Open channel impact-ionization effects in InP-based HEMT's and their dependence on channel quantization and temperature

Summary form only given. Due to the multiplication of channel electrons in the low-bandgap InGaAs channel layer, breakdown effects can seriously limit the operating bias of InP-based HEMTs and their circuit applications. Improvements in both on-state and off-state breakdown voltage can be obtained by adopting a combination of a thin layer of InGaAs and InP in a composite channel structure and reducing the thickness of the InGaAs channel layer. In this way the effective bandgap of the channel can be increased due to channel quantization, and breakdown voltage is enhanced. Despite data on the effects of quantization on the off-state breakdown of metal-insulator doped-channel FET (MIDFETs) and on the on-state breakdown of InP-based HEMTs have been presented, no systematic study of multiplication effects in HEMT devices has been reported. Here we present a detailed study of both on- and off-state breakdown effects in composite channel and conventional InP-based HEMTs. We demonstrate that in both cases breakdown is governed by channel impact ionization so that channel quantization not only increases off-state and on-state breakdown voltage, but effectively reduces the gate current due to generated holes in open channel conditions. The temperature coefficient of the impact ionization rate has been measured; differently from what happens in GaAs-based FETs, a positive coefficient has been found, i.e. carrier multiplication increases on increasing the temperature, following the corresponding reduction in the energy gap. The devices used in this study are based on a MBE-grown heterostructure on semi-insulating InP.