By analogy, the incomplete covalent bond in the vicinity of an acceptor impurity in silicon at low temperatures is treated as a positive charge orbiting the negatively charged impurity atom. Its orbit radius is
and its ionisatioon energy becomes
It so happens that in silicon, so that acceptor and donor ionisation energies are approximately equal, , and we are again justified in assuming that all acceptors impurities in silicon are ionised at room temperature; po = NA- = NA.
Note, however, for a III - V semiconductor such as GaAs,
so 
.
Also,
,so 
;specifically,
, and 
.
