Electronic Fundamental Muhammad Zahid BS Regular 18 March, 17 Transistor Lecture – 6 Electronic Fundamental Muhammad Zahid

LECTURE # 6 BIPOLAR JUNCTION TRANSISTOR By: Muhammad Zahid

BIPOLAR JUNCTION TRANSISTOR The first transistor was invented in Bell Labs (1947): by Bardeen, Brattain, and Shockley Originally made of germanium. The word “transistor” is a combination of the terms “transconductance” and “variable resistor” It can be use as amplifier and logic switch.

BIPOLAR JUNCTION TRANSISTOR The bipolar junction transistor (BJT) is constructed with three doped semiconductor regions separated by two pn junctions Regions are called emitter, base and collector. The term bipolar refers to the use of both holes and electrons as charge carriers in the transistor structure

BIPOLAR JUNCTION TRANSISTOR Base is located at the middle and more thin from the level of collector and emitter There are two types of BJTs construction, the npn and pnp The two junctions are termed the base-emitter junction and the base-collector junction

BIPOLAR JUNCTION TRANSISTOR Base region is lightly doped. The emitter has the highest doping profile. The collector has moderate doping.

BIPOLAR JUNCTION TRANSISTOR The arrow is always drawn on the emitter The arrow always point toward the n-type The arrow indicates the direction of the emitter current: pnp: E B npn: B E IC=the collector current IB= the base current IE= the emitter current

BIPOLAR JUNCTION TRANSISTOR Biasing:- In order for the transistor to operate properly, the two junctions must have the correct dc bias. For both npn and pnp, the base-emitter (BE) junction is forward-biased The base-collector (BC) junction is reverse-biased. This condition is called forward-reverse bias.

BIPOLAR JUNCTION TRANSISTOR These free electrons easily diffuse through the forward biased BE junction into the lightly doped and very thin p-type base region. The base has a low density of holes, which are the majority carriers, as represented by the white circles.

BIPOLAR JUNCTION TRANSISTOR A small percentage of the total number of free electrons injected into the base region recombine with holes and move as valence electrons through the base region and into the emitter region as hole current, indicated by the red arrows.

BIPOLAR JUNCTION TRANSISTOR Most of the free electrons that have entered the base do not recombine with holes because the base is very thin. As the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage.

BIPOLAR JUNCTION TRANSISTOR The free electrons move through the collector region, into the external circuit, and then return into the emitter region along with the base current. The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter.

BIPOLAR JUNCTION TRANSISTOR IN (small) OUT (large)

BIPOLAR JUNCTION TRANSISTOR No current flows Collector current controlled by the collector circuit. (Switch behavior) In full saturation VCE=0.2V. Collector current proportional to Base current The avalanche multiplication of current through collector junction occurs: to be avoided