Heterojunction high speed switching diodes

The Ge-GaAs n-n heterojunction has interesting rectifying properties due to the interface barrier between the two materials. The conduction mechanism of the junction will be described. The fact that the current is carried mainly by the majority carrier allows the diode to switch at high speed without suffering from carrier storage. The transient behavior of the n-n heterojunction shows a switching time less than 10^-9second and no storage time is observed. The switching time is believed to be limited by the circuit impedance involved. The capacitances of these diodes have excellent(V_{D}-V)^{\frac-{1}{2}}dependence indicating the abrupt nature of the junctions. No frequency dependence of the capacitance was found from 50 Kc to 10 Mc. In the forward bias region, no diffusion capacitance was observed. The barrier height was found to be dependent on the orientation of the GaAs surface upon which the Ge was grown. Determination of the barrier height, by current-voltage characteristics, temperature dependence of the reverse current as well as the capacitance all show that the VD's in [111] Ga, [111] As and [110] surfaces decrease in this order which cannot be explained by simple surface dipole moments due to GaAs ionic bond. Measurements on n-p Ge-GaAs heterojunction give apparent barrier heights VD[111]Ga < V_{D}[111]As < V_{D}[110]. Measurements in both n-n and n-p heterojunctions suggest consistently that the Ge-GaAs conduction band discontinuity\Delta E_{c}for the orientations [111] Ga, [111] As and [110] decreases in this order.