Why can’t n + p stay constant when an intrinsic semiconductor is doped?

In intrinsic Si, carrier concentration is of the order of 10^{10} cm^{-3}. If we add both n and p type, it becomes a total of 2 \times 10^{10} cm^{-3}. So, if n+p=2n_i were true the highest doping we could have would not exceed the intrinsic doping and we could never have a low enough resistivity for this to work. Typical doping in semiconductors is 10^{17} cm^{-3}. The reason np stays constant is because when you, e.g. dope the semiconductor with p-type dopant, it "eats up" or recombines with some of the electrons and electron density reduces, but the total product stays the same.

See question on Quora

I like to blog about various topics ranging from education in IITs to social issues in India and US. All opinions and views presented here are my own and not of my employer.

Leave a Reply