Solid-state circuits still create a buzz

In the beginning, there were tubes, in which a stream of electrons flowing through the near-vacuum space inside was controlled by voltage charges on metal plates and grids.

Then there were solid state circuits. Many of us grew up with those buzz-words heralding anything that was purportedly "state of the art." Solid state devices such as transistors control electrons as they flow through solid materials. A diode is perhaps the simplest solid state device, but even the most complex large-scale integrated circuits operate on the same principles.

You can make a radio by rectifying the radio frequency signal from a broadcasting station. Rectification is the conversion of ac to dc. In the case of a rapidly varying RF signal, a rectifier strips off the high frequency ac to leave a pulsating dc, which is varying at an audio frequency rate.

Actually, one of the first rectifiers was solid state; it was made out of a lead-sulfide crystal and a very thin wire called a "cat's whisker." The crystal was not a good conductor, nor was it an insulator. It was somewhere in between — a "semiconductor." The point of the whisker was moved around the crystal until a junction was formed.

A "crystal" radio can even be built from a double edged razor blade and, believe it or not, a bent safety pin. With the point of the pin resting on the anodized part of the blade, the signal is rectified and can actually power sensitive earphones if you are close to a the station.

Current flows in the N-type material in much the same way it flows in a wire; the excess electrons simply move through the material. The P-type material has a positive charge because it has "missing" electrons. It is relatively small like the "cat's whisker." The missing electrons form what some call "holes." Current flows when electrons jump from hole to hole. Some say that the holes flow the opposite direction from the electrons. This is similar to a line of cars in a traffic jam. As each car moves forward, the next car moves up to fill the space left behind. An outside observer might say the spaces are moving backwards, but it's really cars moving forward. The holes are merely the mechanism by which electrons can move in a P-type material.

When the P-type material and the N-type material are joined together, a PN junction is formed. In the N-type material, only the electrons that are close enough to the junction with the P-type material to be attracted by the positive charge are able to cross over the junction and form what is called the "barrier region." The rest of the excess electrons in the N-type material stay where they are, producing a net negative charge.

A PNP transistor is essentially the same thing with the polarities reversed. Just as in the previously discussed NPN transistor, a relatively small forward bias on the emitter-collector junction controls a larger current through the reverse-biased base-collector junction. The convention is to describe the current carriers in an NPN transistor as the electrons and the current carriers in a PNP transistor as the holes. Remembering, however, that the holes are merely spaces where an electron is missing and hole flow is in reality just electrons jumping from one hole to another. The mechanism is essentially the same in both types.

In this article, we have described simple diodes and transistors. However, there are many more variations on both themes. Small changes in mechanical arrangements, bias voltages and materials can make dedicated devices for regulation, impedance matching or high-frequency applications. The same basic theory applies to a small signal device in a cell phone and a high-power switch in a variable frequency motor drive.

Semiconductors are neither pure conductor nor pure insulator; they are somewhere in between.

A semiconductor takes on either an N-type or a P-type characteristic, depending on the material used to dope the substrate.

Diodes are formed by joining N and P-type materials.

Transistors are formed by sandwiching one type of material between layers of the opposite type.

Fig. 1. P-type material contains a large concentration of holes with few electrons; N-type material contains a large concentration of electrons with few holes. A PN junction is formed when the two are joined. The holes from the P-side diffuse into the N-side, while the electrons from the N-side diffuse into the P-side. A negative space charge forms near the P-side and a positive space charge forms near the N-side of the junction. The net current flow across the junction is zero.

Fig. 2. When a negative voltage is applied to the P-type material, the junction is reverse-biased. This increases the size of the barrier region, which drives the impedance very high, making it more difficult to move electrons across the depletion zone of the junction.

Fig. 3. When a positive voltage is applied to the P-type material, the junction is forward-biased. This greatly decreases the size of the barrier region, which drives the impedance very low, and allows significantly more current to flow through the junction.

Fig. 4. A transistor is like a sandwich with the bread being the same type of material and the filling the other type. An NPN transistor has an emitter and a collector made of N-type material with a base made of P-type material.

Fig. 5. When the emitter-base junction is forward biased, there is current flow from the emitter to the base. If there is no connection to the collector, the base-collector current is small because that junction is reverse-biased.

Fig. 6. Placing a large reverse bias on the base-collector junction draws the current flow through the transistor. The relatively small emitter-base current controls a much larger emitter-collector current. When current flows through a load, such as a resistor, connected between the collector and emitter of the transistor, a voltage is developed that is an amplified version of what is applied between the base and emitter.