In previous chapter, you experimented with diffusion assuming there are only one kind of carriers (electrons or hole) and no recombination (infinite lifetime). In reality, electrons and holes may recombine as they diffuse which is the focus of this chapter.
Imagine light is absorbed in the region highlighted by the red box generating electron-hole pairs. Play with the intensity of light (generation rate) and carrier lifetime to see how diffusion and recombination processes work in tandem.
Generation Rate in the Red Box (/cm3/s):
Carrier Lifetime (μs):
Other important factors here are carrier velocity and scattering time as they determine the diffusion coefficient. Play with these factors to see how they affect carrier density once we reach steady state.
Diffusion length is defined as the distance that it takes for the population of carriers to drop by a factor of 1/e. Can you say how carrier lifetime, speed and scattering time affect diffusion length?
Remember that the plots are quite noisy because of the small number of carriers that are visualized.
Carrier Velocity (cm/s):
Scattering Time (fs):
Carrier Lifetime (μs):
The same process can happen in a doped semiconductor. Play with the generation rate and doping concentration and see what is the impact on the electron and hole concentrations.
Generation Rate in the Red Box (/cm3/s):
Acceptor Density (per cm3):
What happens if the doping type changes?
Generation Rate in the Red Box (/cm3/s):
Donor Density (per cm3):