NEGATIVE TRANSCONDUCTANCE IN PARALLEL CONDUCTING SYSTEMS CONTROLLED BY DEVICE GEOMETRY AND MAGNETIC-FIELD

Four-terminal resistance measurements of two-dimensional electron gase s (2DEGs) connected in parallel in GaAs/AlGaAs quantum well (QW) struc tures have been performed. These measurements show regions of positive and negative transconductance, resulting in a peak in resistance clos e to the depletion of one of the 2DEGs. This feature is distinct from that related to resistance resonance and resonant tunnelling phenomena , which occur at matched 2DEG carrier densities. Simulations of the de vice have been performed which confirm that, at zero magnetic field, t his effect is caused by the current flow between the two layers via th e voltage probes. The size of this phenomenon is found to be sensitive to the resistance of the voltage probes as controlled by the device g eometry. The application of a perpendicular magnetic field introduces a second mechanism which leads to a large negative transconductance. T his effect is also seen close to the depletion of one of the 2DEGs and has also been modelled. We are thus able to produce large negative tr ansconductances by merely altering the geometry of the device or the a pplied magnetic field.