Diffusion Capacitance
Definition
Cd = dQ / dV
Where:
- Q = stored charge
- V = applied voltage
Physical Meaning
- Occurs in forward-biased PN junctions
- Charge carriers accumulate instead of disappearing instantly
- This stored charge behaves like capacitance
More current → more stored charge → higher capacitance
Mathematical Derivation
Stored charge:
Q = I × Ļ
Differentiating:
Cd = dQ/dV = d(IĻ)/dV
Assuming Ļ is constant:
Cd = Ļ (dI/dV)
Using Diode Equation
I = Is e^(V / Ī·VT)
Differentiating:
dI/dV = I / (Ī·VT)
Final Formula
Cd = ĻI / (Ī·VT)
Key Insights
- Diffusion capacitance increases with current
- Important in forward bias
- Usually larger than junction capacitance in forward region
Comparison
| Type | Cause | When Important |
|---|---|---|
| Diffusion Capacitance | Stored charge | Forward bias |
| Junction Capacitance | Depletion region | Reverse bias |
Summary
Diffusion capacitance is capacitance due to stored charge in a forward-biased semiconductor junction.