The effect of a magnetic field in the singular enhancement of tunnelin
g near threshold has been investigated in specifically designed double
-barrier resonant tunneling structures. The experimental results prove
that an additional increase of tunnel current is due to impurity-assi
sted tunneling. It is shown that a donor bound state arises as a resul
t of dopant diffusion into the barrier. These states are identified as
strongly localized DX centers. (C) 1998 Elsevier Science Ltd. All rig
hts reserved.