Electrical models for detrimental effects in metamorphic HEMTs

Metamorphic HEMT, MHEMT (InP based layers on GaAs substrate), which are devices still in development, show very promising performances for high bit rate optical links, but still present limitations. In this paper, we propose different new electrical models of detrimental effects affecting the Metamorphic HEMT (MHEMT) functioning. First, we measured devices, particularly in terms of DC characteristics, gate and drain lag effects and on-state and off-state breakdown voltages. Then, we put into evidence the impact ionization mechanism developed in the active layer, and particularly its consequence on the drain current, the gate current and the on-state breakdown voltage. We propose electrical models for the impact ionization, the kink effect, the gate current bell shape model representative of the impact ionization mechanism. Another one permits to take into account the increase of drain current due to the MHEMT burnout (specification of the safe operating area of the MHEMT). All these models have been developed in order to be inserted into a cascade amplifier simulated with ADS software