I'm trying to solve this problem:

A GaAs LED is at \$ 300 K \$ when the current density is 1000 \$A/cm^2\$ . The width of the active region is \$ 1 µm \$. Assume that, for that current, we are in the strong injection regime, and that the bimolecular coefficient is \$10^{-10} cm^3 /s \$. Calculate the cut-off frequency for that current density.

I know the answer is 12.5 MHz.

I know that the cut-off frequency relates do the carrier's lifetime.

$$f_c=\frac{1}{2 \pi \tau}$$

Right, so now I need to calculate the carrier's lifetime. I know that since this is strong injection we would have:

$$\tau=\frac{1}{B \Delta n_0}$$

where B is the bimolecular coefficient. So now I need to calculate \$ \Delta n_0 \$.

My question now is how can I use the temperature, the current density and the width of the active region to calculate \$ \Delta n_0 \$? I feel that I might need some parameters of GaAs at 300 K, but I'm having trouble understanding what parameters.

  • \$\begingroup\$ Maybe PSE gives better response rather than ESE. As the question is close to semiconductor physics. \$\endgroup\$
    – Mitu Raj
    Nov 9, 2020 at 6:06
  • \$\begingroup\$ I have posted there too, no answers and two people voted to close for it being off topic \$\endgroup\$ Nov 9, 2020 at 11:29
  • \$\begingroup\$ Are you sure that it's 12.5 MHz \$\endgroup\$
    – Mitu Raj
    Nov 9, 2020 at 15:34

1 Answer 1


After a bit of digging, I found this:

Equation 9.17 in this chapter

You can manipulate that equation as -







Therefore -


Calculate carrier lifetime - $$\tau=\frac{1}{B \Delta n}\approx\mathbf{12.64 \text{ ns}}$$

Calculate cut-off frequency - $$f_c=\frac{1}{2 \pi \tau}\approx\mathbf{12.6\text{ MHz}}$$

  • 1
    \$\begingroup\$ Thank you so much! I didn't have that equation. That solves it! \$\endgroup\$ Nov 10, 2020 at 1:00

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