The Role of the Base Stack on the AC Performance of GaN Hot Electron Transistor

In this letter, we provide guidelines for the design of the base stack of a GaN-base HET suitable for high-frequency operation, through full band cellular Monte Carlo device simulations. The experimental curves are matched by our model and the transmission coefficient through the base is confirmed to be 0.2. We found that upon moderate reduction of the base thickness and collector barrier height, the transmission coefficient reaches values well above 0.5, hence allowing current amplification. The cutoff frequency of the scaled devices is then calculated from the short circuit current gain and is found to be larger than 100 GHz. Moreover, the cutoff frequency is seen to increase roughly proportionally to the transmission coefficient. This result, and the calculation of the base capacitance, suggests that the cutoff frequency can be well described by the intrinsic delay time C_b/gm.