Theoretical Study on Performance Limit of Cutoff Frequency in Nano-Scale InAs HEMTs Based on Quantum-Corrected Monte Carlo Method

We theoretically study the performance limits of current-gain cutoff frequency, ƒT, for the HEMTs with InAs or In0.70Ga0.30As middle layers in the multi-quantum-well (MQW) channels by means of the quantum-corrected Monte Carlo (MC) method. We calculate the distribution of the delay time along the channel, τ(x), and define the effective gate length, Lg, eff, as the corresponding length to (x). By extrapolating Lg, eff to Lg = 0nm, we estimate the lower limit of Lg, eff, Lg, eff(0). Then we estimate the performance limit of ƒT, ƒT(0), by extrapolating ƒT to Lg, eff(0). The estimated ƒT(0) are about 3.6 and 3.7THz for the HEMTs with InAs middle layers of 5 and 8nm in thickness, and about 3.0THz for the HEMT with In0.70Ga0.30As middle layer of 8nm in thickness, respectively. The higher ƒT(0) in the HEMTs with InAs middle layers are attributed to the increased average electron velocity, vd, in the channel. These results indicate the superior potential of the HEMTs using InAs in the channels. The HEMT with InAs middle layer of 8nm in thickness shows the highest ƒT on condition of the same Lg because of its highest vd. However, the increased total channel thickness results in the longer Lg, eff(0), which leads to the restriction of ƒT(0). Therefore, in order to increase ƒT(0), it is essential to make Lg, eff short in addition to making vd high. Our results strongly encourage in making an effort to develop the HEMTs that operate in the terahertz region.