Analysis of failure mechanisms in velocity-matched distributed photodetectors

The thermal-runaway process in long-wavelength velocity-matched distributed photodetectors (VM DP) with metal-semiconductor-metal photodiodes has been investigated. A three-dimensional numerical electrothermal model has been developed which takes into account the nonlinear thermal properties of the substrate and the nonuniform temperature rise due to self heating. The mode l shows that, owing to its distributed nature, the photodetector is able to operate at high optical-power level before catastrophic failure occurs. Wh en this happens, a highly localised hot spot appears within the device and the characteristic exhibits a typical 'current crush point', where the curr ent rapidly increases with increasing bias voltage. Examples are discussed to highlight the thermal behaviour of the distributed detector and to compa re the model with experimental data.