A high-linearity and high-gain AlGaN/GaN HEMTs with a 100-nm gate were demonstrated. The device employs transitional recessed gate (TRG) along the gate width for millimeter wave power application. The gradually changing gate recess depth was created using transitional dosed photoetching. Accurate etching ensured that the FET-elements have a continued V _ts offset in the local equivalent threshold voltage (V _th ) in different areas. The device exhibits a high I _d,max of 1.12 A/mm and a high peak extrinsic gm of 374 mS/mm with an improved gate swing > 2.6 V, much higher than that of Fin-HEMT. Excellent RF performance was shown, including f _T /f _max = 61.8/148.8 GHz, G _as /G _linear = 9.98/ 12dB at 30 GHz. To the best of our knowledge, this is the best associated gain and linearity performance reported to date for AlGaN/GaN HEMTs. This letter has great potential for high gain and linearity millimeter wave power applications, which are needed for future communication systems.

In this work, low voltage RF power capability on AlGaN/GaN and InAlN/GaN HEMTs is analyzed from the perspective of DC and pulse characteristics, for terminal applications whose operating voltage is usually in the range of 3 to 15 V. Device fabrication is performed on mature AlGaN/GaN heterojunction as well as strongly polarized InAlN/GaN heterojunction, to make a comparison of low voltage RF power capability between two devices. Although it suffers from relatively severe RF dispersion, InAlN/GaN HEMT delivers higher output power density (P _out ) than its opponent, benefiting from the lower parasitic resistance and knee voltage as well as higher output current density. At 8 GHz, P _out of 1.62 W/mm and 1.10 W/mm are achieved for InAlN/GaN and AlGaN/GaN HEMT, respectively, both of which are biased at class AB operation and V _ds of 9 V. However, the tremendous degradation of power added efficiency (PAE) occurs as the higher drain voltage is applied on InAlN/GaN HEMT, as the result of the severe gate leakage. What is more, a higher PAE is more necessary than P _out for terminal applications. Either InAlN/GaN HEMT or AlGaN/GaN HEMT has its own specific voltage range to deliver higher PAE. Concretely, InAlN/GaN HEMT is more suitable to applications with operating voltage not exceeding 6 V, and AlGaN/GaN HEMT is preferred for ones with relatively higher voltage, accompanied by the decent PAE as high as 62% to 66% and moderate P _out to meet the demand of various low voltage terminal applications.

In this paper, AlGaN/GaN Fin-HEMTs with different fin configurations are theoretically and experimentally investigated. A simple physical-based threshold voltage model for AlGaN/GaN Fin-HEMTs is built for qualitative analysis. It is shown that the threshold voltage of AlGaN/GaN Fin-HEMTs depends on the width and height of the fin structure. The peak value and linearity of transconductance and current gain cutoff frequency are improved by reducing the fin length. It is also found that the linearity of transconductance is sensitive to the fin height, which can be attributed to the capacitance control from sidewalls.

In this letter, we explore the impact of configuration parameters for Fin-like high-electron-mobility transistors (HEMTs) formed by partially etching barrier under the gate on improving transconductance (Gm) and cutoff frequency ( f _T ) linearity. It is found that the Gm profile for Fin-like HEMTs can be optimized by choosing appropriate device parameters, including the etching depth ( H _R ) and width ( W _R ) of recess region, as well as the duty ratio ( α) of the planar elements in a periodic unit along the gate width. In general, not only does W _R affect the gate voltage swing (GVS) but also H _R and α have an important role in Gm profile flatness. In addition, the fabricated Fin-like HEMTs shows a GVS of the transconductance plateau larger than 5.6 V and a constant f _T /f _max of 45 GHz/65 GHz over a wide gate voltage range. Furthermore, the proposed architecture also features an exceptional linearity performance at 8 GHz with an output third-order intercept point (OIP3) of 38.5 dBm, whereas that of the planar HEMT is 31 dBm. The device demonstrated in this article has great potential to be a new paradigm for future wireless communication systems where high linearity is essential.

High-frequency enhancement-mode (E-mode) AlGaN/GaN high-electron-mobility transistors (HEMTs) with plasma oxidation technology (POT) were fabricated through plasma-enhanced chemical vapor deposition. POT enables the formation of a thin oxide layer in the gate region, which decreases the gate leakage by at least two orders of magnitude compared with conventional recessed gate HEMTs. Ultra-low leakage was achieved in the fabricated device, with I_off = 9.5 × 10⁻⁷mA/mm and a high ON/OFF ratio of over 10⁹. Good suppression of current collapse was obtained after the application of POT in the access region. The enhancement-mode AlGaN/GaN HEMTs with POT showed an outstanding performance, with a V_th of 0.4 V, a maximum drain current of 965 mA/mm, and an f_max of 272 GHz.

The submicrometer gate HEMT is fabricated with an ultrathin-barrier (UTB) AlGaN/gallium nitride (GaN) combined with in situ SiN passivation. The sheet resistance of the UTB Al _0.2 Ga _0.8 N (4 nm)/GaN heterostructure is effectively reduced by the SiN _x passivation layer grown by metal organic chemical vapor deposition (MOCVD), from 6500 to $312~ \Omega $ /⬜. With the 20-nm stress-engineered in situ SiN, the device not only provides a large output current of 1.05 A/mm but also demonstrates promising potential on the RF applications, which gives AlGaN material two records high cutoff frequency ${f} _{\text {T}}/{f} _{\text {max}}$ of 157 GHz/334 GHz for 100-nm gated device and 211 GHz/379 GHz for 70-nm gated device. During the continuous wave (CW) power measurement at 30 GHz, the 70-nm devices exhibit a large output power of 4.6 W/mm associated with a peak power-added efficiency (PAE) of 48.1% and a gain of 11.6 dB ( ${V} _{\text {ds}} =20$ V), and a high PAE of 53.8% with an output power density of 1.9 W/mm and a gain of 10.8 dB ( ${V} _{\text {ds}} =10$ V), respectively. The huge potential of the UTB-AlGaN/GaN is demonstrated for high-frequency and large-output power applications when it is combined with the in situ SiN, which is necessary for future communication systems.