Lets suppose we have a pn junction diode made of silicon. When it is forward biased we say depletion region shrinks because electrons in the n-region are pushed towards the junction resulting in the decrease of width of positive charges layer also these electrons cross the junction and current starts flowing from n-type to p-type(electron`s flow) and the holes move opposite to the electrons flow, giving the direction of conventional current. There are no holes in the n-type and the electrons are essentially free electrons(in result of pentavent dopping) that exist in conduction band and go to p-type to fill holes and do not leave any hole behind them because they are not coming from a bond but are free electrons.
Holes move opposite to electrons but upto which point inside the diode? How the holes could go into the n-type material? Would they move opposite to electrons and remain only in the p-type and stops near the junction and new electrons are injected into the p-type and process continues.
Best Answer
The distance that minority carriers can move across the other material is called the "diffusion distance". The time that it takes until the minority carrier disappears is called the "minority-carrier recombination lifetime".
The distance / time that minority carriers have is dependent on the number of recombination sites in the base material. Recombination sites are crystal defects.
If your majority carrier has better mobility than your minority carrier, you want to create more recombination sites close to the junction, so that your injected minority carriers are quickly converted to majority carriers.
Since electron mobility is better than hole mobility, you probably want your "diffusion distance" and "carrier recombination lifetime" to be shorter for holes on the N side, and to be longer for electrons on the P side.