# Junction Capacitance in a GaAs Photodiode

1. Aug 14, 2014

### GraemeC

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

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.

2. Aug 14, 2014

### Baluncore

Welcome to PF.
What is the make and model of the photodiode ?
What capacitance do you expect ?

3. Aug 14, 2014

### GraemeC

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!

4. Aug 14, 2014

### marcusl

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?

5. Aug 14, 2014

### GraemeC

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