Design and Fabrication of III-V Semiconductor Devices for Millimetre- and Submillimetre Wave Applications

Doctoral thesis, 1994

Devices (various types of diodes) have been fabricated in several different III-V semiconductors (AlxGa1-xAs, In1-xGaxAs, and InAs). The III- V compound semiconductors lend themselves well to high frequency applications due to the high electron mobility that can be obtained. The current density in a small area Schottky GaAs mixer diode can approach ~109 Am-2 when it is biased at its operating point. The mechanisms responsible for the noise are of great importance to the performance of the mixer. It has been demonstrated that features seen in the current versus voltage characteristic of mixer diodes correspond with features ("humps") in the noise characteristic and that these features can be linked to fabrication procedures. These features were either absent or were very weak in diodes fabricated by the author. Au/n-GaAs diodes showed very good noise characteristics. Fabrication procedures of relevance to mixer diode fabrication are discussed in detail. A simplified analysis of the maximum output power from a resonant tunnelling diode (RTD) is presented. The results of the analysis can be used to estimate the performance using only the measured DC characteristic of the RTD. The author has fabricated a number of small area single and double barrier varactor (SBV, DBV) devices in three different material systems and characterized them by measuring the reflection coefficient, .GAMMA. (=S11), versus frequency with a network analyser. The equivalent circuit elements were then extracted. Multiplier simulations have been performed which show the large potential of SBV multipliers. The measured characteristics and design considerations for SBVs are discussed. A fabrication procedure for a state of the art SBV is proposed.