1. Introduction

If you consider the sensitive nature and the various interacting properties of semiconductors, it should not be surprising to you that solid state devices can be designed for many different purposes. In fact, devices with special features are so numerous and new designs are so frequently introduced that it would be beyond the scope of this chapter to describe all of the devices in use today. Therefore, this chapter will include a variety of representative devices that are used extensively in Navy equipment to give you an idea of the diversity and versatility that have been made possible. These devices have been grouped into three categories: diodes, optoelectronic devices, and transistors. In this chapter each device will be described and the basic operation of each one will be discussed.

2. Diodes

Diodes are two terminal semiconductors of various types that are used in seemingly endless applications. The operation of normal PN-junction diodes has already been discussed, but there are a number of diodes with special properties with which you should be familiar. A discussion of all of the developments in the diode field would be impossible so some of the more commonly used special diodes have been selected for explanation. These include Zener diodes, tunnel diodes, varactors, silicon controlled rectifiers (SCR), and TRIACs.

2.1. The Tunnel Diode

In 1958, Leo Esaki, a Japanese scientist, discovered that if a semiconductor junction diode is heavily doped with impurities, it will have a region of negative resistance. The normal junction diode uses semiconductor materials that are lightly doped with one impurity atom for ten-million semiconductor atoms. This low doping level results in a relatively wide depletion region. Conduction occurs in the normal junction diode only if the voltage applied to it is large enough to overcome the potential barrier of the junction.

In the TUNNEL DIODE, the semiconductor materials used in forming a junction are doped to the extent of one-thousand impurity atoms for ten-million semiconductor atoms. This heavy doping produces an extremely narrow depletion zone similar to that in the Zener diode. Also because of the heavy doping, a tunnel diode exhibits an unusual current-voltage characteristic curve as compared with that of an ordinary junction diode. The characteristic curve for a tunnel diode is illustrated in Figure 1.

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Figure 1. Characteristic curve of a tunnel diode compared to that of a standard PN junction.

The three most important aspects of this characteristic curve are (1) the forward current increase to a peak (IP) with a small applied forward bias, (2) the decreasing forward current with an increasing forward bias to a minimum valley current (IV), and (3) the normal increasing forward current with further increases in the bias voltage. The portion of the characteristic curve between IP and IV is the region of negative resistance. An explanation of why a tunnel diode has a region of negative resistance is best understood by using energy levels as in the previous explanation of the Zener effect.

Simply stated the theory known as quantum-mechanical tunneling is an electron crossing a PN- junction without having sufficient energy to do so otherwise. Because of the heavy doping the width of the depletion region is only one-millionth of an inch. You might think of the process simply as an arc- over between the N- and the P-side across the depletion region.

Figure 2 and Figure 3 shows the equilibrium energy level diagram of a tunnel diode with no bias applied. Note in Figure 2 that the valence band of the P-material overlaps the conduction band of the N-material. The majority electrons and holes are at the same energy level in the equilibrium state. If there is any movement of current carriers across the depletion region due to thermal energy, the net current flow will be zero because equal numbers of current carriers flow in opposite directions. The zero net current flow is marked by a "0" on the current-voltage curve illustrated in Figure 3.

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Figure 2. Tunnel diode energy diagram with no bias (A).
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Figure 3. Tunnel diode energy diagram with no bias (B).

Figure 4 shows the energy diagram of a tunnel diode with a small forward bias (50 millivolts) applied. The bias causes unequal energy levels between some of the majority carriers at the energy band overlap point, but not enough of a potential difference to cause the carriers to cross the forbidden gap in the normal manner. Since the valence band of the P-material and the conduction band of the N-material still overlap, current carriers tunnel across at the overlap and cause a substantial current flow. The amount of current flow is marked by point 2 on the curve in Figure 5. Note in Figure 4 that the amount of overlap between the valence band and the conduction band decreased when forward bias was applied.

