Abstract
The reasons for frequency variation with temperature in X-band Gunn diodes have been examined theoretically and experimentally. Frequency changes arise from the sensitivity of the diode admittance to r. f. voltage, to changes in the velocity field characteristic, the dielectric constant and the expansion coefficient of GaAs and to changes in the nature of the contacts to the diode. The diode cannot be considered in isolation from the circuit because of the heavy coupling between the two. A theory is presented which enables the df/dT to be calculated in terms of the injection locking Q and the loading on the diode from any model of diode operation. The way in which the typical X-band diode operates is largely unknown. The various possibilities are discussed and the values of the parameter Q/f df/dT Gd/God approprite to each mode are calculated. It is shown that a model of the hybrid mode, where the degree of domain build-up varies from slice to slice, can explain the experimentally observed df/dT and in particular the zero values which have been found. Investigations on the temperature variation of the sub-threshold properties of AgSn contacted and n+ GaAs contacted diodes have revealed departures from the ideal which are attributed to the contact regions, but no identifiable effect from this source has been found on the room temperature measurements of From the circuit work a model for some S4 outline packages in reduced height waveguide 16 has been obtained and the existence of a second cavity resonance at low frequencies, between the diode capacitance and the evanescent inductance of the circuit, has been demonstrated. Information oh the harmonic impedances over which the Gunn diode can work has been obtained from measurements on a scaled model of circuit and package and the temperature variation of the dielectric constant of GaAs has been measured.