Normally-Off AlGaN/GaN/AlGaN Double Heterostructure FETs With a Thick Undoped GaN Gate Layer

In this letter, we report on polarization charge engineering enabling normally-off operation for a double-heterostructure Al_0.26Ga_0.74N/GaN/Al_0.07Ga_0.93N-based field effect transistors (DHFETs) using a 35-nm-thick undoped GaN layer underneath the gate metallization. The combined effect of the negative polarization charge induced by the AlGaN back barrier and the undoped GaN gate layer ensures the total depletion of the channel, and provides a positive threshold voltage. The fabricated DHFET exhibits normally-off operation with a threshold voltage of 1.2 V, a maximum drain current density of 370 mA/mm, and a high ON/OFF current ratio of 10⁷, at a gate bias of 7 V. A transistor with gate-drain distance of 6 μm demonstrates 300 V off-state breakdown voltage.