Owing to the high-mobility 2-D electron gas (2DEG) and the wide bandgap of GaN, AlGaN/GaN heterojunction is a suitable platform for high-efficiency power conversion systems and high-frequency power amplifiers. At the same time, AlGaN/GaN on silicon substrate has attracted much attention with its large size and low cost^[1−8]. Moreover, enhancement-mode (E-mode) operation is also preferred for radio-frequency (RF) transistors because only single-polarity power supply is needed for biasing E-mode devices, which improves the compactness and cost-effectiveness of the power amplifier (PA) system. Especially in mobile terminals, E-mode devices play a significant role^[9−14]. Gate recess stands out as a prevalent methodology in the fabrication of E-mode AlGaN/GaN devices^[15−17]. Nevertheless, the reduction of AlGaN barrier thickness from 10−30 nm to below 6 nm by gate recess is still facing huge challenges in improving interface quality and ensuring batch-to-batch repeatability^{[18, 19]}. Therefore, a technical route is proposed for high-uniformity E-mode GaN-on-Si high-electron-mobility transistors (HEMTs) by AlGaN-recess-free process with ultrathin-barrier (UTB) AlGaN (<6 nm)/GaN heterostructure. Additionally, the UTB-AlGaN/GaN heterostructure exhibits good potential to mitigate short-channel effects in mm-wave AlGaN/GaN HEMTs. Due to the intrinsic high aspect ratio between gate length (L_G) and AlGaN barrier thickness (t_AlGaN)^[20−23]. In other words, a thin barrier is indeed required to keep a proper aspect ratio with device size scaling down to extend the operation frequency and enhance high-frequency characteristics. Compared to other technologies, such as gate recess and p-GaN gate, UTB can achieve E-mode devices without a recess process to guarantee the quality of the interface under the gate and access region. Therefore, it is promising to achieve high-performance E-mode devices utilizing UTB platform^{[24, 25]}.

In this work, the AlGaN-recess-free E-mode GaN-on-Si RF HEMTs are fabricated with the UTB-AlGaN/GaN heterostructure. The low-damage remote plasma pretreatment (RPP) is implemented before SiN_x passivation to improve the passivation interface. The E-mode HEMTs exhibit a threshold voltage (V_TH) of +1.1 V with good uniformity over chips. Meanwhile, the HEMTs achieve a high current/power gain cut-off frequency (f_T/f_MAX) of 31.3/99.6 GHz with a 1-µm gate. The fabricated devices also achieve a power added efficiency (PAE) of 52.47% and a reasonable output power density (P_out) of 1.0 W/mm at a frequency of 3.5 GHz^[26−29].

The UTB-AlGaN/GaN HEMT wafer used in this work was grown on a 4-inch high-resistivity (111) Si substrate by metal-organic chemical vapor deposition (MOCVD). The ultrathin-barrier layer consists of 1.5-nm GaN cap layer, 4-nm undoped Al_0.25Ga_0.75N layer and thin (~1 nm) AlN interface enhancement layer. Moreover, the measured thickness of the UTB layer approximates 6.5 nm as shown in Fig. 1(d). It was notable that although there are no observable sharp boundaries among these three layers, the thickness of the different layers is marked according to our design parameters. The cross-sectional schematic view of the device structure is shown in Fig. 1(b). The device fabrication process is initiated with the formation of ohmic contacts. After standard cleaning and acid treatment procedures, an Au-free ohmic stack comprising Ti/Al/Ti/TiN layers with thicknesses of 4/145/26/60 nm, respectively, was deposited through magnetron sputtering. The fabrication process was followed by lift-off steps and a low-temperature annealing treatment at 550 °C in a nitrogen (N₂) atmosphere for 60 s. Remote plasma pretreatments (RPP) were performed on the UTB-AlGaN/GaN heterostructures at 300 °C with NH₃/N₂ in a plasma-enhanced atomic layer deposition (PEALD) system^[19]. Subsequently, a 60-nm SiN_x passivation layer was deposited by plasma-enhanced chemical vapor deposition (PECVD) at 280 °C. A planar isolation procedure was executed utilizing multi-energy nitrogen ion implantation. The Au-free ohmic contact resistance (R_C) with a value of 1.6 Ω∙mm, and a 2-D electron gas (2DEG) sheet resistance (R_2DEG) of 312 Ω/sq are measured by the transfer-length method. This confirms the effective recovery of 2DEG in the access region through PECVD-SiN_x passivation, characterized by a relative dielectric constant ε = 6.4. After passivation recovery, the 2DEG concentration of 1.48 × 10¹³ cm^-2 is revealed by the capacitance−voltage (C−V) measurement, exhibiting sufficient potential to provide substantial current levels. Note that the R_C could be substantially improved by optimizing the Au-free ohmic contact process such as low-damage etching of the AlGaN barrier and increasing the ohmic metallization temperature in future studies^[30].

