Ohmic contact to AlGaN/GaN heterostructures employing hole-array recess of barrier layer is conducted in this paper, in which an Ohmic contact resistance of 0.2 Ωmm with an annealing temperature of 800 °C with 15 nm deep holes recessed on the AlGaN layer before annealing is obtained. The annealing temperature and metal morphology post annealing is compared between Ohmic contacts with and without hole array recess. The annealing temperature required for the Ohmic contact with hole recess to achieve minimum ohmic contact resistance is 40 °C lower when compared to an ordinary Ohmic contact without hole array recess. Additionally metal morphology is significantly improved, such as the reduction of irregular metal particles and metal bumps forming in the annealing process, smaller alloying grains, perfectly straight metal edge and no educts existing at metal sidewalls are realized. The results demonstrated in this paper are beneficial to the performance, yield and reliability of relevant devices and circuits.

In order to suppress the gate leakage current of a GaN high electron mobility transistor (GaN HEMT), a GaN metal-oxide-semiconductor high electron mobility transistor (MOSHEMT) is proposed, in which a metal-oxide-semiconductor gate with high-dielectric-constant HfO 2 as an insulating dielectric is employed to replace the traditional GaN HEMT Schottky gate. A 0.5 μm gate length GaN MOSHEMT was fabricated based on the proposed structure, the Al Ga N 0.28 0.72 barrier layer is partially etched to produce a higher transconductance without deteriorating the transport characteristics of the two-dimensional electron gas in the channel, the gate dielectric is HfO 2 deposited by atomic layer deposition. Current-voltage characteristics and radio frequency characteristics are obtained after device preparation, the maximum current density of the device is 900 mA mm −1 , the source-drain breakdown voltage is 75 V, gate current is significantly suppressed and the forward gate voltage swing range is about ten times higher than traditional GaN HEMTs, the GaN MOSHEMT also demonstrates radio frequency characteristics comparable to traditional GaN HEMTs with the same gate length.

To improve the power-added efficiency (PAE) of the gallium nitride (GaN) high-electron mobility transistor (HEMT) in radio frequency applications, this paper studies the relationship between the nonlinearity of the gate capacitance and the PAE of the GaN HEMTs. The theoretical analysis and simulation results demonstrate that the nonlinearity of the gate capacitance modulates the signal phase at the GaN HEMT input and increases the average drain current, leading to increased power consumption and reduced PAE. Then, an efficiency-enhancement topology for GaN HEMTs that employs the waveformmodulation effect of Schottky diodes to reduce power consumption and improve efficiency is presented. The efficiency-enhancement topology for a 4 × 100-μm GaN HEMT with waveform-modulation diodes is then fabricated. Results of load-pull test demonstrate that the novel topology can increase the PAE of the 4 × 100-μm GaN HEMT by more than 5% at 8 GHz. The novel efficiency-enhancement topology for GaN HEMTs proposed in this paper will be suitable for applications that demand high-efficiency GaN HEMTs or circuits.