage current, which may affect the results obtained when the substitution method is used.
The leakage current may influence the current gain or amplification factor of the transistor. Therefore, a particular transistor might operate properly in one circuit and not in another. This characteristic is more critical in certain applications than in others. As the transistor ages, the amount of leakage current tends to increase. One type of transistor checker used is the semiconductor test set. This test set can be used either for in-circuit or out-of-circuit tests or for collector leakage current or current gain. You should use extreme care when substituting transistors. More and more transistors have specific current and breakdown voltage requirements that may affect how they operate within a given circuit.
Q-12. As a transistor ages, what happens to the leakage current?
9.4. Voltage Checks
Voltage measurements provide a means of checking circuit conditions in a transistorized circuit just as they do in checking conditions in a tube circuit. The voltages, however, are much lower than in a tube circuit. The bias voltage between the base and emitter, for instance, is usually 0.05 to 0.20 volts. When making checks, observe polarity.
9.5. Resistance Checks
Transistors have little tendency to burn or change value because of low voltage in their circuits. They can, however, be permanently damaged by high-voltage conditions that occur when the collector voltage is increased. They can also be permanently damaged when the ambient temperature increases and causes excessive collector current flow. Transistors are easily damaged by high current; therefore, resistance measurements must not be taken with an ohmmeter that provides a maximum current output in excess of 1 milliampere. If you are not sure that the range of ohmmeter you want to use is below the 1 milliampere level, connect the ohmmeter to a milliammeter and check it. See Figure 20 for a method of measuring the current from an ohmmeter.
Resistance measurements usually are not made in transistorized circuits, except when you are checking for open windings in transformers and coils. When a resistance check is required, the transistors are usually removed from the circuit. Resistance checks cannot test all the characteristics of transistors, especially transistors designed for high frequencies or fast switching. The ohmmeter is capable of making simple transistor tests, such as open and short tests.
Refer to NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, for a review of transistor and semiconductor terms and theory.
10. Testing Semiconductors
Unlike vacuum tubes, transistors are very rugged in that they can tolerate vibration and a rather large degree of shock. Under normal operating conditions, they will provide dependable operation for a long period of time. However, transistors are subject to failure when they are subjected to relatively minor overloads. Crystal detectors are also subject to failure or deterioration when subjected to electrical overloads and will deteriorate from a long period of normal use. To determine the condition of semiconductors, you can use various test methods. In many cases you may substitute a transistor of known good quality for a questionable one to determine the condition of a suspected transistor. This method is highly accurate and sometimes efficient. However, you should avoid indiscriminate substitution of semiconductors in critical circuits. When transistors are soldered into equipment, substitution becomes impractical - generally, you should test these transistors while they are in their circuits.
Q-7. What is the major advantage of a transistor over a tube?
Since certain fundamental characteristics indicate the condition of semiconductors, test equipment is available that allows you to test these characteristics with the semiconductors in or out of their circuits. Crystal-rectifier testers normally allow you to test only the forward-to-reverse current ratio of the crystal. Transistor testers, however, allow you to measure several characteristics, such as the collector leakage current (Ic), collector to base current gain (β), and the four-terminal network parameters. The most useful test characteristic is determined by the type of circuit in which the transistor will be used. Thus, the alternating-current beta measurement is preferred for ac amplifier or oscillator applications; and for switching-circuit applications, a direct-current beta measurement may prove more useful.
Many common transistors are extremely heat sensitive. Excess heat will cause the semiconductor to either fail or give intermittent operation. You have probably experienced intermittent equipment problems and know them to be both time consuming and frustrating. You know, for example, that if a problem is in fact caused by heat, simply opening the equipment during the course of troubleshooting may cause the problem to disappear. You can generally isolate the problem to the faulty printed-circuit board (pcb) by observing the fault indications. However, to further isolate the problem to a faulty component, sometimes you must apply a minimal amount of heat to the suspect pcb by carefully using a low wattage, heat shrink gun; an incandescent drop light; or a similar heating device. Be careful not to overheat the pcb. Once the fault indication reappears, you can isolate the faulty component by spraying those components suspected as being bad with a nonconductive circuit coolant, such as Freon. If the alternate heating and cooling of a component causes it to operate intermittently, you should replace it.
