Lesson 5: Semiconductors.



Anything that allows electricity to pass through it is called a conductor, and anything that does not allow electricity to pass through is called an insulator. The next components we will be studying are called Semiconductors, because under certain circumstances they act as one, and other circumstances they act as the other. These devices are made by adding one of two types of impurities to silicon crystals. In its purest state silicon is an insulator, but by adding a slightly negative ("n" type) or positive ("p" type) impurity it becomes a conductor. Then, when you put 2 or more different slices together things really start getting interesting.

The simplest semiconductor is called a Diode, and it is produced by putting a piece of p silicon together with a piece of n silicon. If a battery is connected to the diode, with the positive connected to the p side and negative connectred to the n side then the diode will act as a conductor, but if the polarity is reversed then the diode will act as an insulator. However, if the reverse voltage gets high enough eventually the diode will "breakdown" and electricity will begin to flow in the reverse direction. This is called the "Reverse Breakdown Voltage" and the principle is used in voltage control circuits by what are called Zener Diodes. There are many different types of diodes; some that are called Light Emitting Diodes or LEDs which light up when they are conducting , others are used as Lasers, and specialist (high speed, low forward voltage drop) Schottky switching diodes are in widespread use in computers. There is also a standard configuration for connecting four diodes in power supplies called the Bridge Rectifier available as a single component.

Photos and Circuit symbols of diodes and transistors. Ok, so a diode is made up of two different layers of silicon. The next level uses three layers of silicon set up in the configuration of either NPN or PNP. These triple wafer devices are called Transistors. In simple terms (and to the horror of university lecturers) I like to think of them as voltage controlled resistors. Current flow between the Collector and Emitter is controlled by the voltage appearing at the Base. Below a certain base voltage no current will flow through the collector - emitter path, giving the impression that it is a resistor with infinite resistance. Above a certain base voltage the transistor becomes saturated, and the collector - emitter path appears like a simple piece of wire with zero resistance; a "short circuit".allowing as much current flow as the collector - emitter path can take without burning out. There are several types of transistors, such as the bi-polar that we just looked at, the unijunction, and the field effect transistor which is becoming very common today being used extensively in computers because of the low power required to operate them.

Transistors were first developed in the 1950's, but the biggest development in semiconductors began in the 1970s when several transistors were constructed on a single silicon wafer, together with some diodes, resistors and capacitors. This is called an Intergrated Circuit. Through the years integrated circuits have increased in complexity and miniturization until now entire computers, amplifiers, TVs etc, etc are available on a single "chip".

The next chapter will look at how transistors operate in a circuit. To help you understand this it will benefit you to revise chapter two, paying special attentiopn to how voltages are distributed around a "series" circuit.





We're now ready to move on to the next lesson. Click here to go to the lesson about using a transistor as a simple amplifier, or click here to return to the Electronics for Sound Techs index page.