Optical photolithography was employed to pattern the recessed gate to approximately 1 μm, after which a CHF₃/SF₆ plasma-based low-damage etching process was applied to etch the PECVD-SiN_x by inductively coupled plasma system (ICP). This requires a high alignment accuracy in the lithography step. Further optimization of the process will utilize electron beam lithography to achieve smaller sizes and higher accuracy. It is noteworthy that the etching process will be self-termination upon reaching the UTB-AlGaN surface, attributed to the high etching selectivity between SiN_x and AlGaN with fluorine-based plasma. Then the patterned sample was soaked into wet processing in a diluted HCl solution to eliminate potential oxides and etching byproducts. Finally, a Ni/Au metal stack is deposited by electron-beam evaporation (e-beam) as the gate contact. The fabricated UTB-AlGaN/GaN HEMTs exhibit a gate length/width (L_G/W_G) ratio of 1/75 μm and a gate−source/−drain separation (L_GS/L_GD) of 0.3/0.5 μm, respectively.

Fig. 2 shows the DC characteristics of the fabricated HEMTs. Due to the utilization of the UTB-AlGaN/GaN heterostructure platform, the concentration of 2DEG beneath the gate is reduced, enabling the realization of E-mode devices. The V_TH of the fabricated HEMTs is extracted to be +1.1 V by linear extrapolation. An extrinsic transconductance (G_M) over 200 mS/mm is obtained at drain bias V_DS of 10 V, by the inherent UTB property. The 2DEG within the access region is subsequently resorted by 60-nm PECVD-SiN_x passivation layer and the maximum drain current I_{D, max} reaches 375 mA/mm at gate bias V_GS of +3 V, as shown in Fig. 2(b). To assess the V_TH uniformity of the devices, 50 devices are selected from different areas covering an area of 2.2 × 2.2 cm² on the chip (Fig. 2(c)). It is worth mentioning that the V_TH measured at V_DS = 10 V in Fig. 2(c) remains consistent with the drain voltage biased to 10 V in the load pull measurement. The V_TH values are reasonably uniform, due to the recess-free device fabrication^[17]. The ~0.2 V variation is probably due to the non-uniformity in the epitaxial growth and the warping of the wafer.

In Fig. 2(d), the gate leakage current of the fabricated HEMTs is depicted. The increasing gate leakage under V_GS > +3 V limits the application of a more positive bias^[7]. It is worth noticing that the gate leakage decreases as the drain voltage increases. This could be due to the Schottky gate leakage which is mainly provided by the spillover of 2DEG from the channel to the gate. The depletion region in the channel will expand from the drain side to the source side as the drain voltage increases, resulting in the concentration of 2DEG decreases accompanied by the gate leakage reduction.

The devices’ off-state breakdown voltage (V_BD) reaches 45 V (Fig. 3) with L_GD = 0.5 μm. For measuring the breakdown voltage, we biased the gate at −1 V to fully turn off the device. However, when the drain voltage reaches 45 V, the device experiences a catastrophic breakdown, resulting in severe damage.

