Solid State Secrets, June 1968 Radio-Electronics

Learn-at-home, self-taught curriculums were a big thing in the post-war years of the 20th century. It was seen as a way to earn while you learn, where a person worked a "day job" while studying to be a brain surgeon, nuclear physicist, electrician, auto mechanic, or HVAC technician. It was more convenient and less costly than driving to a campus for regular classes. Many such courses were advertised in technical magazines like Popular Mechanics, Popular Science, and even Radio-Electronics. This 1968 article entitled "Solid State Secrets" was structured like one of those self-taught courses. Having paid for and completed a couple of those courses, I can attest to their value, but ultimately I cannot claim they contributed directly to any job offers. My USAF electronics training got me into the civilian electronics world as a technician, and then earning a BSEE degree advanced me into engineering. Note that the author elected to feature germanium as the semiconductor substrate rather than silicon, even though by 1968 the purification process for silicon had advanced to where it was overtaking germanium in the industry.

Solid State Secrets

Fig. 1 - Atomic structure of semiconductor materials.
a - Pure germanium atoms have four electrons.
b - N-type germanium forms when arsenic doping adds electron.
c - Indium causes electron hole.

Secrets of solid-state devices such as transistors, diodes, silicon controlled rectifiers can be learned without too much difficulty. The programmed text that follows is a modern self-teaching method designed to improve comprehension and retention of the subject matter.

As you read through. each block section, and follow the directions given. you will be led to a better understanding of solid-state principles.

Knowledge breeds knowledge ... if you can cope with this material chances are that you will be stimulated toward additional investigation into the subject. Go to Block 1 and follow the instructions.

To understand what a solid-state device is we must first analyze its general construction. The term semiconductor is used with solid-state devices, Why? Because the elements of which they are made have electrical characteristics somewhere between an insulator and a conductor. Remember that a conductor is a material with many "free" electrons, whereas an insulator has relatively fewer of these free electrons.

The two most common semiconductor materials are silicon and germanium. They are similar in structure and the descriptions to follow are applicable to either in most instances. Only the germanium material will be used for explanation.

In Fig. 1-a the atomic structure of germanium is seen in a simplified way. Orbiting around the center core or nucleus of the atom are four valence electrons. These valence electrons are the important thing to remember. They are rather loosely bound to the center core. As you'll see later, they are used for current carriers.

Fig. 2-a - Zener diodes effectively regulate voltage.
b - Tunnel diode conduction curve.

Two basic modifications are performed on germanium during the semiconductor manufacturing process. First, impurity atoms (such as arsenic) are doped with the basic germanium atoms. The impurity atoms in this case have, not four, but five valence electrons in their orbit. As seen in Fig. 1-b, this causes the basic material to have a few extra electrons distributed within its structure. You recall that electrons by definition are negative charges? Then it should be easy to remember that this material is n-type.

In the second modification to the basic germanium material, impurity atoms (such as indium) are doped in the same manner as before. The impurity atoms of indium have only three valence electrons in their orbit. As seen in Fig. 1-c, the material now has a number of empty "pockets" or "holes" distributed within its structure. It has fewer electrons than the n-type material of Fig. 1-b; therefore by definition it is a more positive material. We call it p-type material.

Question: Which of the following best describes an n-type germanium material?

_ It has fewer electrons than normally found in the basic germanium material. Go to Block 7.

_ It has more electrons than normally found in the basic germanium material. Go to Block 8.

Would you believe ... you're right! The holes in a sense, are "recombined" with electrons from the n-type material.

A simple Zener diode voltage regulator consists of a voltage source, a current-limiting resistor and a Zener diode selected for the desired output voltage. Refer to the schematic of Fig. 2-a. The arrow of the diode always points to the positive voltage source in a circuit such as this. If the output load decreases, the Zener current will increase to maintain a constant output voltage. This is a very simple and effective regulation circuit.

Zener diodes are available with breakdown (operating) voltages anywhere from 2 to several hundred volts. An important parameter of Zener regulators is wattage rating. The wattage is determined by the product of the Zener voltage and maximum Zener current for any particular application.

