Junction Capacitance in a GaAs Photodiode

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SUMMARY

The discussion focuses on estimating the junction capacitance of a GaAs photodiode under varying bias voltages, specifically from 0 to 5V. Key variables include the permittivity of free space (ε0), GaAs dielectric constant (εGgAs), electron mobility (μ), resistivity (ρ), and built-in voltage (Vbi). A participant reported a calculated capacitance of 250pF at 0V, while another participant encountered results in femtoFarads, indicating a significant discrepancy likely due to incorrect variable values or units. The conversation emphasizes the importance of using accurate and consistent data for calculations.

PREREQUISITES
  • Understanding of semiconductor physics principles
  • Familiarity with GaAs photodiode characteristics
  • Knowledge of capacitance calculation formulas
  • Ability to convert and verify units in scientific equations
NEXT STEPS
  • Research the properties of GaAs, including its dielectric constant and electron mobility
  • Learn about the calculation of junction capacitance in semiconductor devices
  • Explore the significance of built-in voltage (Vbi) in photodiodes
  • Investigate common sources for accurate semiconductor material properties
USEFUL FOR

Electrical engineers, semiconductor physicists, and researchers involved in photodiode design and analysis will benefit from this discussion, particularly those focused on junction capacitance calculations and material property verification.

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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