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Figure 4. Tunnel diode energy diagram with 50 millivolts bias (A).
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Figure 5. Tunnel diode energy diagram with 50 millivolts bias (B).

Figure 6 is the energy diagram of a tunnel diode in which the forward bias has been increased to 450 millivolts. As you can see, the valence band and the conduction band no longer overlap at this point, and tunneling can no longer occur. The portion of the curve in Figure 7 from point 2 to point 3 shows the decreasing current that occurs as the bias is increased, and the area of overlap becomes smaller. As the overlap between the two energy bands becomes smaller, fewer and fewer electrons can tunnel across the junction. The portion of the curve between point 2 and point 3 in which current decreases as the voltage increases is the negative resistance region of the tunnel diode.

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Figure 6. Tunnel diode energy diagram with 450 millivolts bias.
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Figure 7. Tunnel diode energy diagram with 450 millivolts bias.

Figure 8 is the energy diagram of a tunnel diode in which the forward bias has been increased even further. The energy bands no longer overlap and the diode operates in the same manner as a normal PN junction, as shown by the portion of the curve in Figure 9 from point 3 to point 4.

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Figure 8. Tunnel diode energy diagram with 600 millivolts bias.
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Figure 9. Tunnel diode energy diagram with 600 millivolts bias.

The negative resistance region is the most important and most widely used characteristic of the tunnel diode. A tunnel diode biased to operate in the negative resistance region can be used as either an oscillator or an amplifier in a wide range of frequencies and applications. Very high frequency applications using the tunnel diode are possible because the tunneling action occurs so rapidly that there is no transit time effect and therefore no signal distortion. Tunnel diodes are also used extensively in high- speed switching circuits because of the speed of the tunneling action.

Several schematic symbols are used to indicate a tunnel diode. These symbols are illustrated in Figure 10, Figure 11, Figure 12, and Figure 13.

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Figure 10. Tunnel diode schematic symbols (A).
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Figure 11. Tunnel diode schematic symbols (B).
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Figure 12. Tunnel diode schematic symbols (C).
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Figure 13. Tunnel diode schematic symbols (D).

2.2. Varactor

The VARACTOR, or varicap, as the schematic drawing in Figure 14 suggests, is a diode that behaves like a variable capacitor, with the PN junction functioning like the dielectric and plates of a common capacitor. Understanding how the varactor operates is an important prerequisite to understanding field-effect transistors, which will be covered later in this topic.

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Figure 14. Varactor diode.

Figure 15 shows a PN junction. Surrounding the junction of the P and N materials is a narrow region void of both positively and negatively charged current carriers. This area is called the depletion region.

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Figure 15. PN junction.

The size of the depletion region in a varactor diode is directly related to the bias. Forward biasing makes the region smaller by repelling the current carriers toward the PN junction. If the applied voltage is large enough (about .5 volt for silicon material), the negative particles will cross the junction and join with the positive particles, as shown in Figure 16. This forward biasing causes the depletion region to decrease, producing a low resistance at the PN junction and a large current flow across it. This is the condition for a forward-biased diode. On the other hand, if reverse-bias voltage is applied to the PN junction, the size of its depletion region increases as the charged particles on both sides move away from the junction. This condition, shown in Figure 17, produces a high resistance between the terminals and allows little current flow (only in the microampere range). This is the operating condition for the varactor diode, which is nothing more than a special PN junction.

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Figure 16. Forward-biased PN junction.
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Figure 17. Reverse-biased PN junction.

As the figure shows, the insulation gap formed by reverse biasing of the varactor is comparable to the layer of dielectric material between the plates of a common capacitor. Furthermore, the formula used to calculate capacitance

\[C = \frac{AK}{d}\]


\(\hspace{15 mm}A =\) plate area
\(\hspace{15 mm}K =\) a constant value
\(\hspace{15 mm}d =\) distance between plates

can be applied to both the varactor and the capacitor. In this case, the size of the insulation gap of the varactor, or depletion region, is substituted for the distance between the plates of the capacitor. By varying the reverse-bias voltage applied to the varactor, the width of the "gap" may be varied. An increase in reverse bias increases the width of the gap (d) which reduces the capacitance (C) of the PN junction. Therefore, the capacitance of the varactor is inversely proportional to the applied reverse bias.

