  Exact $T$-matrix for the delta-function short-range perturbation of the
(3+1)-Dirac equation has been derived. Separability of the potential in the
angular momentum representation is used. A characteristic equation for the
$T$-matric poles determining the bound and resonance states has been obtained.
The calculated $T$-matrix determines transport properties of the narrow-gap and
zero-gap semiconductors.
