Low damage atomic layer etching (ALE) gate recess is developed for fabrication of millimeter-wave AlGaN/GaN high-electron-mobility transistors (HEMTs). Plasma ion induced bombardments to the AlGaN barrier is effectively suppressed by the ALE recess, contributing to a well-controlled recessed surface morphology. The suppressed lattice damage to AlGaN/GaN heterostructure is also reflected by a significantly reduced gate leakage as well as an invisible threshold voltage shift associated with damage induced traps. With a 0.15-μm T-gate fabrication technology, a high power-gain cutoff frequency fMAX of 205 GHz has been achieved. The ALE-recessed AlGaN/GaN HEMTs exhibits a record high power-added-efficiency (PAE) of 43.6% at 40 GHz in a continuous-wave mode. The associated gain and output power density are also remarkably improved compared with controlled HEMTs with conventional gate recess process.

A Ka-band AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) with SiN gate dielectric grown by plasma-enhanced atomic layer deposition (PEALD) is demonstrated. The gate reverse leakage current in a controlled HEMT is reduced by about three orders of magnitude by the PEALD-SiN, contributing to an improved breakdown voltage of 92.5 V (source-drain separation of 2.0 μm) in the fabricated MIS-HEMTs. The MIS-HEMTs also feature small threshold voltage hysteresis in dc transfer and capacitance-voltage characterizations, suggesting a good PEALD-SiN/AlGaN interface. The fabricated MIS-HEMTs deliver a high output power density of 7.16 W/mm and a peak power-added efficiency of 60.3%, respectively, at VDS = 30 V and VDS = 10 V in pulse mode at 28 GHz. PEALD-SiN could be a compelling gate dielectric for fabrication of high-power and high-efficiency MIS-HEMTs.

Plasma enhanced atomic layer deposition (PEALD) SiN gate dielectric is performed on AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT). The fabricated MIS-HEMTs with 5 nm SiN gate dielectric layer exhibits a tiny threshold voltage hysteresis (~ 0.04 V), sufficiently decreased leakage current of ~10.9 A/mm at VGS= -60 V and good threshold voltage (VTH) stability even measured at 200 °C. The increased mobility of 2-D electron gas (2-DEG) and suppressed current collapse are demonstrated, thereby producing a power density of 7.05 W/mm with an associated 34.0 % power-added efficiency (PAE) and a peak PAE of 51.4 % in a continuous-wave mode at 30 GHz.

In this work, characterizations are performed to investigate the stability of an mm-wave GaN metal-insulator-semiconductor HEMT. Bias temperature instability (BTI) experiment conducted at various voltages and temperatures show a highly stable threshold voltage ( ${V}_{\text {TH}}$ ) with both positive BTI (PBTI) and negative BTI (NBTI) attributing to the high quality of SiN gate dielectric. In addition, ${V}_{\text {TH}}$ shift ( ${\Delta } {V}_{\text {TH}}$ ) under reverse-bias step-stress is further monitored, achieving small ${\Delta } {V}_{\text {TH}} < {0.1}$ V and a recoverable behavior below ${V}_{\text {D}}$ , stress = 60 V. After 10 ⁴ s of stress at 175°C, under the OFF-state high drain-bias, ${\Delta } {V}_{\text {TH}}$ is 0.15 V, and a larger ${V}_{\text {TH}}$ shift (0.23 V) is observed under On-state drain-bias of 28 V due to the high electric-field. According to the time-dependent dielectric breakdown prediction, +3.0 V gate voltage stress at a failure rate of 0.01 % can be extrapolated for mm-wave MIS-HEMT. These findings guarantee a good stability for the device utilized in power amplifier applications.

In this work, a pulse-mode N2 plasma surface treatment process was proposed as a means of reducing plasma damage and improving the GaN/GaOx ratio on the surface before SiNx deposition, which further contributes to an enhanced density of 2DEG and a reduced sheet resistance. With the pulsed N2 plasma surface treatment combined with subsequent SiNx passivation, the fabricated GaN HEMTs exhibit negligible current collapse and suppressed leakage current. The improved behavior is attributed to the fact that the pulsed N2 plasma is capable of nitriding the surface and removing carbon contaminants as identified through x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy. Compared to the traditional continuous-wave-mode N2 plasma, the pulsed N2 plasma pre-treatment effectively prevents continuous collisions of the plasma during acceleration, thereby significantly reducing plasma damage. This work offers valuable insights for surface treatment processes in micro- and nanofabrication.