The ratio of varactor capacitance to reverse-bias voltage change may be as high as 10 to 1. Figure 18 shows one example of the voltage-to-capacitance ratio. View A shows that a reverse bias of 3 volts produces a capacitance of 20 picofarads in the varactor. If the reverse bias is increased to 6 volts, as shown in view B, the depletion region widens and capacitance drops to 5 picofarads. Each 1-volt increase in bias voltage causes a 5-picofarad decrease in the capacitance of the varactor; the ratio of change is therefore 5 to 1. Of course any decrease in applied bias voltage would cause a proportionate increase in capacitance, as the depletion region narrows. Notice that the value of the capacitance is small in the picofarad range.

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Figure 18. Varactor capacitance versus bias voltage.

In general, varactors are used to replace the old style variable capacitor tuning. They are used in tuning circuits of more sophisticated communication equipment and in other circuits where variable capacitance is required. One advantage of the varactor is that it allows a dc voltage to be used to tune a circuit for simple remote control or automatic tuning functions. One such application of the varactor is as a variable tuning capacitor in a receiver or transmitter tank circuit like that shown in Figure 19.

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Figure 19. Varactor tuned resonant circuit.

Figure 19 shows a dc voltage felt at the wiper of potentiometer R1 which can be adjusted between +V and −V. The dc voltage, passed through the low resistance of radio frequency choke L2, acts to reverse bias varactor diode C3. The capacitance of C3 is in series with C2, and the equivalent capacitance of C2 and C3 is in parallel with tank circuit L1-C1. Therefore, any variation in the dc voltage at R1 will vary both the capacitance of C3 and the resonant frequency of the tank circuit. The radio-frequency choke provides high inductive reactance at the tank frequency to prevent tank loading by R1. C2 acts to block dc from the tank as well as to fix the tuning range of C3.

An ohmmeter can be used to check a varactor diode in a circuit. A high reverse-bias resistance and a low forward-bias resistance with a 10 to 1 ratio in reverse-bias to forward-bias resistance is considered normal.


What is the main difference in construction between normal PN junction diodes and tunnel diodes?


What resistance property is found in tunnel diodes but not in normal diodes?


When compared to the ordinary diode, the tunnel diode has what type of depletion region?


In the tunnel diode, the tunneling current is at what level when the forbidden gap of the N-type material is at the same energy level as the empty states of the P-type material?


The varactor displays what useful electrical property?


When a PN junction is forward biased, what happens to the depletion region?


When the reverse bias on a varactor is increased, what happens to the effective capacitance?

2.3. Silicon Controlled Rectifier (SCR)

The SILICON CONTROLLED RECTIFIER, usually referred to as an SCR, is one of the family of semiconductors that includes transistors and diodes. A drawing of an SCR and its schematic representation is shown in Figure 20 and in Figure 21. Not all SCRs use the casing shown, but this is typical of most of the high-power units.

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Figure 20. Silicon controlled rectifier (A).
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Figure 21. Silicon controlled rectifier (B).

Although it is not the same as either a diode or a transistor, the SCR combines features of both. Circuits using transistors or rectifier diodes may be greatly improved in some instances through the use of SCRs.

The basic purpose of the SCR is to function as a switch that can turn on or off small or large amounts of power. It performs this function with no moving parts that wear out and no points that require replacing. There can be a tremendous power gain in the SCR; in some units a very small triggering current is able to switch several hundred amperes without exceeding its rated abilities. The SCR can often replace much slower and larger mechanical switches. It even has many advantages over its more complex and larger electron tube equivalent, the thyratron.

