Diffusion Capacitance

DIFFUIOSN CAPACITANCE

The capacitance which exists in a forward biased junction is called diffusion or storage capacitance, C_D. C_D is always larger than transistor capacitance C_T.

This capacitance is caused by the injected charge stored near the junction just outside the transition region.

It is defined as the rate of change of injected charge with applied voltage.

C_D = d Q / d V

dQ - change in number of minority carriers outside the depletion region

d V - change in voltage across the diode

Assumption

P side is more heavily doped than N side. Thus current I is carried across the junction only due to holes moving from P to N side.

The excess minority charge Q existing on N side is given by

Differentiate equation (1) with respect to V

The hole current I is given by I_pn(x) with x = 0

Differentiate P_n(0) with respect to V,

Substitute (2) in (1)

Where, τ - mean life for holes and electron

Diffusion capacitance C_D increases exponentially with forward bias. C_D value from 10 to 1000 PF. C_D is inversely proportional to frequency ie it is high at low frequency and low at high frequency.

Applications of PN Diode

An ideal PN junction diode has zero resistance in forward bias and infinite resistance in reverse bias. Due to these characteristics, the diode has number of applications as follows.

i. Rectifiers in dc power supplies.

ii. Switch in digital logic circuits.

iii. Clipper used in wave shaping circuits in computers, radars, radio and TV receivers.

iv. Clampers used as de restorer in TV receivers.

v. Detector or demodulation circuits.

The PN diode with different doping levels used in

i. Photo detectors (PIN, APD photodiode).

ii. Zener diode in voltage regulators.

iii. Varactor diode in tuners of radio and TV receivers.

iv. LED in digital displays.

v. LASER in optical communication.

vi. Tunnel diodes as microwave oscillator.