Variation of transport properties along the channel of a high electron mobility transistor: a quantum influence

In the theoretical modelling of a high electron mobility transistor (HEMT), it is inherently assumed that there is little variation in quantum confinement along the channel from source to drain and therefore the transport properties are independent of the position and only dependent on the electric field along the channel. In this study, the scattering rates for bulk polar optical phonons, bulk acoustic phonons (through deformation potential) and ionized impurity scattering have been studied theoretically based on Fermi's Golden Rule as a function of position along the channel for a As/GaAs HEMT. For operation in the sub-ohmic regime, it is observed that all three mechanisms exhibit a wide variation in the rates from source to drain due to a varying degree of quantum confinement. Some of the intersubband scattering processes involving a lower and higher subband, which are present at the drain end, are absent at the source end due to the variation of energy eigenvalue differences. A quantitative variation of 10 - 50% was observed in the scattering rates at room temperature such as 1 to 1 adsorption and emission of polar optical phonons. It is important to take into account such variation in scattering rates in a complete device model.