Xin Yu Liu

A 22W Ku band power amplifier based on internal-matched 6mm GaN HEMTs single chip

A Ku-band power amplifier is successfully developed with an internal-matched single chip 6mm AlGaN/GaN high electron mobility transistors (HEMTs). LCL network together with microstrip circuits are used to directly match the impedance of the 6mm GaN HEMTs to 50Ohm without using power combiner. Under the pulsed condition (100μs, 10%), the developed GaN HEMTs power amplifier delivers a 22W saturated output power with 7.6dB linear gain and 26.8% maximum power-added efficiency (PAE) with a drain voltage of 32V and at the frequency of 13.7GHz. To our best knowledge, the achieved high performance power amplifier is the state-of-the-art result ever reported for internal-matched 6mm gate width single chip GaN-based hybrid microwave integrated power amplifier at Ku-band.

Improvement of Breakdown Characteristics of a GaN HEMT with AlGaN Buffer Layer

High-electron-mobility transistors (HEMTs) with a highly resistive two-layer buffer layer (AlGaN/GaN) were grown on 6H-SiC substrates by metalorganic chemical vapor deposition. The characteristics were compared with those of conventional HEMTs utilizing GaN as the high-resistivity buffer. The results of x-ray diffraction and atomic force microscopy indicate that the crystal quality of the HEMT heterostructure is not deteriorated by the AlGaN buffer layer. The direct-current (DC) characteristics of the HEMTs with the two different structures are similar, while the off-state breakdown voltage is enhanced and the mobility of the two-dimensional electron gas is improved by the AlGaN buffer layer. The reasons for the effects of the AlGaN buffer layer are discussed systematically.

Effect of Annealing on the Characteristics of Pd/Au Contacts to p-Type GaN/Al0.45Ga0.55N

The effect of annealing on the characteristics of Pd/Au contacts to p-type GaN/Al0.45Ga0.55N was investigated. The electrical characteristics of Pd/Au contacts and a p-GaN/AlGaN sample were measured after annealing at different temperatures from 550°C to 850°C. