Transistors

Transistor is the most important electronic device. Transistor had been invented in 1948 in bell laboratories but they had been used in 1960s. Before transistors thermionic tubes were used in electronics. Transistor is a three layer semiconductor device. Transistor has a very fast switching capability. The word transistor is the combination of transfer resistor. It means transistor is an electronic device which can change or transfer its resistance. Hence it controls the flow of current by changing its resistance from very high to very low resistance. A transistor can be used as an on an off switch as well as it can be used as a variable resistance to provide limitation to current flow electronically.

Transistors are made of N and P type semiconductor. The semiconductor could be germanium or silicon. In older days germanium was used and now a days silicon is used. Silicon is more stable than germanium at high temperature. There are two main types of injunction transistors. One is NPN and second one is PNP. There are three terminals of a transistor. They are Collector, Emitter and Base. The emitter is the out layer and is forward biased to base. The collector is also the outer layer and has a reverse bias with base. The collector has opposite polarity to the applied voltage and emitter has the same polarity to the applied voltage.

Transistor function and symbol schematics

There are two circuits in a transistor. One is base emitter circuit and the other the collector emitter circuit. When voltage is applied to the base emitter circuit and the voltage passes the value 0.7v in silicon diode and 0.3v in germanium diode, the transistor conducts and current passes between collector and emitter. A small current in the base emitter circuit can control large current in the collector emitter circuit. This is somewhat like a relay operation. But relay is electromagnetic or electromechanical switch whereas transistor is an electronic switch.

Transistor can be used as switch as well as amplifier. If it is used as switch the load is connected in the series of collector or emitter. If it is used as amplifier the load resistance is connected in series of collector or emitter and change in current would be taken as voltage drop on the load resistance.

 Transistors are made in different power ratings. They have different sizes. There are thousands of models designated with different numbers. They have many shapes. Some are small signal transistors and some are power transistors. Some are low frequency and some are high frequency transistors. Some have high gain and some have low gain. There characteristics are mentioned in data books. A single transistor cannot have all characteristics. That is the reason there are thousands of transistor models available for different application. While selecting a transistor for a specific job and application you should keep the requirements and parameters of operation in mind.

There are three biasing method of transistor. Common base, common collector and common emitter. All these three transistor circuit configurations provide different characteristics. All you should know these configuration characteristics and your requirements of operation.