On- and off- state breakdown effects in composite channel and conventional InP-based HEMT's are studied by means of electrical measurements, and electroluminescence spectroscopy. We demonstrate that channel quantization increases off-state and on-state breakdown voltage. The temperature coefficient of the electron impact ionization rate in In0.53Ga0.47As has been studied. Differently from what happens in GaAs-based devices, carrier multiplication increases on increasing the temperature.