Tunnelling and Impact Ionization in Scaled Double Doped PHEMTs

We investigate the dominant breakdown mechanism in aggressively scaled pseudomorphic high electron mobility transistors (PHEMTs) with double delta-doping structure by Monte Carlo device simulations. Two breakdown mechanisms: channel impact ionization and thermionic tunnelling from the gate, are considered for two possible placements of the second delta doping layer either below the channel or between the gate and the first delta doping layer. Thermionic tunnelling starts at very low drain voltages but quickly saturates having a greater effect on those PHEMTs with the second doping layer placed above the original doping. A threshold for impact ionization occurs at larger drain voltages which should assure the reasonable operation voltage scale of double doped PHEMTs. Those double doped PHEMTs with the second delta doping layer placed below the channeldeteriorate faster with the reduction of the channel length due to impact ionization thanthosedeviceswith theseconddopinglayerabove the original doping.