Computer simulation of negative-differential-conductivity effects, including trapping

The computer model is based on a simple approximation of GaAs, and provides for the existence of trapping. The relation between voltage, field, and diode length takes into account the internal voltage, due to doping non-uniformities. The simulation is performed without restrictive assumptions on the field or carrier concentrations at the electrodes. Simulation results focus on parastable diodes, which are stable above threshold because the doping concentration
N
exceeds some critical value N
p
which depends on the electron diffusion
D
, velocity
v
, and differential mobility µ δ, in accordance with the small-signal analytical value
N_{p} = -\epsilonv^{2}/4De \micro \delta
. As
N
increases, the field E
U
in the uniform region of a parastable diode decreases, making parastability less sensitive to doping inhomogeneities. A positive doping gradient near the anode: 1) can make a diode unstable near threshold, and parastable at higher voltages; and 2) causes static negative differential resistance. The ac conductance of a para-stable diode become negative at sufficient modulating voltage levels in the microwave range. The diode can operate in the LSA mode, yet return to parastability when the modulating voltage is removed.