The SCR is an extremely fast switch. It is difficult to cycle a mechanical switch several hundred times a minute; yet, some SCRs can be switched 25,000 times a second. It takes just microseconds (millionths of a second) to turn on or off these units. Varying the time that a switch is on as compared to the time that it is off regulates the amount of power flowing through the switch. Since most devices can operate on pulses of power (alternating current is a special form of alternating positive and negative pulse), the SCR can be used readily in control applications. Motor-speed controllers, inverters, remote switching units, controlled rectifiers, circuit overload protectors, latching relays, and computer logic circuits all use the SCR.

The SCR is made up of four layers of semiconductor material arranged PNPN. The construction is shown in Figure 22. In function, the SCR has much in common with a diode, but the theory of operation of the SCR is best explained in terms of transistors.

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Figure 22. SCR structure (A).

Consider the SCR as a transistor pair, one PNP and the other NPN, connected as shown in Figure 23 and Figure 24. The anode is attached to the upper P-layer; the cathode, C, is part of the lower N-layer; and the gate terminal, G, goes to the P-layer of the NPN triode.

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Figure 23. SCR structure (B).
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Figure 24. SCR structure (C).

In operation the collector of Q2 drives the base of Q1, while the collector of Q1 feeds back to the base of Q2. (Beta) 1 is the current gain of Q1, and (Beta) 2 is the current gain of Q2. The gain of this positive feedback loop is their product, 1 times 2. When the product is less than one, the circuit is stable; if the product is greater than unity, the circuit is regenerative. A small negative current applied to terminal G will bias the NPN transistor into cutoff, and the loop gain is less than unity. Under these conditions, the only current that can exist between output terminals A and C is the very small cutoff collector current of the two transistors. For this reason the impedance between A and C is very high.

When a positive current is applied to terminal G, transistor Q2 is biased into conduction, causing its collector current to rise. Since the current gain of Q2 increases with increased collector current, a point (called the breakover point) is reached where the loop gain equals unity and the circuit becomes regenerative. At this point, collector current of the two transistors rapidly increases to a value limited only by the external circuit. Both transistors are driven into saturation, and the impedance between A and C is very low. The positive current applied to terminal G, which served to trigger the self-regenerative action, is no longer required since the collector of PNP transistor Q1 now supplies more than enough current to drive Q2. The circuit will remain on until it is turned off by a reduction in the collector current to a value below that necessary to maintain conduction.

The characteristic curve for the SCR is shown in Figure 25. With no gate current, the leakage current remains very small as the forward voltage from cathode to anode is increased until the breakdown point is reached. Here the center junction breaks down, the SCR begins to conduct heavily, and the drop across the SCR becomes very low.

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Figure 25. Characteristic curve for an SCR.

The effect of a gate signal on the firing of an SCR is shown in Figure 26. Breakdown of the center junction can be achieved at speeds approaching a microsecond by applying an appropriate signal to the gate lead, while holding the anode voltage constant. After breakdown, the voltage across the device is so low that the current through it from cathode to anode is essentially determined by the load it is feeding.

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Figure 26. SCR characteristic curve with various gate signals.

The important thing to remember is that a small current from gate to cathode can fire or trigger the SCR, changing it from practically an open circuit to a short circuit. The only way to change it back again (to commutate it) is to reduce the load current to a value less than the minimum forward-bias current. Gate current is required only until the anode current has completely built up to a point sufficient to sustain conduction (about 5 microseconds in resistive-load circuits). After conduction from cathode to anode begins, removing the gate current has no effect.

The basic operation of the SCR can be compared to that of the thyratron. The thyratron is an electron tube, normally gas filled, that uses a filament or a heater. The SCR and the thyratron function in a very similar manner. Figure 27 shows the schematic of each with the corresponding elements labeled. In both types of devices, control by the input signal is lost after they are triggered. The control grid (thyratron) and the gate (SCR) have no further effect on the magnitude of the load current after conduction begins. The load current can be interrupted by one or more of three methods: (1) the load circuit must be opened by a switch, (2) the plate (anode) voltage must be reduced below the ionizing potential of the gas (thyratron), (3) the forward-bias current must be reduced below a minimum value required to sustain conduction (SCR). The input resistance of the SCR is relatively low (approximately 100 ohms) and requires a current for triggering; the input resistance of the thyratron is exceptionally high, and requires a voltage input to the grid for triggering action.

