The Optimum Area for Millimetre-Wave Impatt Diodes

Abstract

By considering the loss of a practical millimetre-wave oscillator circuit, the variation of oscillator power wlith diode area for a constant junction temperature rise is examined, the results being scaled from measurements on a known device. It is concluded that the diode area for optimum oscillator power is a function of junction temperature rise and thus diodes for reZiable osciZlators will have different design criteria from devices giving the more spectacular results. Results are presented for silicon single and double-drift and gallium arsenide singZe drift IMPATT diodes over the 30-60 GHz range.The rf power available from IMPATT diodes is primarily limited by two constraints, the permissible junction temperature rise and the circuit loss which determines the efficiency with which the diode can be matched to the load.Most of the device results reported in the literature to date are for near burn-out temperatures at which the power and efficiency may be quite spectacular. For practical applications however the operating temperature must usually be reduced to increase the device life and the design criteria appropriate to the desired operating temperature will not be the same as those for near burn-out temperatures.In this paper we examine the results of changing the diode area and show that if a diode designed to give good power at near burn-out temperatures is operated at reduced current to increase its operating life the resulting power level may be considerably below that which could have been obtained had the diode area been reduced.Let us first consider the permissible operating junction temperature which in turn will determine the diode reliability. In the British Post Office we are concerned with developing a range of IMPATT diode oscillators covering frequencies above 30 GHz for the Millimetric Waveguide Trunk Communications System. One of the important criteria for this system in view of its very large traffic capacity is reliability and our preliminary calculations indicate that in order to meet the requirements of our operational departments we require a median-time-to-failure for the IMPATT oscillators in excess of one million hours.The diode reliability is governed by the contact metallisation used and the junction temperature at which the diode is operated, failure occurring when the contact metals interdiffuse allowing gold to penetrate and short out the junction. Early contact metallisations used were two level typically titanium-gold but more recently, particularly following the work of Hughes [11 a barrier metal is included which greatly reduces the rate of interdiffusion. Table I shows the estimated diode life for various contact metallisation systems based on both the Hughes results and our own Post Office Research Centre, Martlesham Heath, IPSWICH IP5 7RE England 256 measurements. The measured results and assumed activation energy are also given.TABLE I ESTIMATED DIODE LIFE From Table I we can see that to meet the reliability requirements discussed we can use diodes with a platinum barrie...