Generation and Recombination

Question 1

What is the absorption coefficient? Relevance for a solar cell?

Answer 1

The absorption coefficient determines how far into a material light of a particular wavelength can penetrate before it is absorbed.

This is an important parameter as you want to absorb as much photons are possible.

Question 2

What are the excess charge carriers?

Answer 2

Electron-hole pairs are created in a semiconductor when photons with energies exceeding the band gap energy of the semiconductor are absorbed. Usually associated with the three recombination methods.

Question 3

Which excess charge carrier concentration determines recombination (minority or majority)? Why?

Answer 3

Recombination is when an electron recombined with a hole and gives up the energy to produce either light or heat.

Minority?

Question 4

Name the three bulk recombination mechanisms. Which ones are inevitable?

Answer 4

Radiative recombination

Shockly-Read-Hall (SRH) recombination

Auger recombination

Question 5

What is the dependence of the minority charge carrier lifetime of (1)
radiative, (2) SRH, and (3) Auger recombination on the dominant doping
concentration?

Answer 5

GN question

Question 6

What’s the minority and majority charge?

Answer 6

The majority charge carriers carry most of the electric charge or electric current in the semiconductor. Hence, majority charge carriers are mainly responsible for electric current flow in the superconductor.

The charge carriers that are present in small quantity are called minority charge carriers.

Question 7

What is the charge carrier diffusion length?

Why is it important?

Answer 7

Average distance a carrier can move from point of generation until it recombines

Photoexcited carriers must be able to move from their point of generation to where they can be collected.

Question 8

What’s the minority carrier lifetime?

Answer 8

Average time that a carrier can spend in an excited state after e-h+ generation before recombining.

Question 9

What is the state of the semiconductor if we illuminate with light?

Answer 9

If we illuminate it, there’s a constant generation rate Gl.

With equilibrium, we usually talk about thermal equilibirum. This is when T>0K, the crystal lattice vibrations lead to the generation of e-h pairs at a generation rate Gth.