Principles of Electricity

Lesson 48 - Transistors

Recall that in a semiconductor diode, we have 2 regions of DOPED semiconductive material. One region is doped positive, and the other region is doped negative. There is also a junction, where the two regions are joined.

When a diode is forward biased, it conducts electricity easily, like a ball rolling down a hill. When it is reverse biased, it is extremely resistive to current flow, as the ball is rolling uphill, and is much harder to get over the hump.

Remember also, that we had diode tubes, which operated in a similar manner. They would conduct electricity in one direction easily, but would not conduct in the opposite direction.

When we added another element to a tube, we created a triode, which would not only allow electricity to flow, but could also amplify the signal. Reason tells us that if we add another element to a semiconductor diode, that a similar effect should take place. In December of 1947, Scientists at Bell Laboratories would prove this theory correct.

With the addition of a 3rd semiconductive layer, joined at a second P-N junction, W. H. Brattain made the world famous comment, " We've Got Gain! " implying that this 3 layer device could amplify!

With proper bias applied, there is a small hill to overcome at the first P-N junction
(approximately 0.7 Volts for Silicon, 0.3 Volts Germanium), which is the normal characteristic for any semiconductor diode.

But then the electrons reach the peak of the hill at the second P-N junction, and have a fast run downward. There is an increase in flow downhill, and the electrons act like a waterfall, pouring into the collector. It may seem at first, that a transistor is like 2 diodes placed back to back, and in resistance checks will even resemble this. Actually though, 2 diodes back to back will not operate like a transistor in circuit.

A diode only has 2 semiconductive regions, and therefore has 2 leads. A transistor, on the other hand, has 3 regions, and must have 3 leads. To the left is a photo of a small signal transistor. Just as you must know which end is which on a diode, a transistor has markings which identify which lead is which. The three leads are called the Emitter, Base, and Collector. The Emitter is the lead that current enters into. It can be compared to the Cathode of a tube. The Collector is the lead that current exits from. It can be compared to the Plate of a tube. Finally, the Base is the controlling lead, and is comparable to the Control Grid of a tube. It might help to remember that electrons are emitted at the Emitter, collected at the Collector, and controlled by the Base.

Transistors come in many different packages, and while they are NOT always marked so that you will know which lead is which, they are by no means standard either! One transistor may have the emitter on the left, another may have it in the middle. Transistors often do, however have identifying marks, and can be referenced to find out which lead is which. In any case - it is ALWAYS best to check the specification sheet for any given transistor before using it. Doing so will save you a load of heartache.

Of the many kinds of transistors there are, probably the most commonly used is the Small Signal, Bipolar transistor, as pictured above.

Bipolar transistors come in two flavors: PNP and NPN. This is because the semiconductive material can be laid out in ( basically ) two different ways.

If we look closely at how a bipolar transistor is made, we can understand more easily how this can be. The illustration to the left is a cutaway of a semiconductor transistor. Try to visualize this as being circular ( button shaped ) from the top view, with 3 layers, one upon another.

Transistors are built in layers by very precise machines. Each layer is added to the layer below it. We begin with a single layer ( or substrate ), and add layers on top of it. If we begin with a layer of N type semiconductor ( on the bottom ), the second layer would be P type, followed by another N type. We say that transistors, and other semiconductive devices, are "grown" in this manner. The second layer ( in this case a P type ), is very thin, along the order of 800 micrometers ( μM ) or less.

As shown by the blue line, electric current enters via the N type emitter substrate layer, passes through the ( red ) P type base substrate layer, until it reaches the N type collector substrate layer. The gold colored lines represent the leads that connect the transistor to outside circuitry.

If we were to reverse the N and P layers, we would have a PNP transistor, with the base being N type, and the emitter and collector being P type material.

The schematic diagram symbol for a bipolar transistor is shown to the right. Notice that the only difference between an NPN and PNP type transistor, is the direction of the arrow. To remember which is which, just keep in mind that the NPN is Not Pointing to the base.
( NP = Not Pointing ) Otherwise, the two symbols are identical. The EMITTER is ALWAYS the ARROW, the base is always the line ( think baseline ), and the collector is the one left over.

Rules for Bias Connections

This is important! Pay Attention!!

The Emitter - Base connection is always FORWARD biased. This means more Positive voltage goes to P type & more Negative to N type. Also, for a Silicon transistor, there must be at least a 0.7 Volt DC bias across the emitter-base junction in order for the transistor to be active. Many times, when without a schematic, I have been able to repair a circuit simply by looking for 0.7 VDC across the E-B of every transistor in the circuit. If it doesn't have at least 0.7 V across it, it isn't turned on! Of course, the bias is 0.3 Volts DC for Germanium Transistors, so you must also know a little about the transistor itself. When in doubt - look up the number on the transistor, and read its specification sheet. A good generic resource for this is the NTE(or ECG) cross reference guide. This guide helps you find generic equivalent transistors and chips for brand name parts, as well as providing internal schematics, etc for microchips and such.

The Collector - Base connection is always REVERSE biased. This means more Positive goes to N type & more Negative goes to P type. You must be wondering now, how the Collector-Base junction can be reverse biased while the Emitter - Base junction is forward biased?

The answer lies in the words "More Positive" and "More Negative". You see, electronics is more of a relative science than an exact science. Is 5 Volts D.C. positive or negative? Well, it's more positive than 2 Volts D.C., but less positive than 9 Volts D.C. Did I lose you yet?

It's simple.

Let's try plotting it out on a number line:

Assuming a Silicon NPN transistor:
We know that the the Emitter-Base junction must be FORWARD biased (Positive to P type doping & Negative to N type doping). So the Emitter must be more Negative and the Base must be more Positive. We know then, that the Base must be 0.7 Volts ( minimum ) Higher than the emitter.

So if, say, the Emitter is at 3 Volts, ( I just picked that number at random). We plot 3 Volts on the number line.

If the Base has to be 0.7 volts Higher than the Emitter, then the base has to be at least 3.7 Volts.
( 3 + .7 = 3.7 ) So we plot that on the number line.

So far so good! Now comes the tricky part. In a NPN transistor, the Base is P type doping, while the collector is N type. But we want it to be REVERSE biased ( Positive to N & Negative to P ), so we want the N doped collector to be MORE POSITIVE than the P doped Base, which is at 3.7 Volts. So any voltage above 3.7 would work. Let's say, 5 Volts. ( We plot that on the number line ).

So in order to turn on this NPN transistor, we would need the following voltages:
Emitter = 3.0 Volts
Base = 3.7 Volts
Collector = 5.0 Volts.

This is why, in the picture above, I have a minus sign ( - ) next to the base, a plus sign ( + ) next to the base, and TWO plus signs ( ++ ) next to the collector. It demonstrates the relative polarity of each terminal.

We have a pattern then, that while it is an NPN transistor, it is biased N-P-PP, with the COLLECTOR being the MOST POSITIVE point.

If we go through the same logic for the PNP transistor, we would find that it needs to be biased P-N-NN, with the COLLECTOR being the MOST NEGATIVE point.

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