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Figure 27. Comparison of an SCR and a thyratron.

The applications of the SCR as a rectifier are many. In fact, its many applications as a rectifier give this semiconductor device its name. When alternating current is applied to a rectifier, only the positive or negative halves of the sine wave flow through. All of each positive or negative half cycle appears in the output. When an SCR is used, however, the controlled rectifier may be turned on at any time during the half cycle, thus controlling the amount of dc power available from zero to maximum, as shown in Figure 28. Since the output is actually dc pulses, suitable filtering can be added if continuous direct current is needed. Thus any dc operated device can have controlled amounts of power applied to it. Notice that the SCR must be turned on at the desired time for each cycle.

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Figure 28. SCR gate control signals.

When an ac power source is used, the SCR is turned off automatically, since current and voltage drop to zero every half cycle. By using one SCR on positive alternations and one on negative, full-wave rectification can be accomplished, and control is obtained over the entire sine wave. The SCR serves in this application just as its name implies—as a controlled rectifier of ac voltage.


The SCR is primarily used for what function?


When an SCR is forward biased, what is needed to cause it to conduct?


What is the only way to cause an SCR to stop conducting?

2.4. Triac

The TRIAC is a three-terminal device similar in construction and operation to the SCR. The TRIAC controls and conducts current flow during both alternations of an ac cycle, instead of only one. The schematic symbols for the SCR and the TRIAC are compared in Figure 29. Both the SCR and the TRIAC have a gate lead. However, in the TRIAC the lead on the same side as the gate is "main terminal 1," and the lead opposite the gate is "main terminal 2." This method of lead labeling is necessary because the TRIAC is essentially two SCRs back to back, with a common gate and common terminals. Each terminal is, in effect, the anode of one SCR and the cathode of another, and either terminal can receive an input. In fact, the functions of a TRIAC can be duplicated by connecting two actual SCRs as shown in Figure 30. The result is a three-terminal device identical to the TRIAC. The common anode-cathode connections form main terminals 1 and 2, and the common gate forms terminal 3.

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Figure 29. Comparison of SCR and TRIAC symbols.

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Figure 30. Back to back SCR equivalent circuit.

The difference in current control between the SCR and the TRIAC can be seen by comparing their operation in the basic circuit shown in Figure 31 and Figure 32.

In the circuit shown in Figure 31, the SCR is connected in the familiar half-wave arrangement. Current will flow through the load resistor (RL) for one alternation of each input cycle. Diode CR1 is necessary to ensure a positive trigger voltage.

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Figure 31. Comparison of SCR and TRIAC circuits (A).

In the circuit shown in Figure 32, with the TRIAC inserted in the place of the SCR, current flows through the load resistor during both alternations of the input cycle. Because either alternation will trigger the gate of the TRIAC, CR1 is not required in the circuit. Current flowing through the load will reverse direction for half of each input cycle. To clarify this difference, a comparison of the waveforms seen at the input, gate, and output points of the two devices is shown in Figure 33.

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Figure 32. Comparison of SCR and TRIAC circuits (B).
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Figure 33. Comparison of SCR and TRIAC waveforms.

The TRIAC is similar in operation to what device?


When used for ac current control, during which alternation of the ac cycle does the TRIAC control current flow?

2.4.1. Solid-State Relays

As versatile as electromechanical relays can be, they do suffer many limitations. They can be expensive to build, have a limited contact cycle life, take up a lot of room, and switch slowly, compared to modern semiconductor devices. These limitations are especially true for large power contactor relays. To address these limitations, many relay manufacturers offer "solid-state" relays, which use an SCR, TRIAC, or transistor output instead of mechanical contacts to switch the controlled power. The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED light source inside the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power. This optical isolation between input to output rivals the best that electromechanical relays can offer.


