  We explore the nature of the electroluminescence (EL) emission of purely
n-doped GaAs/AlGaAs resonant tunneling diodes (RTDs) and the EL evolution with
voltage. A singular feature of such a device is unveiled when the electrical
output current changes from high to low and the EL on-off ratio is enhanced by
2 orders of magnitude compared to the current on-off ratio. By combining the EL
and current properties, we are able to identify two independent impact
ionization channels associated with the coherent resonant tunneling current and
the incoherent valley current. We also perform the same investigation with an
associated series resistance, which induces a bistable electrical output in the
system. By simulating a resistance variation for the current-voltage and the
EL, we are able to tune the EL on-off ratio by up to 6 orders of magnitude. We
further observe that the EL on and off states can be either direct or inverted
compared to the tunneling current on and off states. This electroluminescence,
combined with the unique RTD properties such as the negative differential
resistance (NDR) and high frequency operation, enables the development of high
speed functional opto-electronic devices and optical switches.
