TRANSISTORS

The transistor is a solid state device,whose operation depends upon the flow of electric charge carriers within the solid.Transistor is capable of amplification and in most respect it is analogous to a vacuum triode. The main difference between the two is that the transistor is a current controlled device whereas vacuum triode is a voltage controlled device.The transistor is only about 6 decade old, yet it has replaced vacuum tubes in almost all applications.The reasons are obviously its advantages over vacuum tubes such as compact size,light weight,rugged construction,more resistive to shocks and vibrations,instantaneous operation (no heating required),low operating voltage,high operating efficiency (no heat loss) and long life with essentially parature and frequency.However,transistor,in comparison to vacuum triodes, have  some drawbacks also such as loud hum noise,restricted operating temperature (up to 75°C) and operating frequency (up to a few MHz only).

It consists of a silicon or germanium (preferably silicon because of its smaller cutoff current  ICBO ,smaller variations in ICBO due to variations in temperature and higher operating temperature) crystal in which a layer of N-type material is sandwiched between two layers of P-type material,as shown in fig.Alternatively a transistor may consist of a layer of p-type material sandwiched between two layers of N-type material,as shown in fig.In the former case the transistor is referred to as a P-N-P transistor and in latter case,as an N-P-N transistor.Each type of transistor has two P-N junctions-one junction between the emitter and base,called the emitter-base junction or simply the emitter junction and the other junction between the base and collector,called the collector-base junction or simply the collector junction.Thus a transistor is like two P-N junction diodes connected back to back.The two junctions give rise to three regions provided with three terminals called the emitter,base and collector,as shown in fig.The emitter base,collector correspond in a general way to the the cathode, grid and plate or anode of vacuum triode.