Thermal Recombination and Generation

[@sallyportillo2755]

You know what it's likeable about you, the voice and the font type used, this is highly complex module but it looks like in kindergarten classroom.

[@Heroofutopia]

Wise words: constants almost always represent our ignorance!

[@codewithlax]

Sir the number of traps Nt means number of trapped levels or number of trapped states?

[@JordanEdmundsEECS]

Good question! And a subtle one. Nt refers to the trap states density (number of trapped states). We assume all these traps are roughly at the same energy level, which is approximately true but quantum mechanically false.

[@user-ge8hj9br6w]

4:50, how probability of almost 1?

[@ilanmer1234]

thanks a lot. could you please explain why in the Et level the probabillity to find an electron is almost one?

[@suruchiverma48]

One last thing, the equilibrium state is also for a doped material? And the non equilibrium state is just a bit more doped?

[@JordanEdmundsEECS]

Yes, 'equilibrium' just refers to a state where we have no applied voltage or *external* source of carriers. A doped semiconductor can be in equilibrium.

[@suruchiverma48]

And finally, what exactly is low level injection? Is it any externally applied bias like voltage or radiation or a specific type? How is it done?

[@JordanEdmundsEECS]

Also a great question, this is when the 'injected' carries (the ones created by non-thermal processes) are much lower than the dominant dopant concentration. So if my donor concentration is 10^18 cm^-3, 'low-level injection' means my injected carriers produce a concentration much smaller than that.

[@suruchiverma48]

That was clear. But my question was how are they injected? Through what method? Light or voltage application, what else?

[@JordanEdmundsEECS]

Usually we just deal with those two cases. You could also have carriers diffuse via other areas of the semiconductor even if you don’t apply a voltage directly to some area.

[@suruchiverma48]

@@JordanEdmundsEECS ok thanks

[@suruchiverma48]

Thanks a lot for answering all my queries. Can you explain why you have an added impurity? We could've done the same thing with no trapped states. Is it because that's how it generally is because manufacturing pure silicon cells is difficult therefore rare?

[@JordanEdmundsEECS]

Trapped states are minimized as much as possible during fabrication but are still present at very low concentrations, and are the primary cause of recombination at lower carrier densities (Shockley-Reed Hall recombination).

[@suruchiverma48]

Does this cp depend on the material? Because it looks like it is inversely proportional to lifetime. Doesn't lifetime depend on number of holes? Am confused.

[@JordanEdmundsEECS]

Yes, Cp depends on the material. This video is just a simplified first picture, and ignores the doping-dependence of things like carrier lifetime. In this simplified picture, carrier lifetime is just a constant, and the rate of change of carriers is equal to the number of carriers over the lifetime. In reality, these parameters will have doping (and even carrier concentration) dependence, and this simplified model is just the first step to understanding recombination.

[@suruchiverma48]

Sir, it's v clear now. But, in non equilibrium state, why should the constant of proportionality remain CP for recombination?

[@JordanEdmundsEECS]

Great question! In general, it won't (this is just an overly simplified picture). However, for doped semiconductors and low-level injection this is approximately true (this is called Shockley-Reed-Hall recombination).

[@user-ge8hj9br6w]

can we say generation rate is inversely proportional to majority carriers and directly proportional to minority carriers due to mass action law in play?

[@vlad7780]

Sir, first of all, thank you, for all your videos, they are really great. I have just one question: why there is no donor level Ed in band structure? Or Et include it, as some average?

[@JordanEdmundsEECS]

In general, you are 100% correct, there will be a donor level in the band structure. There will also be so-called 'trap' levels, extra energy levels due to impurities (such as gold) within the semiconductor. We *generally* don't draw these levels when talking about the band structure because it would make the diagrams quite messy.

[@vietanhngo8893]

Thank you