Question

Diffusion capacitance of PN Junction Diode ______.
1. increase with increasing current and increasing temperature
2. increase with increase current and decreasing temperature
3. decrease with decreasing current and decreasing temperature
4. decrease with increasing current and increasing temperature

Answer 1

Correct Answer - Option 2 : increase with increase current and decreasing temperature

Explanation:

Diffusion Capacitance:

1. Diffusion capacitance is due to the transfer of minority carries during the forward bias

2. The minority carriers diffuse from one end of the junction to the other, causing variation of charge with applied voltage

3. This leads to capacitance, it is present only in forward bias and is significantly higher than depletion capacitance in the forward bias

The diffusion capacitance is given by the formula:

\({C_d} = \frac{{{I_{DQ}}\times\tau }}{{2{V_T}}}\)   ---(1)

IDQ = Quiescent current of the diode.

τ = Minority carrier lifetime

VT = Thermal voltage

So from equation (1), we can say that the diffusion capacitance of the PN Junction Diode increases with an increase in current as C_d ∝ I_DQ and 

also, C_d ∝ 1/V_t, where,

\(V_t=\frac{T}{11600}\)

Hence diffusion capacitance of the PN Junction Diode increases with a decrease in temperature.

So option (2) is the correct answer.

Depletion Capacitance:

• Depletion capacitance is due to the storage of charges in reverse bias junction, which acts like parallel plate capacitance with the forward voltage
• In the forward the depletion width decreases which increases depletion capacitance
• It is less than diffusion capacitance in the forward bias mode