In Situ Crystalline AlN Passivation for Reduced RF Dispersion in Strained‐Channel AlN/GaN/AlN High‐Electron‐Mobility Transistors

Abstract

The recent demonstration of ≈ 2 W mm−1 output power at 94 GHz in AlN/GaN/AlN high‐electron‐mobility transistors (HEMTs) has established AlN as a promising platform for millimeter‐wave electronics. The current state‐of‐art AlN HEMTs using ex situ‐deposited silicon nitride (SiN) passivation layers suffer from soft gain compression due to trapping of carriers by surface states. Reducing surface state dispersion in these devices is thus desired to access higher output powers. Herein, a potential solution using a novel in situ crystalline AlN passivation layer is provided. A thick, 30+ nm‐top AlN passivation layer moves the as‐grown surface away from the 2D electron gas (2DEG) channel and reduces its effect on the device. Through a series of metal‐polar AlN/GaN/AlN heterostructure growths, it is found that pseudomorphically strained ≤ 15 nm thin GaN channels are crucial to be able to grow thick AlN barriers without cracking. The fabricated recessed‐gate HEMTs on an optimized heterostructure with 50 nm AlN barrier layer and 15 nm GaN channel layer show reduction in dispersion down to 2 − 6 % compared with 20 % in current state‐of‐art ex situ SiN‐passivated HEMTs. These results demonstrate the efficacy of this unique in situ crystalline AlN passivation technique and should unlock higher mm‐wave powers in next‐generation AlN HEMTs.

24.4 W/mm X-Band GaN HEMTs on AlN Substrates With the LPCVD-Grown High-Breakdown-Field SiN x Layer

“…By increasing the maximum drain current, we confirmed that the drain current with bias stress was also significantly increased compared to the HEMT in [8]. Reduction in drain current was evaluated between the two devices by dividing I d with bias stress by I d without stress at drain voltage of 5V [5], [10]. For the bias stress of V gs =−5 V and V ds =0 V, 6% and 10% drain current reduction was observed for the HEMT in this study and the HEMT in [8], respectively.…”

2.2 W/mm at 94 GHz in AlN/GaN/AlN High‐Electron‐Mobility Transistors on SiC

“…Long‐term efforts aim at in situ passivation of the AlN/GaN/AlN heterostructure. [ 23 ] Potential MBE‐grown passivation materials include crystalline and amorphous AlN, as well as SiN.…”

“…Partial relaxation of the AlN barrier layer is expected due to 0.6% strain in the 200 nm GaN channel. [ 23 ] Transport characteristics were measured via Hall effect at room temperature, showing a mobility of and sheet concentration of . The HEMT processing started with regrown ohmic contacts.…”

Very High Density (>1014 cm−2) Polarization‐Induced 2D Hole Gases Observed in Undoped Pseudomorphic InGaN/AlN Heterostructures

“…The presence of intrinsic polarization [1] in III-nitride semiconductors have been used to generate high mobility 2D electron gases (2DEGs) by utilizing the positive polarization discontinuity across a heterointerface since the early 1990s. These have yielded as-grown 2DEG densities [2,3] ranging from low-5 × 10 12 to high 5 × 10 13 cm −2 without the need of donor dopants. These 2DEGs have a high room temperature mobility of 1400-1800 cm 2 V −1 s −1 resulting in low sheet resistances <200 Ω per sq.…”