Shortly after the PN junction showed how it was possible to control the amount and direction of current that is flowing through some part of the material, the search was on for a device that would allow for more precise control. This device is known as a transistor and its development (in about 1950) enabled all aspects of the semiconductor industry including the transistor radio, which was the 1960 equivalent of your I-phone - it was a big deal - kids in school got kicked out of classes for listening to their transistor radio - ask your grandparents about the transistor radio.

A basic transistor consistes of the three components shown below, the base, the emitter and the collector:


basic transistor

transistor array

The development of the Transistor.

This is the first step in the development of an integrated circuit (IC). An IC contains millions of transistors. In turn, IC's will lead to component electronics (e.g. motherboards).

Basically a transistor is a solid state semiconductor device that can be used for:

• amplification of an electric current
• current switching
• voltage stabilization
• signal modulation

In other words, if you consider the properties of an electric current as information, a transistor is a device that allows you to control the flow of that information and/or the rate of information flow. The basic conceptual idea is that a transistor acts like a valve. Based on the input voltage, the transistor can control the amount of current that is drawn from that connected voltage supply.

The first idea for transistor was patented in 1926 but there was no possible technology available at the time to actually produce one. The first practical working transistor was built in 1947 by Bardeen, Brattain and Shockely in 1947. They subsequently won the Nobel Prize in Physics.

A transistor is created by using three layers rather than the two layers used in a diode. You can create either an NPN or a PNP sandwich. Therefore, a transistor looks like two diodes back-to-back. You'd imagine that no current could flow through a transistor because back-to-back diodes would block current both ways. And this is true. However, when you apply a small current to the center layer of the sandwich, a much larger current can flow through the sandwich as a whole. This gives a transistor its switching behavior. A small current can turn a larger current on and off and this acts as an amplifier of the current .

Let's review the diode structure again, using another example:

Even though N-type silicon by itself is a conductor, and P-type silicon by itself is also a conductor, the combination shown in the diagram does not conduct any electricity. The negative electrons in the N-type silicon get attracted to the positive terminal of the battery. The positive holes in the P-type silicon get attracted to the negative terminal of the battery. No current flows across the junction because the holes and the electrons are each moving in the wrong direction.

If you flip the battery around, the diode conducts electricity just fine. The free electrons in the N-type silicon are repelled by the negative terminal of the battery. The holes in the P-type silicon are repelled by the positive terminal. The associated circuit for this current generating configuration would be as shown to the right. Thus elecricity can be turned off or on by simply switching the voltage

Transistors are composed of three parts a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply.

By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier.

The same process can be used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts ON, less than five volts OFF. The high speed of computers today is a direct result of the ability to now manufacture and mount millions of transistors in a very small area.

A transistor's ON state can be encoded as a 1 and the OFF state can be encoded as a 0. In this animated example there are 4 transistors (4 BITS) and their relative ON/OFF states generates a binary code that is recognized as a number. All transistors off, for instance, is 0, while the second and fourth transistors turned to their ON state would be the number 5. A possible 16 numbers can be produced by this configuration.
As we have discussed, adding certain types of impurities to the silicon in a transistor changes its crystalline structure and enhances its ability to conduct electricity. Silicon containing boron impurities is called p-type silicon-p for positive or lacking electrons. Silicon containing phosphorus impurities is called n-type silicon-n for negative or having a majority of free electrons. The basic NPN sandwich is shown to the right
The On/Off states of a Transistor: Step 1: A transistor consists of three terminals: the source, the gate and the drain (also called, the gate, the base and the collector).
Step 2: In the n-type transistor, both the source and the drain are negatively charged and sit on a positively charged well of p-silicon.
Step 3: Apply a positive voltage to the gate. This causes electrons in the p-type silicon in the bottom substrate to be attracted to the area right under the gate. Now we have a pure electron channel between the source and the drain meaning that electrons now flow through the top layer of our material.
Step 4: Now apply a positive voltage to the drain. This acts to pull the electrons from the source to the drain. In this state, the transistor is now ON.
Last Step: Remove the voltage from the gate. Electrons are now no longer attracted to the area under the gate and the flow channel becomes impaired (positive charge appears there). The electrical pathway is now broken and the transistor is OFF.

Your clock speed on your processor is essentially a measure of how fast these applied voltages can be turned on and off to the millions of transistors on your processor.

Economies of scale are realized when these single n-type transistors can be made arbitrarily small from a master wafer free of impurities. In turn, this manufacturing ability keeps Moore's law going.

Please read this history of the development of the transistor -- if you don't I will take away your I-phone and replace it with a transistor radio ...