Strong negative transconductance in in-plane-gate transistors written by focused-ion-beam implantation

A strong negative transconductance is investigated in in-plane-gate transistors written by focused-ion-beam implantation in the two-dimensional electron gas in modulation-doped AlGaAs/GaAs heterostructures. This occurs in a configuration where two in-plane gates G1 and G2 used to control the current through a channel lying between them are biased with different voltages V _g1 and V_g2. When the voltage V_g2<0 is held constant, the current through the channel can be reduced to zero by increasing V_g1 beyond a critical value. In an earlier study this effect was attributed to velocity modulation. It is found, however, that in this regime V_g1 causes a very small current I_g2 to flow across gate G2. It is observed that changes in I_d are correlated to changes in I_g2 and thus conclude that I_g2 is responsible for the strong negative transconductance.