What else can we do with these p- and n-type materials? Another item we should briefly mention is the tunnel diode. Its construction is identical to an ordinary diode's, with one exception: more impurity atoms are added to the basic material. This causes the diode's forward conduction characteristic to appear as in Fig. 2-b. Section A is a "negative-resistance" region. Notice that the current actually decreases while the voltage is increasing. This characteristic of the tunnel diode is used for many applications, especially in the amplification of high-frequency signals.

Fig. 3 - Load line (AB) is drawn on transistor collector operating curves to find circuit operating characteristics, then dc operating point is selected.

Question: If the Zener diode in Fig. 3-a is a 10-volt device and the maximum current through it is 1 ampere, what is its wattage dissipation?

It could get pretty confusing if we tried to analyze an amplifier under its dynamic conditions without some type of graph or chart to tell us what happens at any specific time. We know the collector current, base current and collector voltage are all related to each other. For instance, if base current is increased, collector current increases and collector-to-emitter voltage decreases. This can be seen by referring to the transistor "load-line" drawing of Fig. 3.

The load line, once established, will tell us the dynamic operating characteristics of a particular transistor type. The transistor manufacturer usually supplies the basic graph and the user then plots the load line to suit his requirements. Much knowledge of transistor action can be obtained by learning the fundamentals of this graph. The steps used to draw the load line of Fig. 3 are:

1. Determine collector voltage when collector current is zero (With no collector current flowing, there is no voltage drop across the 1200-ohm resistor. Hence the full -30 volts appears at the collector. This is shown as point A)

2. Determine collector current when collector voltage is zero. (If the collector is at zero volts the transistor would have to be turned full on. Thus for practical purposes, the 1200-ohm resistor is the only limit to collector current.

4. Determine a dc operating point on this line. (A linear area is usually chosen where the base-current change is equal on each side . of the operating point.)

Refer to Fig. 3. You will see that when the base-current change is 20 μA, the collector current will change a total of 12 mA. Likewise, the change in collector voltage will be 15 volts.

This is a good time to mention two terms encountered in transistor theory. The first is beta (β), which is simply the ratio between the base-to-emitter current and the collector current. In our example, it would be 12mA 12 mA/20 μA = 600. This example would represent a very high-gain transistor circuit.

The second term encountered is alpha. This is the ratio between the collector and emitter currents. Since these currents are almost identical (the emitter contains the small additional base current) this ratio is generally close to unity.

Fig. 4-a - Field-effect transistors (FETs) have advantage of high input impedance. Negative bias on gate (G) is used to regulate electron flow (arrows).
b - Arrow direction on gate indicates this is an n-channel FET. If arrow is reversed, it is p-channel FET with p-type material in its body.

If the base current of a transistor is increased, the collector current will decrease.

Your answer is right ... for a transistor (ordinary two-junction type), but not for a unijunction transistor. A unijunction has one emitter and two base leads. Go to Block 6.

One problem when using ordinary transistors in certain circuits is the loading effect they present. For instance, it is difficult to match the high impedance of a crystal transducer to the considerably lower input impedance of a conventional transistor base circuit. This problem is eliminated by the field-effect transistor (FET). Just as its name implies, it uses a field as shown in Fig. 4-a to control current :flow through its main section. As shown, the drain is made positive with respect to the source, so electrons flow from source to drain. The gate is biased negative - and the more negative it becomes, the more the field blocks current flow to the drain.

Operation is very similar to a vacuum-tube grid; each has a very high input impedance.

If the FET body is made of n-type material, the device is called an n-channel FET. If the body is made of p-type material, it is called a p-channel FET. FET elements are called source, gate and drain. They perform much the same functions as their names imply.

Fig. 4-b shows a typical n-channel FET amplifier circuit. If the arrow on the gate lead were pointing in the other direction, the device would be a p-channel type. Remember the great similarity between the field-effect transistor and its grandfather, the vacuum tube.

Fig. 5-a - With positive battery terminal connected to anode, electrons flow to p material, holes move to n material.
b - No current flows when polarities are reversed (c); if voltage is increased to breakdown point, current jumps sharply.

Sorry about that ... Go back to Block 1 and refer to Fig. 1-b. Select another answer.

