  We find that, under appropriate conditions, electrons can pass a barrier
etched across a two dimensional electron gas (2DEG) by field emission from the
GaAs/AlGaAs heterojunction into a second, low-density 2DEG formed deep in the
substrate. The current-voltage characteristics exhibit a rapid increase in the
current at the field emission threshold and intrinsic bistability above this
threshold, consistent with a heating instability occurring in the second 2DEG.
These results may explain similar behaviour recently seen in a number of
front-gated devices by several groups.
