Calculator inputs
Plotly graph
The chart shows how junction capacitance changes with reverse bias across the selected grading profile.
Formula used
Depletion capacitance definition
Cj = εsA / W
Here, εs = εrε0, A is junction area, and W is depletion width.
Abrupt doping-based zero-bias capacitance
Cj0 = A √[ q εs NA ND / (2 (NA + ND) Vbi) ]
This version is useful when the device geometry and doping concentrations are known.
Bias dependence
Cj(V) = Cj0 / (1 + VR/Vbi)m
Use m = 0.5 for abrupt junctions, m = 1/3 for linearly graded junctions, or enter a custom coefficient for fitted data.
How to use this calculator
- Select a calculation mode. Use doping mode when you know NA and ND. Use zero-bias mode when measured Cj0 is already available.
- Choose a material or enter a custom relative permittivity.
- Set the junction profile. Abrupt and graded modes auto-fill the grading coefficient.
- Enter the junction area, built-in voltage, reverse bias, and graph limit.
- Press Calculate capacitance. The result appears above the form with summary cards, export buttons, and a bias graph.
- Review the example table and formulas below to compare your result with a practical bias sweep.
Example data table
This table shows a sample capacitance sweep using the current model settings when available. Otherwise, it uses default sample values.
| Reverse Bias (V) | Capacitance (formatted) | Capacitance (pF) |
|---|---|---|
| 0.00 | 198.791517 pF | 198.7915 |
| 1.00 | 127.562319 pF | 127.5623 |
| 2.00 | 101.219686 pF | 101.2197 |
| 3.00 | 86.466127 pF | 86.4661 |
| 5.00 | 69.664166 pF | 69.6642 |
| 7.00 | 59.937898 pF | 59.9379 |
| 10.00 | 50.845788 pF | 50.8458 |
Frequently asked questions
1. What does PN junction capacitance represent?
It represents the small-signal capacitance created by charge storage in the depletion region. As bias changes, depletion width changes, so capacitance changes too.
2. Why does reverse bias reduce capacitance?
Reverse bias widens the depletion region. Since capacitance is inversely related to depletion width, a wider depletion region produces a smaller junction capacitance.
3. When should I use the doping mode?
Use doping mode when you know device area, built-in voltage, permittivity, and both doping concentrations. It is a physics-based route to estimate zero-bias and biased capacitance.
4. When is the zero-bias mode more useful?
Use zero-bias mode when you already have measured or datasheet capacitance at zero bias. Then apply the profile coefficient to estimate capacitance across reverse-bias values.
5. What grading coefficient should I choose?
Use 0.5 for abrupt junctions and one-third for linearly graded junctions. Choose a custom value when you are fitting measured capacitance-voltage data from a real device.
6. Which units does this page use internally?
Area is converted to cm² internally because common semiconductor formulas use F/cm for permittivity and cm⁻³ for doping concentrations. Results are displayed in practical units.
7. Is this calculator suitable for forward bias?
This page is designed for depletion capacitance under zero or reverse bias. Strong forward-bias diffusion capacitance is a different effect and is not modeled here.
8. Can I use this for learning and quick design checks?
Yes. It is useful for studying trends, comparing materials, and making quick estimates. Final device design should still be checked against measured data and detailed simulations.