Transistor

TRANSISTOR… A transistor is a semiconductor device with two back to back PN junctions developed in a single piece of semiconductor. The word transistor drives from two words transfer and resistor. Transistor is basically a resistor that amplifies electric signals. There are two types of transistors (a) NPN and (b) PNP. A transistor has three distinct regions and two junctions, and the there are two types of charge carriers viz, electrons and holes. A transistor is called as Bipolar junction transistor (BJT). In NPN transistor, a layer of p-type material is developed between two layers of Ntype semiconductor of the same material. Similarly in PNP transistor a layer of N type semiconductor is sandwiched between two layers of p-type semiconductor of the same material. General arrangement of two types of transistor is given below. There are two junctions in a transistor they are Base-Emitter junction and BaseCollector junction. The three regions of the

transistor are (1) Emitter – which is heavily doped and is having the second largest area, function of this region is to supply majority charge carriers to the base. (2) Base – is the lightly doped region is having the least area. It is the central region of the transistor. (3) Collector - has the maximum area and moderately doped. Its function is to collect all the majority charge carriers that travel through the base from the emitter. This region is made larger in order to support effective heat dissipation. Symbols for NPN and PNP transistor are given below, the terminal with the arrow head is the emitter and the common terminal is the base. The arrow head direction on the emitter denotes the direction of conventional current flow.

WORKING OF NPN TRANSISTOR… For normal working of a transistor the base-emitter junction must be forward biased at a lower voltage (Vee) and the base collector junction must be reverse biased at a higher potential (Vcc). Normal connection diagram for a NPN transistor is as given below. Due to the forward biasing, electrons flow easily from the emitter to the base. Since the base region is very thin and lightly doped majority of the electrons diffuse through the base and reach the collector. Only a small number of electrons will combine with the holes in the base (5%) and get neutralized. The remaining electrons (95%) reach the collector, due to the higher reverse voltage. As soon as the electrons reach the collector, they are captured by the positive collector voltage. For every electron flowing out of the collector and entering the positive terminal of Vcc, an electron enters from the negative terminal of Vee.

The flow of the electrons from emitter to base constitutes the emitter current (Ie). A small number of holes are lost on the base region due to recombination with electrons. This loss of holes per second can be treated as the flow of an equal number of electrons from the base to the positive terminal of Vee, which yields the base current (Ib). The electrons reaching the collector are flowing towards the positive of Vcc, giving rise to the collector current (Ic). From the figure it is clear that the base and collector current in NPN transistor will flow into the transistor and the emitter current will flow out from the transistor. SO we have the relation Ie=Ic+Ib, since the base current is very small (1000 times smaller than collector or emitter current) we can have Ie ≈ Ic. Usually Ie ≈ Ic = several mA and Ib = µA.

WORKING OF PNP TRANSISTOR… For normal working of a transistor the base-emitter junction must be forward biased at a lower voltage (Vee) and the base collector junction must be reverse biased at a higher potential (Vcc). Normal connection diagram for a PNP transistor is as given below. Due to the forward biasing, holes flow easily from the emitter to the base. Since the base region is very thin and lightly doped, most of the holes diffuse through the base and reach the collector region. Only a small number (5%) of holes combine with the electrons in the base region and get neutralized. The remaining holes (95%) will reach the collector, due to the higher reverse voltage Vcc. For each hole reaching the collector, an electron is liberated from the negative of Vcc and neutralizes the hole. To compensate the loss of holes in the collector region an equal number of electrons and holes are generated by the breakage of covalent bonds in the emitter region. Electrons are

attracted to the positive of Vee and the holes remain in the emitter region. In a PNP transistor the current is due to the motion of holes inside the transistor, while it is due to the motion of electrons in the external circuit, this is due to the fact that in the external circuit the connecting wires are conductors and conductors do not support holes in their structure. The flow of the holes from emitter to base constitutes the emitter current (Ie). A small number of electrons are lost in the base region due to recombination with holes. This loss of holes can be treated as the flow of an equal number of electrons from the negative of Vee to the base, which yields the base current (Ib). The holes that cross the base reach the collector and this constitutes the collector current (Ic). As in NPN transistor here also we have the relation Ie=Ic+Ib, since the base current is very small (1000 times smaller than collector or emitter current) we can have Ie ≈ Ic.