Junction Capacitance in a GaAs Photodiode

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Discussion Overview

The discussion revolves around estimating the junction capacitance of a GaAs photodiode as a function of varying bias voltages. Participants are exploring the relevant equations and parameters needed for accurate calculations, including the dielectric constant, mobility of electrons, resistivity, and built-in voltage.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks assistance in estimating junction capacitance, noting discrepancies in their calculations compared to expected values.
  • Another participant requests the make and model of the photodiode and the expected capacitance value for context.
  • A participant mentions a previous calculation of 250pF at 0V, contrasting with their current results in femtoFarads.
  • There is a suggestion that the original poster has identified a potential issue but has not provided detailed calculations for further assistance.
  • The original poster requests confirmation of the correctness of their equations and specific values for various parameters related to the photodiode.

Areas of Agreement / Disagreement

The discussion remains unresolved, with participants expressing different expectations for capacitance values and seeking clarification on the equations and parameters involved.

Contextual Notes

Participants have not provided specific calculations or detailed assumptions, which may affect the accuracy of the discussion. There is also a lack of consensus on the expected capacitance values and the correct parameters to use.

GraemeC
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I am trying to get an estimation the junction capacitance of my GaAs Photodiode with respect to varying bias voltages.

My Semiconductor Physics online textbook gives the following equations

Junction1.PNG


with
ε0 = permittivity of free space
εGgAs = GaAs dielectric constant
μ = Mobility of electrons
ρ = resitivity
Vbi = built-in voltage
VA = Bias voltage (in my case 0 - 5V)
A = photodiode area (in my case 0.3μm2)

Whenever, I work through the calculations I get answers that are many orders of magnitude off what I expect

I think the main issue is just getting the correct values(in the correct units) for each of the variables as I have been pulling them in from different sources.

Any help would appreciated.
 
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Welcome to PF.
What is the make and model of the photodiode ?
What capacitance do you expect ?
 
It's custom part.
My colleague did a calculation a while back and came up 250pF at 0V
I'm getting femtoFarads which is definitely not right!
 
You haven't shown us your work. You seem to have diagnosed your problem is, but haven't taken the step of fixing it. What exactly are you asking for?
 
Confirmation that the equations I've given are correct and then values for
ε0 = permittivity of free space
εGgAs = GaAs dielectric constant
μ = Mobility of electrons
ρ = resitivity
Vbi = built-in voltage
 

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