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.

We investigated the growth behavior of GaN grown on AlN along with V/III ratio and pressure variation, and found out the lateral growth regime for the fully coalesced channel layer of the AlN-based double-hetero structure high electron mobility transistor (HEMT). When the V/III ratio increases and pressure decreases, compressive stress in the GaN channel increases, and pit formation occurs to release the stress. The AlN-based HEMT structure was grown and the device was fabricated with an optimized channel layer. The two-dimensional electron gas mobility, sheet density, and sheet resistance were 1480 cm2 V−1 s−1, 1.32 × 1013 cm−2, and 319 Ω/sq., respectively, at room temperature. The device was characterized; direct current output result showed that the maximum current was ∼620 mA mm−1, on-resistance was 6.4 Ω mm, transconductance was ∼140 mS mm−1, and current on/off ratio was ∼104, respectively.

No abstract