Microwave circuit characteristics of bulk GaAs oscillators

The microwave circuit characteristics of bulk GaAs transit-time mode and limited space-charge accumulation (LSA) mode oscillators have been evaluated experimentally and theoretically. Experimental measurements were performed with a waveguide-coaxial microwave circuit having two experimental degrees of freedom in which the circuit radiation impedance at the device contacts was evaluated by a dyadic Green's function method. Experiments conducted in three rectangular waveguide circuits at fixed bias voltage have established that the LSA mode frequency tuning range is determined by the magnitude and variation of the circuit series inductive reactance XLrelative to the device low-field resistance R0. At a bias voltage which is twice threshold the tuning range is given by1/2.4≤ XL/R₀≤ 2.4. No fixed, linear equivalent circuit characterizes the LSA mode. Analysis and experimental results indicate that the device impedance of small-signal transit-time mode oscillators changes from a passive parallel RC impedance well below threshold to an impedance just above threshold which can be approximated by a series RLC circuit. The series L and C decrease linearly with transit-time mode harmonic order number.