TRANSISTOR

TRANSISTOR

Transistor is a three terminal device which transforms current flow from a low resistance path to a high resistance path. This transfer of current through resistance path given the name to the device “transfer resistor” or Transistor. It consists of junctions of P and N type semiconductors. There are many types of transistors with difference in the number of junctions, structure and characteristics.
Eg. UJT, BJT, FET, MOSFET etc.

BIPOLAR JUNCTION TRANSISTORS:

BJT is a three terminal device consisting of two P N junctions. A basic two-junction semiconductor must necessarily have one type of region sandwiched between two of the other type. There are two types of standard transistors, NPN and PNP. NPN transistor consists of a narrow P-type region between two N-type regions. Similarly a PNP transistor consists of a narrow N-type region between two P-type regions. So there are three regions in a transistor and the three regions are emitter (E), base (B), and collector (C), respectively.









The layer from which the emission of charge carriers takes place is called Emitter, the second layer through the carrier pass through is called Base and the third layer which collects the charge carriers is called Collector. Although the Collector and emitter are of the same type of materials, they cannot be interchanged as their functions are different. They have different physical and electrical properties. The base region is very thin compared to collector and emitter. The collector is made physically larger than emitter as it has to collect all the injected carriers and to withstand the large reverse voltage. In modern versions of this device, the emitter region is heavily doped with the appropriate impurity, while the base region is very lightly doped. The collector region has a moderate doping level.



Eg: BC 107, SL 100, BC 558, BF 195, 2N2222 etc.



UNBIASED TRANSISTOR:
Transistor which is not connected to any external source is called an unbiased transistor.


Under this condition the carriers across the junction produces two depletion layers. The barrier potential for Si is 0.7V and that for Ge is 0.3V. Since the three regions Emitter, Base, collector have different doping levels, the depletion layer do not have the same width. Since the Emitter region is heavily doped the EB junction penetrates slightly in to the Emitter but penetrates deeply in to the base. Similarly, the CB junction penetrates slightly in to the Collector but penetrates deeply in to the base. The EB junction width is less than the CB junction width.