Being solid-state devices, there are no moving parts to wear out, and they are able to switch on and off much faster than any mechanical relay armature can move. There is no sparking between contacts, and no problems with contact corrosion. However, solid-state relays are still too expensive to build in very high current ratings, and so electromechanical contactors continue to dominate that application in industry today.

One significant advantage of a solid-state SCR or TRIAC relay over an electromechanical device is its natural tendency to open the AC circuit only at a point of zero load current. Because SCR’s and TRIAC’s are thyristors, their inherent hysteresis maintains circuit continuity after the LED is de-energized until the AC current falls below a threshold value (the holding current). In practical terms what this means is the circuit will never be interrupted in the middle of a sine wave peak. Such untimely interruptions in a circuit containing substantial inductance would normally produce large voltage spikes due to the sudden magnetic field collapse around the inductance. This will not happen in a circuit broken by an SCR or TRIAC. This feature is called zero-crossover switching.

One disadvantage of solid state relays is their tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail "open." In either case, it is possible for a relay to fail in the other mode, but these are the most common failures. Because a "fail-open" state is generally considered safer than a "fail-closed" state, electromechanical relays are still favored over their solid-state counterparts in many applications.

2.5. IGBT [TODO]

2.6. Optoelectronic Devices

OPTOELECTRONIC devices either produce light or use light in their operation. The first of these, the light-emitting diode (LED), was developed to replace the fragile, short-life incandescent light bulbs used to indicate on/off conditions on panels. A LIGHT-EMITTING DIODE is a diode which, when forward biased, produces visible light. The light may be red, green, or amber, depending upon the material used to make the diode.

Figure 34 shows an LED and its schematic symbol. The LED is designated by a standard diode symbol with two arrows pointing away from the cathode. The arrows indicate light leaving the diode. The circuit symbols for all optoelectronic devices have arrows pointing either toward them, if they use light, or away from them, if they produce light. The LED operating voltage is small, about 1.6 volts forward bias and generally about 10 milliamperes. The life expectancy of the LED is very long, over 100,000 hours of operation.

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Figure 34. LED.

LEDs are used widely as "power on" indicators of current and as displays for pocket calculators, digital voltmeters, frequency counters, etc. For use in calculators and similar devices, LEDs are typically placed together in seven-segment displays, as shown in Figure 35 and Figure 36. This display uses seven LED segments, or bars (labeled A through G in the figure), which can be lit in different combinations to form any number from "0" through "9." The schematic, view A, shows a common-anode display. All anodes in a display are internally connected. When a negative voltage is applied to the proper cathodes, a number is formed. For example, if negative voltage is applied to all cathodes except that of LED "E," the number "9" is produced, as shown in Figure 37. If the negative voltage is changed and applied to all cathodes except LED "B," the number "9" changes to "6" as shown in Figure 38.

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Figure 35. Seven-segment LED display (A).

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Figure 36. Seven-segment LED display (B).
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Figure 37. Seven-segment LED display examples (A).

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Figure 38. Seven-segment LED display examples (B).

Seven-segment displays are also available in common-cathode form, in which all cathodes are at the same potential. When replacing LED displays, you must ensure the replacement display is the same type as the faulty display. Since both types look alike, you should always check the manufacturer’s number.

LED seven-segment displays range from the very small, often not much larger than standard typewritten numbers, to about an inch. Several displays may be combined in a package to show a series of numbers, such as the one shown in Figure 39.

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Figure 39. Stacked seven-segment display.

Another special optoelectronic device in common use today is the photodiode. Unlike the LED, which produces light, the photodiode uses light to accomplish special circuit functions. Basically, the PHOTODIODE is a light-controlled variable resistor. In total darkness, it has a relatively high resistance and therefore conducts little current. However, when the PN junction is exposed to an external light source, internal resistance decreases and current flow increases. The photodiode is operated with reverse-bias and conducts current in direct proportion to the intensity of the light source.