And now that you know what n-type and p-type semiconductor materials are, you may well say, "So what?" Let's take a block of n-type and a block of p-type germanium and place them together, as shown in Fig. 5-a. The positive terminal of the battery is connected to the p-type material, the negative terminal to the n-type. Now the fun begins. The junction is said to be forward-biased and the "extra" electrons in the n-material will move in the direction of the junction. Since they are loosely bound in the valence bond, they may leave each nucleus rather easily. When a free electron reaches the junction, it will exchange places with a hole. (That is, it will fill a hole in the p-type material and leave a hole in the n-type material from whence it came.)

This always sounds a little confusing at first. Just remember that the electrons flow from the n- to the p-type material, and the hole flow is the opposite. Some of you younger readers may call this exchange of positions a "happening." Actually it's just what takes place in an ordinary solid-state diode.

Fig. 5-b shows the characteristic curve for a typical diode. The amount of forward current that flows is directly dependent upon the forward voltage applied across the diode terminals.

If we reverse the battery connections to the n- and p-type materials - as in Fig. 5-c, a very interesting thing takes place. The free electrons and holes are actually pulled away from the junction. Current flow ceases, except for a very small leakage current. Obviously the diode conducts in only one direction.

So far, we have been assuming the reverse voltage applied to the diode is insufficient to cause a breakdown of the pn junction. If we continue increasing this voltage, however, a breakdown will take place - as shown in Fig. 5-b. A heavy current flows and, due to the sharpness of the breakdown, a very small voltage change takes place. This characteristic is used in Zener or breakdown diode applications. Since silicon gives the sharpest breakdown curve, this material is used for most Zener diodes.

Fig. 6-a - Internal make-up and circuit connections for three-layer npn transistor. Junction A is reverse biased, junction B is forward biased.
b - Schematic notation for npn transistor circuit.

Fig. 7 - Three commonly used amplifier circuits. a - Common emitter has high voltage, current and power gain. b - Common collector: no voltage gain, high current and low power gain. c - Common base: high voltage gain, low current gain, and medium power gain.

Question: When a pn junction is forward-biased, the holes do which of the following:

_ Flow into the junction where they are subsequently occupied by an electron from the n-type material. Go to Block 3.

Your answer is not correct. Return to Block 4, study Fig. 3 and then go directly to Block 15.

Your answer is correct! If the Zener has a 10-volt drop across it and 1 ampere of current, the dissipation is 10 watts (P = IE). For this example, at least a 15-watt Zener would be used.

By now you are probably wondering what happened to the good old transistor encountered in everything these days. So far we have been discussing semiconductor devices that are constructed of one slice of n-type material and one slice of p-type material. These can all be classified as two-layer diodes. Suppose we sandwich a thin slice of the p-type material between two pieces of n-type, as shown in Fig. 6-a. With the battery voltage applied as shown, junction A has a reverse bias applied to it. Junction B is forward-biased. The current that will flow through resistor R_L depends on the amount of bias at these two junctions. If V_BE is increased (as would be the case with an input signal) current through R_L will increase.

A pnp transistor is constructed in the same manner as the npn, except that a thin slice of n-type material is sandwiched between two of p-types, The operation of both transistor types is identical. In fact, an npn may be directly interchanged with a pnp (of similar characteristics) if the supply voltage polarity is reversed. (In practice this is seldom done, as polarized capacitors might be damaged.)

An important thing to remember about transistors is that the base-to-collector junction is reverse-biased, while the base-to-emitter is forward-biased. For any normal transistor configuration this must be true. Notice that Fig. 6-b is the schematic version of Fig. 6-a and represents the bias conditions just mentioned.

Fig. 7 shows the three most common amplifierss used in transistor work. Note the characteristics associated with each. A great deal of practical knowledge can be gained by becoming familiar with these three configurations.

Question: In a typical transistor circuit, which of the following statements would be correct?

_ The base-to-emitter junction is reverse-biased, while the base-to-collector is forward-biased. Go to Block 17.

_ The base-to-emitter is forward-biased, while the base-to-collector is reverse-biased. Go to Block 4.

_ Both the base-to-emitter and base-to-collector junctions are forward-biased. Go to Block 10.

Would you believe ... Wrong! For our purposes, it is best to consider the holes as actually moving. Return to Block 8 and select another answer.

Sorry about that ... your answer is not correct. Go back and review Block 15 and then select another answer.

In a sense this may be true, but it is not the correct answer here. Better return to Block 6 and select another answer.

