Over the course of the past few years, transistors based on gallium nitride (GaN) and related III-N materials have proven to be clear contenders for solid-state power amplifier applications in the commercial and military market spaces. In microwave technology nodes of lateral GaN high-electron-mobility transistor (HEMT) technology (with gate lengths greater than 0.25 microns), the material-driven electrical performance of GaN has been demonstrated with exceptional reliability in production-released processes.

While a power density of several W/mm can be easily achieved at S- and X-band with GaN HEMTs based on an AlGaN/GaN heterojunction, it generally becomes more difficult to obtain the same output power level at higher frequency nodes. This is primarily due to limitations on the operating voltage that can be used; as device dimensions scale shorter, internal electric fields increase and field management techniques, such as source-connected field plates, generally are not used because they introduce undesirable parasitic capacitance. To address these shortcomings, significant technical research and development has been carried out over the past few years focusing on extending GaN HEMT frequency performance into the millimeter wavelength (MMW) range of 30 – 300 GHz.

To prevent the onset of short-channel effects in short gate length devices, vertically-scaled barrier heterostructures have been investigated. In a number of examples, materials growth development of novel III-N materials has enabled heterojunctions with desirable two-dimensional electron gas transport properties. In this talk, we will review a number of recent developments in the community related to MMW GaN HEMT technology. In addition to reported literature, we will discuss NRL’s internal materials and MMW device development using molecular beam epitaxy. Novel HEMT heterostructures based on AlN, Sc_xAl_1-xN, and In_xAl_1-xN (metal- and N-polar) will be discussed.