The present study investigated the Mg doping effect in the gallium nitride (GaN) buffer layers (BLs) of AlGaN/GaN high-electron-mobility transistor (HEMT) structures grown on semi-insulating 4H-SiC substrates by metal organic chemical vapor deposition. When the Mg concentration was increased from 3 × 1017 to 8 × 1018 cm−3, the crystal quality slightly deteriorated, whereas electrical properties were significantly changed. The buffer leakage increased approximately 50 times from 0.77 to 39.2 nA at −50 V with the Mg doping concentration. The Mg-compensation effect and electron trapping effect were observed at Mg concentration of 3 × 1017 and 8 × 1018 cm−3, respectively, which were confirmed by an isolation leakage current test and low-temperature photoluminescence. When the BL was compensated, the two-dimensional electron gas (2DEG) mobility and sheet carrier concentration of the HEMTs were 1560 cm2 V−1 s−1 and 5.06 × 1012 cm−2, respectively. As a result, Mg-doped GaN BLs were demonstrated as a candidates of semi-insulating BLs for AlGaN/GaN HEMT.

Herein, the effect of crystal quality of AlN buffer layer on AlGaN/GaN/AlN double‐heterostructure high‐electron‐mobility transistor (DH‐HEMT) is investigated. The material quality of the GaN channel and the AlGaN barriers, such as the dislocation density and the interface roughness, deteriorates, and the 2D electron gas (2DEG) mobility decreases as the threading dislocation density (TDD) of the AlN buffer increases. It is also revealed that the thickness and the Al mole fraction of the AlGaN barrier are affected by the strain variation of the GaN channel depending on the TDD of the AlN buffer. The variation of the compressive strain of the GaN channel is responsible for the 2DEG density change by affecting the barrier condition and the piezoelectric polarization charge. Low‐temperature Hall effect measurement reveals that the interface roughness scattering is a dominant factor for the mobility of the DH‐HEMT, which is ≈2–6 × 103 cm2 (V s)−1.

In this paper, the authors report the effect of the AlxGa1−xN buffer layer on the structural and electrical properties of an AlGaN/GaN/AlxGa1−xN double heterojunction high electron mobility transistor (HEMT). As the Al composition of the buffer layer increased, the two-dimensional electron gas (2DEG) confinement of the channel was shown to improve, which was confirmed by the simulation. The AlGaN buffer HEMT showed improved structural characteristics, such as the surface morphology, crystal quality, and interface roughness compared with the conventional HEMT with a C-doped GaN buffer. A slight decrease in 2DEG characteristics owing to the negative polarization charge was observed. However, in the breakdown voltage characteristics, comparable results were obtained as 652 V for the HEMT with C-doped GaN, 624 V for the HEMT with an Al0.044Ga0.956N buffer, and 642 V for the HEMT with an Al0.088Ga0.912N buffer, although the AlGaN buffers were not doped for semi-insulating.

The AlGaN/AlN/GaN/AlN double‐heterostructure high‐electron mobility transistor (DH‐HEMT) on sapphire substrate is introduced, and its direct current (DC) and radio frequency (RF) characteristics to the conventional GaN‐based single‐heterostructure HEMT (SH‐HEMT) on SiC substrate are compared. The devices having the two‐finger gate are fabricated with gate width of 200 μm and gate length of 500 nm. The DC performance of the DH‐HEMT shows a transconductance of 0.233 S mm−1 and a maximum drain current density of 0.93 A mm−1, comparable with that of the SH‐HEMT. There is less‐pronounced kink‐effect in the DC I–V characteristics, whereas the off‐state subthreshold current is approximately four orders of magnitude higher than that of SH‐HEMT. A pulsed I–V shows a greatly suppressed slump ratio Z1 and Z2 of 1.6% and 4.3% for the DH‐HEMT. It is shown that the performances of a small‐ and a large‐signal characteristics of the DH‐HEMT are equivalent to the GaN SH‐HEMT: the current gain cutoff frequency (fT) and the maximum oscillation frequency (fmax) are 20.1 and 47.6 GHz, and the output power density and the power added efficiency (PAE) at the peak PAE, at 20 V drain voltage and 3.5 GHz frequency, are 3.83 W mm−1 and 57.2%, respectively.

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