Small-signal RF characteristics of the fabricated E-mode HEMTs are also characterized using a network analyzer, as shown in Fig. 4(a). A high f_T/f_MAX of 31.3/99.6 GHz was obtained by extrapolating H21 and MAG using a −20 dB/decade slope with pad de-embedding. This shows the promising potential of the UTB platform for RF applications. Fig. 4(b) plots the evolution of f_T/f_MAX and gain at 3.5 GHz with V_GS at drain bias V_DS = 10 V. Both f_T and gain increase slowly and saturate at V_GS > +2 V with V_GS increased from 1 to 3.5 V. The variation trend of the f_T and gain show satisfactory linearity.

Because the devices will undergo high voltage stresses before returning to the linear region during the load-pull measurement, the current collapse is measured to evaluate the reduction of drain current under the large signal operation. Current collapse characteristics of the fabricated E-mode UTB-AlGaN/GaN HEMTs were evaluated by AMCAD BN106-AM200 pulsed current−voltage (I−V) measurement system and corresponding test software (Fig. 5(a)). The measurement setup consists of a pulse width of 0.2 μs and pulse period of 10 μs as shown in Fig. 5(b). The off-to-on switching time is 0.04 μs. To ensure the significance of the pulsed I−V measurement, a quiescent bias point correlating with the load pull measurement was chosen. An invisible current collapse occurs in the linear region under a quiescent bias of V_{GS, Q} = 0 V, V_{DS, Q} = 15 V. An abnormal negative I_D is observed under pulse bias with V_GS = 3 V, which may be attributed to the gate to drain parasitic capacitors charging current at high gate voltage during pulse testing. According to the pulse I−V measurement results and our previous results^{[23, 28, 29]}, the combination of RPP and PECVD-SiN_x is an effective passivation method for the UTB-AlGaN/GaN HEMTs. In addition, the recess-free process will lead to better interface quality in the access region, which may suppress current collapse.

The power performance of the fabricated HEMTs with 1-μm gate was also characterized at 3.5 GHz using continuous-wave (CW) load-pull measurements. The E-mode HEMTs were biased at Class-AB condition under V_DS of 10 and 13 V. A maximum output power density (P_out) of 1.0 W/mm was obtained shown in Fig. 5(c). The tuned source impedance of the device is 34.25 + 167.24i and the tuned load impedance is 381.16 + 312.25i. A peak PAE of 52.47% was obtained under a different bias to optimize the PAE in Fig. 5(d). The tuned source impedance of the device is 62.88 + 168.72i and the tuned load impedance is 458.0 + 561.69i.

Table. 1 summarizes the reported RF performance of E-mode GaN HEMTs on Si substrate accomplished via different fabrication technologies^{[31, 32]}. Significant improvements in RF performance were observed in the fabricated E-mode UTB-AlGaN/GaN HEMTs on Si when compared to conventional p-GaN E-mode HEMTs. The enhancement can be attributed to the exceptional transconductance of the UTB-AlGaN/GaN HEMTs, which are facilitated by the reduced gate-to-channel distance. Additionally, the advantage of this UTB will help mitigate short channel effects when further reducing the gate length. Further research on reduction of R_C of the Au-free ohmic connect, suppression of gate leakage and scaling down of the gate length are underway.

E-mode AlGaN/GaN high electron mobility transistor power amplifiers, exhibiting reasonable radio frequency performance, have been fabricated on a UTB AlGaN (<6 nm)/GaN heterostructure grown on a silicon substrate, featuring Au-free ohmic contacts. The realized E-mode HEMTs with gate length of 1 μm and source−drain distance of 1.8 μm, exhibit a threshold voltage of +1.1 V, cut-off frequency and maximum oscillation frequency of 31.3/99.6 GHz, a peak output power of 1.0 mW/mm, and the power-added efficiency of 52.47% in continuous wave power characterization. The UTB-AlGaN (<6 nm)/GaN-on-Si heterostructure emerges as a compelling technology for millimeter-wave power amplifiers and mobile terminal applications.