  In amorphous materials, acceptor and donor impurities rarely dope the system
(shift the Fermi level). We find out why in a-Si:H. We report simulations on B
and P doping of a-Si:H and a-Si. We analyze the Electronic Density of States
(EDOS) with concentrations ranging from 1.6% to 12.5% of B or P in a-Si. The
results indicate that tetrahedral B and P are effective doping configurations
in a-Si, but high impurity concentrations introduce defect states. Clustered B
or P also introduced mid-gap states. For a-Si:H, we report that both B(3,1) and
P(3,1) (B or P atom bonded with three Si atoms and one H atom) are effective
doping configurations. We investigate H passivation in both cases. There exists
a "hydrogen poison range" for which H can modify the dopant configuration and
suppress doping. For B doping, nearby H prefers to stay at the bond-center of
Si-Si, leaves B four-fold and neutralizes the doping configuration; for P
doping, nearby H spoils the doping by making tetrahedral P three-fold.
