Question

what happened to the electrical effect when the donor and acceptor impurities are equal in a semiconductor?Do they cancel each other? if so, what is it's mechanism?​

Answer 1

Let us assume that your semiconductor is p-doped; in this case, when you add an n-dopant, the electrons donated by this dopant will recombine with the previously existing mobile holes, meaning that the concentration of free holes will slowly decrease and approach that of an intrinsic semiconductor. In other terms, your semiconductor will lose its "p-type behaviour" and return an intrinsic semiconductor (at least in terms of free charge concentration).

When the concentration of n-dopant is exactly the same as the concentration of the previously inserted p-dopant, as pointed out by Dr. Kornblum, the concentration of free electrons reaches that of free holes and the semiconductor (theoretically) returns intrinsic.

However, it should be noted that:

1) As observed by Dr. Krauss, the presence of dopant atoms inside the semiconductor crystal lattice alters the lattice periodicity thus decreasing the mobility of charges; in other words, your counterdoped semiconductor will have a lower "intrinsic" charge mobility than the pristine, undoped intrinsic semiconductor;

2) As observed by Dr. Kornblum, counterdoping is based on the assumption that all your dopants are perfectly ionised, meaning that once inside the semiconductor each donor (acceptor) atom is able to release a free, mobile electron (hole). This is an approximation of what really happens but, depending on the semiconductor and on the dopants, it does not always hold true;

3) It is experimentally impossible to control EXACTLY the quantity of dopants penetrating the semiconductor, so it is virtually impossible to dope the semiconductor with the precise quantity of a certain dopant necessary to counterdope the previous doping.

As a consequence of all these reasons, if the semiconductor is already doped you can try to counterdope it to bring it back to an "intrinsic-like" state but practically you will never be able to obtain exactly the same mobility you can have with the same pristine, undoped semiconductor.