Frank Brunner’s research while affiliated with Ferdinand-Braun-Institut and other places

In this study, the impacts of fabrication technology and epitaxial layer design on the transconductance ( g m ) of radio frequency AlGaN/GaN high‐electron‐mobility transistors (HEMTs) are examined. Optimization of the SiN x passivation and AlGaN barrier design of 150 nm gate HEMTs enhances the extrinsic (at V ds = 10 V) and intrinsic (at V ds = 15 V) transconductance from ≈0.47/0.65 to ≈0.62/1.1 S mm ⁻¹ . Notably, an extrinsic g m of 0.70 S mm ⁻¹ at V ds = 5 V is achieved, setting a new benchmark for the extrinsic transconductance of AlGaN/GaN HEMTs designed for the K/Ka frequency range with a breakdown voltage exceeding 100 V.

The monolithic integration of the GaN-on-Si HEMTs is challenging since the back-gating effects caused by a common substrate degrade the device performance. In this paper, we introduce novel GaN-on-AlN/SiC power HEMTs. Compared with conventional GaN-on-Si HEMTs, the presented transistors show immunity to back-gating effects, thereby enabling monolithic integration of power devices without degradation of performance. Discrete power HEMTs are characterized systematically regarding both static and switching characteristics with a focus on the impact of the substrate potential. Furthermore, monolithic GaN-on-AlN/SiC half-bridges and monolithic GaN-on-AlN/SiC bidirectional switches are fabricated using the same device technology. The switching characteristics of these monolithically integrated devices are also investigated for different substrate terminations. It is demonstrated that both discrete and monolithically integrated devices achieve stable and fast switching and show a satisfactory back-gating immunity.

In this work, Si implantation and activation for lowering the ohmic contact resistance (Rc) of mm-wave GaN HEMTs has been investigated. Various combinations of annealing temperature/duration and implantation doses were tested. Dopant activation was performed using a modified procedure in an MOCVD tool, involving fast temperature ramping and annealing the samples for 8 minutes at 1150 °C. Thereby, ~ 0.02 ± 0.01 Ω·mm contact resistance was achieved on a fully doped region and ~ 0.1 ± 0.02 Ω·mm when only the source and drain contact region was n-type doped. For comparison, a well-established alloyed Ti/Al/Ni/Au ohmic contact scheme without implantation, was used as reference resulting in an average Rc ~ 0.34 ± 0.12 Ω·mm on the same wafer. Besides the three times lowered contact resistance the implanted contacts also showed a significantly improved on-wafer homogeneity.

AlN-based GaN-channel HEMTs with MOCVD-grown AlGaN/GaN/AlN epistack were realized on AlN substrates, achieving 400 mA/mm current density at V _GS = 1 V and 125 V/μm breakdown voltage (V _Br ) scaling. Unlike for similar AlN-on-SiC devices, the high V _Br scaling also applies above 1000 V. This is attributed to the significantly reduced AlN-buffer defect density. A record power density of 1.17 GW/cm ² is extracted from a device with 2.2 kV breakdown voltage. High-voltage switching transients at 0.6 A / 464 V off-state voltage show dispersions effects attributed to the AlN-buffer/ GaN-channel interface quality.

Defect-selective etching with molten Ba(OH) 2 /MgO etch drops was performed on c-plane AlGaN layers covering the entire composition range between GaN and AlN. Regardless of the aluminum content, the etchant produced shallow, hexagonal etch pits with depth-to-diameter ratios of 1/10 – 1/100. Two predominant types of etch pits were observed, which differed in size. In addition, the etch rate decreased from the center to the edge of the area exposed to the etch drops, providing a radially symmetric variation in etch pit size. For all AlGaN compositions, the positions of the etch pits correlate perfectly with the positions of the dark luminescence spots in cathodoluminescence measurements. Areas on the AlGaN samples that were not exposed to the etching procedure showed identical dark spots with the same size and density as those in the etched regions. Additionally, the density of etch pits and dark spots corresponded to the density of threading dislocations in the AlGaN layers. These observations demonstrate that the density of threading dislocations in c-plane AlGaN layers can be determined by destructive defect-selective etching with Ba(OH) 2 /MgO and etch pit counting, as well as by nondestructive counting of the dark spots in cathodoluminescence images.

The development of processes for epitaxial growth of vertical gallium nitride (GaN) drift layers enabling 1.2 kV breakdown voltage on low‐cost sapphire substrates is presented in comparison to GaN bulk substrates. The targeted blocking capability demands drift layers with a thickness of 10 μm and low but controllable n‐type doping. Using a growth rate of 2.5 μm h ⁻¹ the concentration of unintentionally incorporated carbon is sufficiently low to adjust the n‐type carrier concentration to ≈1 × 10 ¹⁶ cm ⁻³ for all types of substrates. To assess GaN drift region properties in terms of forward bias conductivity and reverse bias blocking strength, a quasi‐vertical p‐n‐diode structure is utilized. Bow reduction of GaN‐on‐sapphire structures is achieved using a stealth laser scribing process. Breakdown voltages higher than 1600 V and a specific on‐state resistance as low as 0.7 mΩ cm ² are obtained with diodes fabricated on GaN substrates. Similar structures grown on sapphire show breakdown voltages of about 1300 V due to higher levels of current leakage. Comparing different types of substrates, a direct correlation between dislocation density in the drift layer with the leakage current in p‐n diodes is deduced.

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH3 or H2 plasma pre-treatment is studied. Planar MIS capacitors are used to investigate the electrical properties and robustness of the gate insulators. In vacuo x-ray photoelectron spectroscopy (XPS) is used to study the changes in chemical composition after every surface treatment. XPS shows that all plasma pre-treatments efficiently remove all carbon contamination from the surface, but only NH3 plasma is observed to additionally remove the native oxide from the n-GaN surface. The water precursor step in the ThALD process does not completely remove the CH3 ligands of the trimethylaluminum precursor step, which might electrically be associated with a reduced forward bias robustness. The O2 plasma step in the PEALD process is associated with the removal of carbon and a tremendous increase of the O content in the GaN surface region. Electrically, this strongly correlates to an enhanced forward bias robustness and an increased forward bias hysteresis, respectively. The ThALD/PEALD stack method mitigates the shortcomings of both ALD processes while maintaining its advantages. Electrical measurements indicate that the stack method alongside NH3 plasma pretreatment provides the best characteristics in terms of hysteresis, threshold voltage, forward bias robustness, and interface trap density of states.

AlN-based semiconductor devices are considered to outperform lateral AlGaN/GaN HFETs for power-electronic switching applications due to the high AlN-material breakdown field strength. We present an AlGaN/GaN/AlN-HFET transistor without any compensation doping in the AlN-buffer layer. Breakdown voltage scaling as function of the gate-drain separation of 140 V/µm and power figure-of-merit of 2.4 GW/cm2 were achieved which is superior to most other GaN device technologies. 120 mΩ power transistors demonstrated 10 A switching transients up to 950 V off-state voltage and thus meet basic requirements for kW-range power switching. The origin of still present dispersion effects during high voltage switching could be attributed to a high structural defect density at the AlN-buffer / GaN channel material interface.