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.