transistor It is semiconductor device consisting of a thin layer of one type of semiconductor (p type or n type) sandwitched in between two other relatively thick layers of the other type. A transistor can either p-n-p or n-p-n. Three successive layers are called emitter, base and collector (see fig. t13). The base is lightly doped in comparison with emitter and collector and its thickness is ~ 4 m m.

Consider the n-p-n transistor which is more common. In fig. t14, an n-p-n transistor connected into a circuit is shown. The emitter base junction is forward biased and base collector junction is reverse biased. Current will flow through the transistor when the base emitter voltage is greater than 0.6 V. It is needed to overcome the potential barrier formed at the junction. The emitter current I_E is the sum of the base current I_B and the collector current I_c.

I_E = I_B + I_c

I_C is usually 98% of I_E..

Transistor characteristics: Three characteristics are usually measured which are shown in fig. t15 a-c.

(a) The variation of base current, I_B with base emitter voltage, V_BE.

(b) The variation of collector current, I_C with base current, I_B.

(c) The variation of collector current with collector emitter voltage, V_CE.

Fig. t15 b shows that changing the base current, I_B by a few microampere produces a large variation in the collector current (approximately 100 times). Thus amplification action of the transistor is current controlled rather than voltage controlled. The current gain is denoted by h_FE,

h_FE = (D I_C/ D I_B) (t11)

Its usual value lies between 50 to 150 for n-p-n silicon transistor.