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Characteristics of NPN Transistor.


structurally; (BJT) it is a semiconductor with three layers of alternate doping

functionally; it is an electric component that can be used as an electric switch and signal amplifier

NPN transistor;

It is a bipolar junction transistor that consists of p-type doping sandwiched between two n-type dopings. it has three terminals:-

  1. emitter; it is heavily doped N region

  2. base; it is lightly doped, narrow P region

  3. collector; it is lightly doped, broad N region

proper configuration of NPN transistor in common Emitter mode:-

1. the E-B junction should be forward biased so that large current flows through emitter for small current through the base to act as a amplifier           

ie. Vb > Ve

2. the B-C junction should be reverse biased so that current only flows if the breakdown voltage is applied to act as a switch

ie Vb < Vc

from 1 and 2, we get

Vc > Vb > Ve


=> it is relation between Veb ()input voltage)  and Ib (input current) at constant Vce

=> since, E-B junction is forward biased, a large base current is obtained when the base-emitter voltage exceeds the current knee voltage

=> the knee voltage also increases with the increase in emitter-collector voltage


  • it is the relation between Vce (output voltage) and Ic (output current) at constant Ib

  • in saturation mode (ie when Vce is very low),  Ic increases actively with the increase in Vce

  • in activation mode (ie when Vce is high) Ib is independent of Vce

  • in cutoff mode (when Ib = 0) small leakage current is obtained


  • it is the dependence of Ic on Ib

  • Ic increases linearly with increase in Ib

  • beta = Ic/Ib (slope) is the current gain

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