Q-8. Name two major disadvantages of transistors.
10.1. Transistor Testing
When trouble occurs in solid-state equipment, you should first check power supplies and perform voltage measurements, waveform checks, signal substitution, or signal tracing. If you isolate a faulty stage by one of these test methods, then voltage, resistance, and current measurements can be made to locate defective parts. When you make these measurements, the voltmeter impedance must be high enough that it exerts no appreciable effect upon the voltage being measured. Also, current from the ohmmeter you use must not damage the transistors. If the transistors are not soldered into the equipment, you should remove the transistors from the sockets during a resistance test. Transistors should be removed from or reinserted into the sockets only after power has been removed from the stage; otherwise damage by surge currents may result.
Transistor circuits, other than pulse and power amplifier stages, are usually biased so that the emitter current is from 0.5 milliampere to 3 milliamperes and the collector voltage is from 3 to 15 volts. You can measure the emitter current by opening the emitter connector and inserting a milliammeter in series. When you make this measurement, you should expect some change in bias because of the meter resistance. You can often determine the collector current by measuring the voltage drop across a resistor in the collector circuit and calculating the current. If the transistor itself is suspected, it can be tested by one or more of the methods described below.
10.1.1. Resistance Test
You can use an ohmmeter to test transistors by measuring the emitter-collector, base-emitter, and base-collector forward and reverse resistances. A back-to-forward resistance ratio on the order of 100 to 1 or greater should be obtained for the collector-to-base and emitter-to-base measurements. The forward and reverse resistances between the emitter and collector should be nearly equal. You should make all three measurements for each transistor you test, because experience has shown that transistors can develop shorts between the collector and emitter and still have good forward and reverse resistances for the other two measurements. Because of shunting resistances in transistor circuits, you will normally have to disconnect at least two transistor leads from the associated circuit for this test. Exercise caution during this test to make certain that current during the forward resistance tests does not exceed the rating of the transistor — ohmmeter ranges requiring a current of more than 1 milliampere should not be used for testing transistors. Many ohmmeters are designed such that on the R × 1 range, 100 milliamperes or more can flow through the electronic part under test. For this reason, you should use a digital multimeter. Be sure you select a digital multimeter that produces enough voltage to properly bias the transistor junctions.
Q-9. When you are using an ohmmeter to test a transistor, what range settings should be avoided?
10.1.2. Transistor Testers
Laboratory transistor test sets are used in experimental work to test all characteristics of transistors. For maintenance and repair, however, it is not necessary to check all of the transistor parameters. A check of two or three performance characteristics is usually sufficient to determine whether a transistor needs to be replaced. Two of the most important parameters used for transistor testing are the transistor current gain (beta) and the collector leakage or reverse current (Ic).
The semiconductor test set (Figure 21) is a rugged, field type of tester designed to test transistors and semiconductor diodes. The set measures the beta of a transistor, resistance appearing at the electrodes, reverse current of a transistor or semiconductor diode, shorted or open conditions of a diode, forward transconductance of a field-effect transistor, and condition of its own batteries.
In order to assure that accurate and useful information is gained from the transistor tester, the following preliminary checks of the tester should be made prior to testing any transistors.
With the POLARITY switch (Figure 21) in the OFF position, the meter pointer should indicate exactly zero. (When required, rotate the meter adjust screw on the front of the meter to fulfill this requirement.) When measurements are not actually being made, the POLARITY switch must always be left in the OFF position to prevent battery drain.
Always check the condition of the test set batteries by disconnecting the test set power cord, placing the POLARITY switch in the PNP position and placing the FUNCTION switch first to BAT.1, then to BAT.2. In both BAT positions the meter pointer should move so as to indicate within the red BAT range.
