No abstract

No abstract

No 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...

A two stage ITAPATT diode amplifier with an output power of 1OW has been developed for use in a digital radio relay system operating in the 10.7 to 11.7 GHz freauency band. The amplifier consists of a 6 dB gain out-put stage, combining the outputs of two 4W high efficiency IIVEPATT diodes, preceeded by a similar stage with 10 dB gain using two 1.5-2W TPb,TPATT diodes. The combining circuit is based on Rucker's [1 symmetrical multidevice combining circuit and is realised in coaxial troughline. The practical details of the combining circuit are discussed in a subsequent secti-on which also describes the RF performance of the circuit. DEVELOPMZENT OF HIGH EFFICIENCY INEPATT DIODESIn order to realise a 1OW output power with the minimuim number of active devices, it has been necessary to develop high efficiency IMIPATT diodes. Single diodes give output powers in the 2-3W range with 20% efficiency. However, if two or three diodes are bonded in parallel in one package, output powers of 4-5W can be obtained reproducibly. It is then a simple matter to use combining ci.rcuits to reach the desired 10W output power.The diodes are fabricated at the Post Office Research Centre[2] from GaAs material with a high-low dopitg profile,nd a drift region doping density of 6 + 0.5 x 10 atoms cm .The width of the high-doped region is adjusted chemically to yield a DC breakdown voltage of 28-32v. Such a tight specification is necessary to avoid discontinuous changes of amplifier gain with RF drive associated with the premature collection mode in the upper part of the 10.7 -11.7 GHz frequency band.Devices destined for the output stage of the amplifier consist of arrays of two 160m or three 140 eu-m diameter diodes capable of an output in the 4.5-5W range with a DC to RF conversion efficiency of at least 20%. The diodes are symmetrically interconnected by a perforated gold tape which is thermocompression bonded to the toD contacts of the diode and the metallised surface of a low profile ceramic ring package.The lower power requirements of the driver stage allow devices consisting of one 160 or 140 ,czm diameter diode to be used.