Search

Ohmic contacts are formed by recess etching the source and the drain regions with line and dot patterns of a few hundred nanometers on an AlGaN/GaN heterostructure and treating the surface with tetramethylammonium hydroxide (TMAH) prior to ohmic metal deposition. Electrical characterizations show that ohmic behavior is obtained at a low annealing temperature of 700 ◦ C as the direct contact area to the two-dimensional electron gas (2DEG) is increased via the etched patterns, and the ohmic contact is further improved when the interface is treated with TMAH. The analyses based on transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and atomic force microcopy confirm the presence of an improved ohmic contact interface and morphology, suggesting a novel and viable route towards the fabrication of AlGaN/GaN-based devices with improved controllability and reliability.

As the data transfer rates in various mobile systems increase, the needs for 5G wireless communication systems have also been correspondingly increased. For the enhancement of data transfer rates, 28 GHz, one of the high frequency bands proposed by Korea for global standards, was chosen for 5G systems. While GaN-based high-electron-mobility transistors (HEMTs) are very promising candidates for 5G network applications due to their usage in high output power amplifiers, typical GaN HEMTs only operate in a normally-on state. However, for successful operations of mobile handsets of 5G systems, a normally-off device operating under positive gate bias is required, which provides advantages of decreased circuit complexity and system cost, as well as potential for use in the domain of digital circuits. For positive gate bias operation, metal-insulator-semiconductor (MIS)-HEMT devices were fabricated by using the ETRI GaN MIS-HEMT process. We designed and fabricated MIS-HEMTs and characterized their performances. In addition to the adaptation of a gate recess technique, we employed an Al2O3 gate insulator to shift the threshold voltage in GaN MIS-HEMTs. The power amplifier (PA) monolithic microwave integrated circuit (MMIC) was designed and operated under a positive gate bias for 5G mobile handsets and exhibited a maximum output power of 29.5 dBm, a power gain of 11 dB, and a power added efficiency (PAE) of 11% at frequencies of 26 and 28 GHz.

We fabricated a 0.18-μm gate-length AlGaN/GaN high electron mobility transistor (HEMT) on SiC substrate fabricated by using two-step gate recessing which was composed of inductively coupled plasma (ICP) dry etching with a gas mixture of BCl3/Cl2 and wet chemical etching using the oxygen plasma treatment and HCl-based cleaning. The two-step gate recessing process exhibited an etch depth of 4.5 nm for the AlGaN layer and the clean surface of AlGaN layer at the AlGaN/gate metal contact region for the AlGaN/GaN HEMT structure. The recessed 0.18 μm × 200 μm AlGaN/GaN HEMT devices showed good DC characteristics, having a good Schottky diode ideality factor of 1.25, an extrinsic transconductance (g m ) of 345 mS/mm, and a threshold voltage (V th ) of −2.03 V. The recessed HEMT devices exhibited high RF performance, having a cut-off frequency (f T ) of 48 GHz and a maximum oscillation frequency (f max ) of 130 GHz. These devices also showed minimum noise figure of 0.83 dB and associated gain of 12.2 dB at 10 GHz.

In this paper, an enhanced-mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMT) was developed by using 4-inch GaN HEMT process. We designed and fabricated Emode HEMTs and characterized device performance. To estimate the possibility of application for millimeter wave applications, we focused on the high frequency performance and power characteristics. To shift the threshold voltage of HEMTs we applied the Al2O3 insulator to the gate structure and adopted the gate recess technique. To increase the frequency performance the e-beam lithography technique was used to define the 0.15 um gate length. To evaluate the dc and high frequency performance, electrical characterization was performed. The threshold voltage was measured to be positive value by linear extrapolation from the transfer curve. The device leakage current is comparable to that of the depletion mode device. The current gain cut-off frequency and the maximum oscillation frequency of the E-mode device with a total gate width of 150 um were 55 GHz and 168 GHz, respectively. To confirm the power performance for mm-wave applications the load-pull test was performed. The measured power density of 2.32 W/mm was achieved at frequencies of 28 and 30 GHz.

We fabricated a 0.17-μm gate-length AlGaN/GaN high electron mobility transistor (HEMT) on SiC recessed by using ICP gate etching with a gas mixture of BCl3/Cl2. The ICP gate recess process exhibited an etch rate of 20 nm/min for the AlGaN layer and the clean surface characteristics at the etched AlGaN/gate metal interface for the AlGaN/GaN HEMT devices. The recessed 0.17 μm × 200 μm AlGaN/GaN HEMT devices showed good DC characteristics, having a high gateto-drain breakdown voltage of -177 V, a good Schottky diode ideality factor of 1.32, an extrinsic transconductance (g m ) of 245 mS/mm, and a threshold voltage (V th ) of -2.6 V. The recessed 0.17 μm × 200 μm AlGaN/GaN HEMT devices also exhibited high RF performance, having a cut-off frequency (f T ) of 46 GHz and a maximum oscillation frequency (f max ) of 150 GHz.

SiC substrates and GaN/AlGaN epitaxial layers were etched by using an inductively-coupled plasma (ICP) in order to electrically connect the backside metal to the source electrode pads on the front-side of an AlGaN/GaN high-electron-mobility transistor (HEMT) device. The shape of an etched via hole was a tapered cylinder with a larger diameter at the entrance. The surfaces on the bottom and a wall of the via hole were very smooth in comparison with the results on the most recently published papers. Through optimization of the etching conditions, the etch-stops on the GaN epi-layer and on the source pad were successfully completed. The electrical properties of the device with the backside via holes were not critically different from those of the device without the backside via holes. In this paper, the fabrication process and the electrical properties of AlGaN/GaN HEMTs on SiC substrates with taper-shaped backside via holes are reported for the first time.

Low ohmic contact resistance and smooth AlGaN/GaN surface morphology were obtained on a 2-inch c-plane sapphire substrate by using stepwise annealing at three different temperatures. Metallization was performed under deposition of a composite metal layer of Ti/Al/Ni/Au with thickness variations of 30 nm/100 nm/30 nm/100 nm (MS1), 15 nm/220 nm/40 nm/50 nm (MS2), 20 nm/120 nm/55 nm/45 nm (MS3), 15 nm/120 nm/40 nm/50 nm (MS5) and 20 nm/200 nm/55 nm/45 nm (MS6) and of Au/Ge/Au/Ge/Pt with thicknesses of 120 nm/60 nm/120 nm/60 nm/85 nm (MS4), respectively. After multi-layer metal stacking, a rapid thermal annealing (RTA) process was applied with a stepwise annealing temperature profile of 400°C for 180 s, 900°C for 30 s and 940°C for 30 s. We obtained a minimum specific contact resistance of ρc = 4.3 × 10−7 cm2 for MS1, ρc = 1.0 × 10−4 cm2 for MS2, ρc = 3.2 × 10−6 cm2 for MS3, ρc = 5.4 × 10−8 cm2 for MS5 and ρc = 1.1 × 10−7 cm2 for MS6.