Figure 40 shows a photodiode with its schematic symbol. The arrows pointing toward the symbol indicate that light is required for operation of the device. A light source is aimed at the photodiode through a transparent "window" placed over the semiconductor chip. Switching the light source on or off changes the conduction level of the photodiode. Varying the light intensity controls the amount of conduction. Because photodiodes respond quickly to changes in light intensity, they are extremely useful in digital applications such as computer card readers, paper tape readers, and photographic light meters. They are also used in some types of optical scanning equipment.

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Figure 40. Photodiode.

A second optoelectronic device that conducts current when exposed to light is the PHOTOTRANSISTOR. A phototransistor, however, is much more sensitive to light and produces more output current for a given light intensity that does a photodiode. Figure 41 shows one type of phototransistor, which is made by placing a photodiode in the base circuit of an NPN transistor. Light falling on the photodiode changes the base current of the transistor, causing the collector current to be amplified. Phototransistors may also be of the PNP type, with the photodiode placed in the base-collector circuit.

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Figure 41. Phototransistor.

Figure 42 illustrates the schematic symbols for the various types of phototransistors. Phototransistors may be of the two-terminal type, in which the light intensity on the photodiode alone determines the amount of conduction. They may also be of the three-terminal type, which have an added base lead that allows an electrical bias to be applied to the base. The bias allows an optimum transistor conduction level, and thus compensates for ambient (normal room) light intensity.

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Figure 42. 2-terminal and 3-terminal phototransistors.

An older device that uses light in a way similar to the photodiode is the photoconductive cell, or PHOTOCELL, shown with its schematic symbol in Figure 43. Like the photodiode, the photocell is a light-controlled variable resistor. However, a typical light-to-dark resistance ratio for a photocell is 1:1000. This means that its resistance could range from 1000 ohms in the light to 1000 kilohms in the dark, or from 2000 ohms in the light to 2000 kilohms in the dark, and so forth. Of course, other ratios are also available. Photocells are used in various types of control and timing circuits as, for example, the automatic street light controllers in most cities.

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Figure 43. Photocell.

The photovoltaic cell, or solar cell, is a device which converts light energy into electrical energy. An example of a solar cell and its schematic symbol are shown in Figure 44. The symbol is similar to that of a battery. The device itself acts much like a battery when exposed to light and produces about .45 volt across its terminals, with current capacity determined by its size. As with batteries, solar cells may be connected in series or parallel to produce higher voltages and currents. The device is finding widespread application in communications satellites and solar-powered homes.

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Figure 44. Solar cell.

When it is necessary to block the voltage between one electronic circuit and another, and transfer the signal at the same time, an amplifier coupling capacitor is often used as shown in Figure 45. Although this method of coupling does block dc between the circuits, voltage isolation is not complete. A newer method, making use of optoelectronic devices to achieve electrical isolation, is the optical coupler, shown in Figure 46. The coupler is composed of an LED and a photodiode contained in a light-conducting medium. As the polarity signs in Figure 46 show, the LED is forward biased, while the photodiode is reverse biased. When the input signal causes current through the LED to increase, the light produce by the LED increases. This increased light intensity causes current flow through the photodiode to increase. In this way, changes in input current produce proportional changes in the output, even though the two circuits are electrically isolated.

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Figure 45. DC blocking with a coupling capacitor.

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Figure 46. Optical coupler.

The optical coupler is suitable for frequencies in the low megahertz range. The photodiode type shown above can handle only small currents; however, other types of couplers, combining phototransistors with the SCR, can be used where more output is required. Optical couplers are replacing transformers in low-voltage and low-current applications. Sensitive digital circuits can use the coupler to control large current and voltages with low-voltage logic levels.


What type of bias is required to cause an LED to produce light?


When compared to incandescent lamps, what is the power requirement of an LED?


In a common anode, seven-segment LED display, an individual LED will light if a negative voltage is applied to what element?


