Solve diffusion capacitance with exact diode charge relations. Check approximations across temperature and bias conditions. Get exportable results, formulas, steps, and practical guidance here.
Diffusion capacitance comes from stored minority carrier charge in a forward biased diode.
ID = IS(eV/(ηVT) - 1)
VT = kT/q
QD = τTID
CD = dQD/dV
Substituting the diode current equation gives:
CD = τTISeV/(ηVT) / (ηVT)
For strong forward bias, a practical form is:
CD ≈ τTID / (ηVT)
| Case | IS (A) | V (V) | η | τT (s) | T (K) | Approx. CD (F) |
|---|---|---|---|---|---|---|
| Silicon Diode A | 1.0e-12 | 0.65 | 1.8 | 5.0e-7 | 300 | 1.26e-11 |
| Silicon Diode B | 2.0e-12 | 0.70 | 2.0 | 1.0e-6 | 300 | 2.92e-11 |
| Warm Junction Device | 5.0e-12 | 0.75 | 1.5 | 2.0e-6 | 320 | 1.81e-8 |
Diffusion capacitance matters in forward biased semiconductor junctions. It appears because injected carriers store charge inside the device. That charge changes with voltage. The change creates an effective capacitance. This calculator helps you estimate that value quickly and clearly.
Many tools show only a final number. This page also shows the derivation path. That is useful for students, engineers, and circuit designers. You can connect diode current, stored charge, and capacitance in one workflow. That makes the result easier to verify.
The diode current begins with the exponential junction equation. The thermal voltage depends on temperature. The ideality factor adjusts the exponential response. Transit time links current to stored charge. Once charge is expressed in terms of voltage, differentiation gives diffusion capacitance. This is the main physics idea behind the model.
In forward bias, minority carriers cross the junction and remain stored for a short time. That short time is the transit time. More current usually means more stored charge. More stored charge means larger diffusion capacitance. For strong forward conduction, the approximation becomes very convenient. It reduces the formula to current, transit time, ideality factor, and thermal voltage.
Temperature changes the thermal voltage. A warmer device has a higher thermal voltage. That affects the exponent and the final capacitance. The area multiplier is also useful. It lets you model larger junctions or parallel device sections without rewriting the base saturation current.
Diffusion capacitance influences switching speed, small signal behavior, and transient response. It can affect detector circuits, analog front ends, rectifiers, and high speed semiconductor networks. Designers often compare exact and approximate values during bias selection. Students use the same comparison to understand when the simplified expression is valid.
This calculator gives both views. It reports thermal voltage, exact current, stored charge, derivative terms, and capacitance values. That makes it useful for learning, checking homework, and building reliable semiconductor models.
It is the capacitance caused by stored minority carrier charge in a forward biased diode or junction device. It rises as injected charge increases.
The approximation works best during strong forward bias. In that region, diode current is much larger than saturation current, so the simplified form becomes reliable.
Temperature changes thermal voltage. Thermal voltage changes the exponential current term. That directly changes stored charge and diffusion capacitance.
Transit time represents how long injected carriers stay stored before recombining or leaving the region. Larger transit time usually increases diffusion capacitance.
Ideality factor adjusts the diode equation to match device behavior. It changes the exponent and affects both exact current and capacitance estimates.
Yes. This file includes an optional measured current input. That value is used in the approximate capacitance expression when you provide it.
Usually no. Under reverse bias, diffusion capacitance becomes very small. Junction or depletion capacitance is normally more relevant in that condition.
Showing both values helps you compare theory and engineering simplification. It also helps you judge whether forward bias is strong enough for the approximation.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.