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Biasing

Biasing means the way of connecting emf source to P-N junction diode. It is of the following two types:

Forward biasing

The positive terminal of the battery is connected to P-crystal and the negative terminal of battery is connected to N-crystal (Fig. 13).
 
108109.png
Fig. 13
  • In forward biasing, the width of depletion layer decreases.
  • In forward biasing, resistance offered RForward 10 – 25
  • Forward bias opposes the potential barrier and for V > VB, a forward current is set up across the junction.
  • The current is given by 107021.png, where is = Saturation current, In the exponent, e = 1.6 × 10–19 C, k = Boltzmann’s constant

Cut-in (knee) voltage

The voltage at which the current starts to increase rapidily (Fig. 14). For Ge, it is 0.3 V and for Si, it is 0.7 V.
 
In Fig. 14, df stands for diffusion and dr stands for drift.
 
108120.png
Fig. 14

Reverse biasing

Positive terminal of the battery is connected to N-crystal and the negative terminal of the battery is connected to P-crystal.
  • In reverse biasing width of depletion layer increases
  • In reverse biasing resistance offered RReverse 105
  • Reverse bias supports the potential barrier and no current flows across the junction due to the diffusion of the majority carriers.
108132.png
Fig. 15
 
(A very small reverse currents may exist in the circuit due to the drifting of minority carriers across the junction)

Break down voltage

Reverse voltage at which the break down of semiconductor occurs. For Ge, it is 25 V and for Si, it is 35 V (Fig. 16).
 
108148.png
Fig. 16




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