What is the resistance level of a photodiode in total darkness?


What type of bias is required for proper operation of a photodiode?


What is a typical light-to-dark resistance ratio for a photocell?


What semiconductor device produces electrical energy when exposed to light?

3. Transistors

Transistors are semiconductor devices with three or more terminals. The operation of normal transistors has already been discussed, but there are several transistors with special properties that should be explained. As with diodes, a discussion of all the developments in the transistor field would be impossible. The unijunction transistor (UJT) and the field effect transistor (FET) will be discussed because of their widespread application in Navy equipment. Many other special transistors have been developed and will be discussed in later NEETS modules.

3.1. The Unijunction Transistor (UJT)

The UNIJUNCTION TRANSISTOR (UJT), originally called a double-based diode, is a three- terminal, solid-state device that has several advantages over conventional transistors. It is very stable over a wide range of temperatures and allows a reduction of components when used in place of conventional transistors. A comparison is shown in Figure 47 and Figure 48. Figure 47 is a circuit using conventional transistors, and Figure 48 is the same circuit using the UJT. As you can see, the UJT circuit has fewer components. Reducing the number of components reduces the cost, size, and probability of failure.

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Figure 47. Comparison of conventional transistors and UJT circuits (A).
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Figure 48. Comparison of conventional transistors and UJT circuits (B).

The physical appearance of the UJT is identical to that of the common transistor. As shown in Figure 49, both have three leads and the same basic shape; the tab on the case indicates the emitter on both devices. The UJT, however, has a second base instead of a collector.

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Figure 49. Transistor and UJT.

As indicated in the block diagram shown in views A and B of Figure 50, the lead differences are even more pronounced. Unlike the transistor, the UJT has only one PN junction. The area between base 1 and base 2 acts as a resistor when the UJT is properly biased. A conventional transistor needs a certain bias level between the emitter, base, and collector for proper conduction. The same principle is true for the UJT; it needs a certain bias level between the emitter and base 1 and also between base 1 and base 2 for proper conduction.

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Figure 50. Transistor and UJT structure.

The normal bias arrangement for the UJT is illustrated in Figure 51. A positive 10 volts is placed on base 2 and a ground on base 1. The area between base 1 and base 2 acts as a resistor. If a reading were taken between base 1 and base 2, the meter would indicate the full 10 volts as shown in Figure 52. Theoretically, if one meter lead were connected to base 1 and the other lead to some point between base 1 and base 2, the meter would read some voltage less than 10 volts. This concept is illustrated in Figure 53. Figure 54 is an illustration of the voltage levels at different points between the two bases. The sequential rise in voltage is called a voltage gradient.

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Figure 51. UJT biasing (A).

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Figure 52. UJT biasing (B).

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Figure 53. UJT voltage gradient (A).

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Figure 54. UJT voltage gradient (B).

The emitter of the UJT can be viewed as the wiper arm of a variable resistor. If the voltage level on the emitter is more positive than the voltage gradient level at the emitter-base material contact point, the UJT is forward biased. The UJT will conduct heavily (almost a short circuit) from base 1 to the emitter. The emitter is fixed in position by the manufacturer. The level of the voltage gradient therefore depends upon the amount of bias voltage, as shown in Figure 55.

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Figure 55. Forward bias point on UJT voltage gradient.

If the voltage level on the emitter is less positive than the voltage gradient opposite the emitter, the UJT is reverse biased. No current will flow from base 1 to the emitter. However, a small current, called reverse current, will flow from the emitter to base 2. The reverse current is caused by the impurities used in the construction of the UJT and is in the form of minority carriers.

More than 40 distinct types of UJTs are presently in use. One of the most common applications is in switching circuits. They are also used extensively in oscillators and wave-shaping circuits.


The UJT has how many PN junctions?


The area between base 1 and base 2 in a UJT acts as what type of common circuit component?


The sequential rise in voltage between the two bases of the UJT is called what?


What is the normal current path for a UJT?