Transit-time negative conductance in GaAs bulk-effect diodes

It is well known that GaAs bulk-effect devices generate RF power by velocity-modulating the charge carriers. We have found through a small-signal analysis and computer simulations that RF power could also be generated by density modulation of the carriers if the diode is subcritically doped ( cm^-2) and carrier injection at the cathode is limited to appropriate values. This second power-generating mechanism is transit-time dependent and is optimum when the transit angle is approximately 3π/2. Under optimum conditions, when the two power-generating mechanisms are combined, the device efficiency exceeds what is predicted by the static velocity-electric field characteristic. Computer results indicate that for sinusoidal operation a peak-to-valley current ratio of 3 to 1 is achievable for an intrinsic peak-to-valley velocity ratio of 2.5 to 1. Experimental and computed results agree qualitatively.