Electrostatic characterization and ballistic limit evaluation of high electron mobility transistors (HEMTS) using quantum mechanical approach

Abstract

The High Electron Mobility Transistor or the HEMT is one of the most promising candidates for next generation high speed, low-power logic applications. In the highly scaled regime of operation, characterization of HEMT requires full incorporation of quantum mechanical (QM) effects. In this work the electrostatic and transport characteristics are analyzed for the generic, delta doped and spacer layered types of two single channels, such as In0.53Ga0.47As and In0.70Ga0.30As HEMTs, and two multi-quantum-well (MQW) channels, namely In0.70Ga0.30As and InAs MQW HEMTs. The study shows that though delta doping increases the carrier density, on-state current and channel conductance of single channel HEMTs, it has negligible influence on the performance of MQW HEMTs. On the other hand, the addition of spacer layer decreases carrier density, drive current and channel conductance for all variants of the device. Evaluation of transport issues however shows that spacer layer can increase the mobility of HEMTs both in the long and short channel limits by reducing scattering. This work also presents a novel extraction method of the voltage at which parallel conduction initiates in highly scaled HEMTs. This entirely quantum mechanical technique defines two deterministic parameters VEmin and Qratio for each device. Comparison of these parameters indicates that stronger the quantum mechanical confinement, higher the voltage for the onset of parallel conduction. For this reason in InAs MQW HEMT, which has the deepest quantum well, parallel conduction occurs at above 0.25V whereas in the In0.53Ga0.47As HEMT it occurs at above 0.10V. Also with the onset of parallel conduction, the channel conductance in MQW HEMT can decrease by more than a factor of 1.5 whereas for single channel HEMT the conductance remains almost constant. However the MQW structure is found to reduce mobility of the HEMT in the short gate length limit. Nevertheless the mobility in all types of HEMTs is ballistic for up to 100 nm gate length, whereas in highly scaled Si devices the value is around 20 nm. The study shows that the strong confinement of MQW HEMTs, particularly of InAs MQW HEMT, causes the charge density, on-state current and channel conductance to be higher than other HEMTs at least by a factor of 1.20. Finally analyses of strain effects show that charge density, current and channel conductance are overestimated if strain is neglected in highly scaled HEMTs. Strain however can increase the mobility of the device in the short gate length limit, which is in accordance with the technology of strain enhanced channel materials like SiGe, Ge, SiN and GaAs.