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Guoyong Duan THREE-DIMENSIONAL EFFECTS AND SURFACE BREAKDOWN ADDRESSING EFFICIENCY AND RELIABILITY PROBLEMS IN AVALANCHE BIPOLAR JUNCTION TRANSISTORS

Although avalanche switching has been known since the 1950s, a trustworthy one-dimensional physical interpretation of the practically interesting high-current mode ("secondary breakdown") in a Si avalanche transistor has appeared only within the last decade and thanks to numerical onedimensional and two-dimensional physics-based device modelling. A good fit with experimental waveforms has been achieved only for high-current, long-duration pulses (~100 A/7 ns), however, and modelling fails in the case of shorter pulses in a range that is of greater practical importance. One significant finding in this thesis is that reliable modelling of a Si avalanche transistor is in general impossible without taking account of three-dimensional effects. The task is a challenging one, as it is being put forward for the first time and state-of-the-art simulators are unable to model three-dimensional avalanche dynamics with an external circuit included (i.e. in “MixedMode”). Thus a smart approach was adopted which allowed the main features of the three-dimensional transient to be explained using a two-dimensional simulator and compared with the experimental data. The focus was on a trade-off of between high switching efficiency in an avalanche transistor (high-speed switching with a lower residual voltage as occurs at extremely high current densities) and device reliability as determined by local overheating during a single pulse, similarly resulting from high current density. This denotes the practical importance of the work performed here, as the current density is directly affected by three-dimensional dynamic processes. The second task performed in this thesis concerns the reliability of the GaAs avalanche transistors developed recently in the Electronics Laboratory and demonstrated of unique (superfast) switching and high-power-density sub-THz emission for mm-wave imaging and radars. Critically important for this new device is the limitation originating from premature breakdown at the surface of the GaAs p-n junction with a high density of surface states. Two of the results of this work are also fairly challenging: (i) the mechanism of "soft" surface breakdown intrinsic to all GaAs transistor mesas was interpreted in terms of the surface trapping of avalanchegenerated electrons as suggested here, and (ii) passivation of the surface with a chalcogenide glass was suggested, as this allows the premature surface breakdown to be suppressed completely, an effect that has proved to be caused by a large negative surface charge formed on the “U centres” intrinsic to a chalcogenide glass.