Another type of semiconductor device is the unijunction transistor (UJT). This device has no collector element in the normal sense. It consists instead of two base leads and one emitter. Its construction is shown in Fig. 8-a. The body consists of a piece of n-type material. The resistance of this silicon is relatively high between the base 1 and base 2 connections; normally very little current will flow between these elements.

But suppose we apply a voltage between the emitter and base 1 junction - a voltage high enough to forward-bias the junction. Then the resistance between the emitter and base 1 becomes very small. When this happens, the resistance of the body (from base 1 to base 2) is suddenly lowered and current flow increases. This effect is put to good use in timing and pulse-generation circuits. As an example, some recent video pattern generators use a UJT for timing functions.

Fig. 8-a - Unijunction transistor has two base leads and one emitter. Normally high resistance between B1 and B2 can be quickly lowered by emitter voltage, permit-ting current to flow.
b - Timing circuit utilizes this resistance-breakdown effect.

Fig. 9-a - A four-layer diode, or pnpn switch. At certain level of applied voltage across terminals junction 2 breaks down, resulting in very low resistance between anode and cathode. Current pulse that results is available as an output.
b - In thyristor or silicon controlled rectifier, this breakdown is controlled by bias on the gate.

- RF Cafe

Fig. 10-a - Basic integrated circuit (IC) consists of n-type-material with 4 p-type junctions.
b - Schematic of (a) shows four diodes with cathodes common.

Fig. 8-b shows the unijunction transistor symbol and a typical timing circuit. As capacitor C1 charges through resistor R1, the voltage eventually becomes great enough to forward-bias the emitter-to-base 1 junction. This causes the capacitor to discharge suddenly through base 1 and resistor R2. When the capacitor discharges, the cycle starts all over again. The values of R1 and C1 determine the time between output pulses.

After seeing the various configurations and arrangements of the two basic n- and p-type materials and their applications, can there be more? Yes! You remember that a pnp transistor is made of a thin slice of n-type material sandwiched between two p-types, Suppose we add another n-type to this arrangement, as shown in Fig. 9-a. This is called a four-layer diode, or pnpn switch.

By taking advantage of the biased junctions and leakage currents across these junctions, several valuable devices have been developed. The first is exactly as shown in Fig. 9-a. When the applied voltage across this four-layer device reaches a certain level, junction 2 will actually break down, and a very low resistance will exist across the terminals. The point at which this occurs is relatively sharp and a current pulse is available as an output.

The characteristics of a four-layer diode are similar to a thyratron tube; each has a distinct firing (or breakdown) point. Once the device is turned on, it will remain on until the anode voltage is removed or reduced to a very low level. The breakdown may be controlled by attaching another lead as shown in Fig. 9-b. This device is known as a thyristor or silicon controlled rectifier (SCR). It has found its way into a multitude of uses such as electronic switching, motor speed control, light dimmers, etc. Like the thyratron's control grid, the SCR's gate loses control once conduction takes place.

Your answer is not correct. A reverse bias would cause the positive holes to flow away from the junction. Return to Block 8 and try again.

Your answer is incorrect. As described in Block 3, the wattage is the product of the Zener voltage and current (10 watts). Go directly to Block 11.

You have chosen the correct answer once again. Since we began at Block 1, we have touched upon ordinary diodes, four-layer diodes, silicon controlled rectifiers, ordinary transistors, unijunction and field-effect transistors, and Zener and tunnel diodes. All these do a better job and take up less space than we could have imagined just a few years ago. But even so, the latest trend is even more amazing. Someone apparently opened the case of a transistor and discovered a great deal of wasted space there. In the integrated circuit many components are now put inside a case the size of a single transistor. These components are all made of semiconductive materials like those we have been discussing. Transistors, capacitors, diodes and resistors are constructed on a small chip that would fit under your thumbnail.

Figure 10-a shows a very simple integrated circuit of four diodes with common cathode connections. Fig. 10-b is the same circuit shown schematically.

Use your imagination and you will see that many entire circuits could (and in fact are) constructed in a very small area. This technique is the future of electronics. And after this, you may wonder what could possibly come next. Would you believe ... L.S.D.?*' R-E

Solid State SecretsJune 1968 Radio-Electronics

Solid State Secrets

Solid State Secrets
June 1968 Radio-Electronics