BETA MEASUREMENTS.—If the transistor is to be tested out of the circuit, plug it into the test jack located on the right-hand side below the meter shown in Figure 21. If the transistor is to be tested in the circuit, it is imperative that at least 300 ohms exist between E-B, C-B, and C-E for accurate measurement. Initial settings of the test set controls are as follows:
FUNCTION switch to BETA
POLARITY switch to PNP or NPN (dependent on type of transistor under test)
RANGE switch to X10
Adjust METER ZERO for zero meter indication (transistor disconnected)
|The POLARITY switch should remain OFF while the transistor is connected to or disconnected from the test set. If you determine that the beta reading is less than 10, reset the RANGE switch to X1 and reset the meter to zero.
After connecting the yellow test lead to the emitter, the green test lead to the base, and the blue test lead to the collector, plug the test probe (not shown) into the jack located at the lower right-hand corner of the test set. When testing grounded equipment, unplug the 115 vac line cord and use battery operation. The beta reading is attained by multiplying the meter reading times the RANGE switch setting. Refer to the transistor characteristics book provided with the tester to determine if the reading is normal for the type of transistor under test.
ELECTRODE RESISTANCE MEASUREMENTS.—Connect the in-circuit probe test leads to the transistor with the yellow lead to the emitter, the green lead to the base, and the blue lead to the collector. Set the FUNCTION switch to the OHMS E-B position, and read the resistance between the emitter and base electrode on the center scale of the meter.
To read the resistance between the collector and base and the collector and emitter, set the FUNCTION switch to OHMS C-B and OHMS C-E. These in-circuit electrode resistance measurements are used to correctly interpret the in-circuit beta measurements. The accuracy of the BETA X1, X10 range is ±15 percent only when the emitter-to-base load is equal to or greater than 300 ohms.
Ic MEASUREMENTS.—Adjust the METER ZERO control for zero meter indication. Plug the transistor to be tested into the jack or connect test leads to the device under test. Set the PNP/NPN switch to correspond with the transistor under test. Set the FUNCTION switch to Ic and the RANGE switch to X0.1, X1, or X10 as specified by the transistor data book for allowable leakage. Read the amount of leakage on the bottom scale, and multiply this by the range setting figure as required.
DIODE MEASUREMENTS.—Diode qualitative in-circuit measurements are attained by connecting the green test lead to the cathode and the yellow test lead to the anode. Set the FUNCTION switch to DIODE IN/CKT and the RANGE switch to X1. (Ensure that the meter has been properly zeroed on this scale.) If the meter reads down scale, reverse the POLARITY switch. If the meter reads less than midscale, the diode under test is either open or shorted. The related circuit impedance of this test is less than 25 ohms.
PRECAUTIONS.—Transistors, although generally more rugged mechanically than electron tubes, are susceptible to damage by excessive heat and electrical overload. The following precautions should be taken in servicing transistorized equipment:
Test equipment and soldering irons must be checked to make certain that there is no leakage current from the power source. If leakage current is detected, isolation transformers must be used.
Ohmmeter ranges that require a current of more than 1 milliampere in the test circuit are not to be used for testing transistors.
Battery eliminators should not be used to furnish power for transistor equipment because they have poor voltage regulation and, possibly, high ripple voltage.
The heat applied to a transistor, when soldered connections are required, should be kept to a minimum by using a low-wattage soldering iron and heat shunts (such as long-nose pliers) on the transistor leads.
All circuits should be checked for defects before a transistor is replaced.
The power should be removed from the equipment before replacing a transistor or other circuit part.
When working on equipment with closely spaced parts, you will find that conventional test probes are often the cause of accidental short circuits between adjacent terminals. Momentary short circuits, which rarely cause damage to an electron tube, may ruin a transistor. To avoid accidental shorts, a test probe can be covered with insulation for all but a very short length of the tip.