2.3 Normally-Off HEMT

The properties of GaN and AlN and their heterostructures have encouraged the research of AlGaN/GaN based transistors for various applications in the last decade. Consequently, outstanding results have been reported for the depletion mode (D-mode) high electron mobility transistors. However, for several applications enhancement mode (E-mode) devices are essential.

In analog electronics E-mode devices supersede the negative voltage supply and also assure a safe state in case of power loss. In digital electronics, they allow complementary FET-based logic [39]. Normally-off operation is also a requirement for automotive applications (e.g. hybrid vehicles) [33]. Despite the interest in E-mode operation, the excellent results as in D-mode devices remain to be demonstrated.