| In this thesis an experimental study was made of the effect of an Al(,x)Ga(,1-x)As heterojunction cathode on the (80-100) GHz performance of a second harmonic GaAs Gunn diode. Substituting Al(,x)Ga(,1-x)As for the n('+) cathode end of a conventional Gunn device provides, under forward bias, an energy step in the conduction band which injects electrons into the active layer with nearly the kinetic energy necessary for transfer to the upper valley. Thus the usual "dead zone" with its positive resistance is eliminated, while at the same time a velocity overshoot effect is produced which raises the peak current of the device.;In addition to the microwave study a new calculation was made of heterojunction capacitance and thermionic current which includes the effects of subband quantization in the 2-dimensional electron gas. The calculation was developed into a simple experimental method for diagnosing energy discontinuity, doping in the Al(,x)Ga(,1-x)As, and other parameters of the heterojunction.;The efficiency, power, frequency of oscillation and electron average velocity were studied for devices with different active layer lengths of 2 (mu)m, 1.5 (mu)m, 1.0 (mu)m and 0.5 (mu)m and were compared to the results from conventional n('+)nn('+) Gunn devices with the same lengths. The 2 (mu)m heterojunction cathode devices achieved 1.5% conversion efficiency and 50 mW peak power at 85 GHz which were more than twice the values obtained from the conventional devices. Results for both types of devices were limited by n('+) substrate losses and it is estimated that improvement by a factor of 5 could be achieved by reduction of the substrate thickness. Results of average electron velocity measurement were in agreement with the predictions of Monte-Carlo calculations and also momentum and energy conservation equations utilizing average-energy dependent relaxation times obtained from steady state Monte-Carlo theory. |
