How do you accurately determine the capacitance of a p-n junction?

Hi Team,

I’m Darshan Hiranandani, I’m working on a project that involves measuring the capacitance of a p-n junction. Could anyone provide suggestions or best practices for accurately determining this capacitance?

Regards
Darshan Hiranandani

Are you interested in determining this for junctions in forward bias or reverse bias?

Hey!

To accurately determine the capacitance of a p-n junction, follow these steps:

  1. Set Up the Circuit: Connect the p-n junction (diode) in reverse bias. This means the positive terminal of the power supply to the n-side and the negative terminal to the p-side.
  2. Measure Reverse Bias Voltage: Apply a known reverse bias voltage to the diode.
  3. Use an LCR Meter: An LCR meter can measure the capacitance directly. Connect it across the diode terminals while it’s in reverse bias.
  4. Frequency Considerations: Ensure the LCR meter operates at a suitable frequency, typically in the range of 1 kHz to 1 MHz, as capacitance can vary with frequency.
  5. Capacitance-Voltage Profiling: For more detailed analysis, measure the capacitance at different reverse bias voltages to plot a C-V curve. This helps in understanding the junction properties.

By following these steps, you can accurately determine the capacitance of your p-n junction.

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@reesewrenley gives excellent advice. An LCR meter (or bridge) is a great tool to use, but be aware of their limitations as well as the nature of p-n junctions.

First of all, it’s very important to minimize the effects of parasitic capacitance, resistance, and inductance of the test leads and/or the junction’s “environment”. Special test figures are available for most discrete components, but whatever the conditions it’s important to fully calibrate the test equipment. If the phase angle is more than a few degrees off from -90° it’s a good idea to try to find the resistance responsible for the measures error.

Some LCR meters may give inaccurate readings in the presence of DC, so if your meter is affected it may be a good idea to use a DC-blocking capacitor. This capacitor will appear in series with the junction’s capacitance, so calculation will be necessary. Of course, it’s imperative that any additional capacitors used be “well-behaved” at the frequencies and voltages they are subjected to.

Also, since a p-n junction’s capacitance depends on voltage, it’s necessary to minimize the test signal amplitude swing. ESR meters effectively calculate an average capacitance over the total voltage swing, and since capacitance is not a linear function of voltage the error induced by the voltage swing is greater with higher test signal amplitudes.

Finally, the effect of test frequency on the junction capacitance of small-signal diodes is usually much less of a problem than the offset created by lead inductance.