Photodiode carrier transit time

[Infidel22]

Hello, I am having a bit of trouble with the following problem:

Assuming a built in voltage equal to the bandgap voltage, calculate the slowest possible transmit time for a carrier with 5V reverse bias applied to the GaAs photodiode.

I would be able to do this problem without difficulty if it wasnt for the part mentioning "Bandgap Voltage." I am familiar with bangap energy and how to calculate it, but I have no idea how to figure out the bandgap voltage for a given material. Can anyone give me any hints on this?

Thanks!

 

[OlderDan]

hhhmmmmm Is this something more complicated than the basic relationship between electrical energy and potential? Energy gaps are typically measured in eV because electric potential is energy per charge

 

[thepatientmental]

Hi there, I understand your confusion with the mention of "bandgap voltage." The bandgap voltage refers to the voltage at which the energy gap between the valence band and the conduction band is closed and no current can flow through the material. In other words, it is the voltage at which the material becomes an insulator. This voltage is typically equal to the bandgap energy divided by the charge of an electron (or hole). In the case of GaAs, the bandgap voltage is approximately 1.42V.

To calculate the slowest possible transit time for a carrier with 5V reverse bias applied to the GaAs photodiode, you will need to use the Shockley-Read-Hall (SRH) recombination equation. This equation takes into account the carrier lifetime, which is the average time a carrier spends in the material before recombining. The longer the lifetime, the slower the transit time.

To find the carrier lifetime, you can use the expression: τ = 1/(Nt + Ns), where Nt is the trap density and Ns is the surface recombination velocity. These values can be found in material databases or from experimental measurements.

Once you have the carrier lifetime, you can plug it into the SRH equation along with the applied voltage and the bandgap voltage to calculate the transit time. Remember to convert all units to SI units before plugging into the equation.

I hope this helps guide you in solving the problem. Good luck!