This work reports on high-performance enhancement-mode (E-mode) AlN/GaN Schottky gate HEMT (AlN SGHEMT) for millimeter-wave applications. Utilizing an ultrathin 4-nm barrier of AlN tuned by the mechanical stress from in-situ SiN, and self-terminated etching technique, to form the E-mode AlN SGHEMT. As a result, the proposed device demonstrated positive threshold voltage ( V-Th) of 0.53 V, high maximum drain current density ( Id- max) of 1.19 A/mm, and maximum transconductance ( G(m- max)) of similar to 0.61 S/mm. Load-pull test was carried out at 30 GHz, which illustrated the ability of device to deliver a saturated output power density ( P-sat) of 5.59 W/mm at a drain-source voltage ( V-ds) of 25 V. The excellent results highlight a new approach to obtain mmW RF E-mode GaN HEMTs at Ka-band.

Improved radio-frequency (RF) power performance of InAlN/GaN high electron mobility transistor (HEMT) is achieved by optimizing the rapid thermal annealing (RTA) process for high-performance low-voltage terminal applications. By optimizing the RTA temperature and time, the optimal annealing condition is found to enable low parasitic resistance and thus a high-performance device. Besides, compared with the non-optimized RTA HEMT, the optimized one demonstrates smoother ohmic metal surface morphology and better heterojunction quality including the less degraded heterojunction sheet resistance and clearer heterojunction interfaces as well as negligible material out-diffusion from the barrier to the channel and buffer. Benefiting from the lowered parasitic resistance, improved maximum output current density of 2279 mA⋅mm −1 and higher peak extrinsic transconductance of 526 mS⋅mm −1 are obtained for the optimized RTA HEMT. In addition, due to the superior heterojunction quality, the optimized HEMT shows reduced off-state leakage current of 7 × 10 −3 mA⋅mm −1 and suppressed current collapse of only 4%, compared with those of 1 × 10 −1 mA⋅mm −1 and 15% for the non-optimized one. At 8 GHz and V DS of 6 V, a significantly improved power-added efficiency of 62% and output power density of 0.71 W⋅mm −1 are achieved for the optimized HEMT, as the result of the improvement in output current, knee voltage, off-state leakage current, and current collapse, which reveals the tremendous advantage of the optimized RTA HEMT in high-performance low-voltage terminal applications.

>GaN-based HEMTs have significant advantages, making them a prime choice for high-frequency, high-power switching [1]. However, the inherent poor linearity of GaN HEMT has yet to be solved, which can be reflected by the nonlinearity of the transconductance (Gm) profile [2]. This makes it hard to achieve high data transmission efficiency and less signal distortion in the wireless communication system [3].

In this work, a high linearity AlGaN/GaN HEMT integrated dual threshold coupling (DT) technology and Schottky–Ohmic drain (SOD) were fabricated and analyzed. Since the architecture of DT synthesized planar- and recess-HEMT periodically along the gate width, SOD alleviated the peak electric field (E-field) around the gate region and the peak transconductance (Gm-max) of 248 mS/mm with the associated transconductance plateau of ∼4.0 V at Vds = 28 V was obtained, which is evidently flatter than that of HEMT without the SOD structure. Attributed to the improved Gm linearity at high E-field, the DT-SOD HEMT exhibits the predicted linearity figure of merit of 5–13 dB when biased at class AB operation, which is ∼10 dB higher than that of DT-HEMT. Moreover, the fabricated device yields a nearly constant fT/fmax of 47/118 GHz over a wide gate voltage, and load-pull measurements at 30 GHz reveal that these devices deliver output power density (Pout) of 7.8 W/mm with the associated 1-dB compression point (P1dB) of 28.5 dBm at Vds = 28 V. The experimental results indicate that the employment of DT technology and SOD structure is an attractive approach to enhance the linearity at high E-field for millimeter wave devices.

We demonstrate a novel Si-rich SiN bilayer passivation technology for AlGaN/GaN high electron mobility transistors（HEMTs） with thin-barrier to minimize surface leakage current to enhance the breakdown voltage. The bilayer SiN with 20-nm Si-rich SiN and 100-nm Si 3 N 4 was deposited by plasma-enhanced chemical vapor deposition（PECVD） after removing 20-nm SiO 2 pre-deposition layer. Compared to traditional Si 3 N 4 passivation for thin-barrier AlGaN/GaN HEMTs, Si-rich SiN bilayer passivation can suppress the current collapse ratio from 18.54% to 8.40%. However, Si-rich bilayer passivation leads to a severer surface leakage current, so that it has a low breakdown voltage. The 20-nm SiO 2 pre-deposition layer can protect the surface of HEMTs in fabrication process and decrease Ga–O bonds, resulting in a lower surface leakage current. In contrast to passivating Si-rich SiN directly, devices with the novel Si-rich SiN bilayer passivation increase the breakdown voltage from 29 V to 85 V. Radio frequency（RF） small-signal characteristics show that HEMTs with the novel bilayer SiN passivation leads to f T /f max of 68 GHz/102 GHz. At 30 GHz and VDS= 20 V, devices achieve a maximum P out of 5.2 W/mm and a peak power-added efficiency（PAE） of 42.2%. These results indicate that HEMTs with the novel bilayer SiN passivation can have potential applications in the millimeter-wave range.

In this work, high linearity AlGaN/GaN HEMTs accomplished by implementing the dual-threshold coupling technique (DT HEMTs) have been studied. It was found that the transconductance (Gm) profile for DT HEMTs at high operation voltage was sever compressed induced by the high electric field (E-field), and the phenomenon could be optimized by modulating the α, which was defined as the duty ratio of planar- and recess-elements in a period width. Consequently, the fabricated composite device with α = 0.7 yielded the gate voltage swing (GVS-Gm) of 4.5 V at Vds = 6 V, while GVS-Gm of 3.97 V at Vds = 25 V was obtained by DT HEMTs whose α is 0.5. Meanwhile, For the DT HEMTs with α = 0.5, the predicted output third-order intercept (OIP3) of 39 dBm among a wide gate voltage range at Vds = 25 V was achieved, and the large signal performance at 8 GHz delivered Pout of 4.7 W/mm with 1.9 dB improvement in gain compression (Gcom), and 2 dB enhancement in 1-dB compression point (P1-dB), respectively, at Vds = 25V. The suppressed Gcom and enhanced P1-dB indicates that the DT HEMTs with α = 0.5 is capable for linearity requirements at high operation voltage.