Characteristics of NPN Transistor.
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
It is a bipolar junction transistor that consists of p-type doping sandwiched between two n-type dopings. it has three terminals:-
- emitter; it is heavily doped N region
- base; it is lightly doped, narrow P region
- 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
INPUT CHARACTERISTICS
=> 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
OUTPUT CHARACTERISTICS
- 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
TRANSFER CHARACTERISTICS
- it is the dependence of Ic on Ib
- Ic increases linearly with increase in Ib
- beta = Ic/Ib (slope) is the current gain