Carrier confinement and overshoot velocities in GaAs and InP based HEMTs

High speed HEMT devices operate in the electron overshoot velocity regime. Extrinsic electron transit velocities above 2×10⁷ cm/s as defined by the f_T*L_g- product have been observed. To obtain optimum RF conditions under these circumstances, the aspect ratio L_g/t plays an important role as for example in determining the f_max/f_T-ratio. The aspect ratio is a geometrical figure of merit, which fails however to address current injection into the buffer layer, which is thought to be substantially reduced by using a deep QW-channel. The authors address the question whether in QW-channel FET devices the aspect ratio alone is still the dominating figure of merit or whether this has to be modified, taking also into account various degrees of carrier confinement. Here, the confinement of hot carriers in the drift region is considered through changes in the small signal parameters especially the output conductance G_ds