Biploar transistor:www.ecmagic.blogspot.com

A transistor is basically a Si on Ge crystal containing three separate regions. It can be either NPN or PNP type fig. 1. The middle region is called the base and the outer two regions are called emitter and the collector. The outer layers although they are of same type but their functions cannot be changed. They have different physical and electrical properties. In most transistors, emitter is heavily doped. Its job is to emit or inject electrons into the base. These bases are lightly doped and very thin, it passes most of the emitter-injected electrons on to the collector. The doping level of collector is intermediate between the heavy doping of emitter and the light doping of the base. The collector is so named because it collects electrons from base. The collector is the largest of the three regions; it must dissipate more heat than the emitter or base. The transistor has two junctions. One between emitter and the base and other between the base and the collector. Because of this the transistor is similar to two diodes, one emitter diode and other collector base diode.

Fig .1

When transistor is made, the diffusion of free electrons across the junction produces two depletion layers. For each of these depletion layers, the barrier potential is 0.7 V for Si transistor and 0.3 V for Ge transistor.

The depletion layers do not have the same width, because different regions have different doping levels. The more heavily doped a region is, the greater the concentration of ions near the junction. This means the depletion layer penetrates more deeply into the base and slightly into emitter. Similarly, it penetration more into collector. The thickness of collector depletion layer is large while the base depletion layer is small as shown in fig. 2.

Fig. 2

If both the junctions are forward biased using two d.c sources, as shown in fig. 3a. free electrons (majority carriers) enter the emitter and collector of the transistor, joins at the base and come out of the base. Because both the diodes are forward biased, the emitter and collector currents are large.

Fig. 3a

Fig. 3b

If both the junction are reverse biased as shown in fig. 3b, then small currents flows through both junctions only due to thermally produced minority carriers and surface leakage. Thermally produced carriers are temperature dependent it approximately doubles for every 10 degree celsius rise in ambient temperature. The surface leakage current increases with voltage.