Theoretical Investigation on the Evolution of 2DEG for Design of E-Mode p-GaN HEMTs

In this paper, a theoretical study of p-GaN/Al x Ga 1-x N/GaN heterostructures is conducted systematically. In order to efficiently deplete the two-dimensional electron gas (2DEG) at the lower Al x Ga 1-x N/GaN interface and to achieve a reasonable threshold voltage (V th ), the dependence of 2DEG sheet densities on variables, such as p-GaN cap layer thickness (t cap ) and hole concentration (N cap ), Al x Ga 1-x N barrier layer thickness (t bar ) and Al component x, is investigated. Given t bar and x, the calculated results show that, when t cap or N cap is less than its minimum value, enhancement mode (E-Mode) cannot be realized and the device can only work in depletion-mode (D-Mode); when t cap and N cap are larger than the minimum value, the 2DEG under the gate may not completely depleted, and the device may still work in D-Mode; in order to obtain E-Mode p-GaN HEMT devices, the combination of t cap and N cap must be in the E-Mode region. Given t bar and x, the calculated results show that, when the t bar is thinner than the critical thickness corresponding to the specific x, the device will work in E-Mode; when the x is high enough and the t bar is thick enough so that the cap layer cannot completely deplete the 2DEG under the channel, the device will work in D-Mode. Only if the t cap , N cap , x and t bar are all set in E-Mode region can the E-Mode be realized. The obtained insight offers indications for the structure design of the normally-off p-GaN HEMTs.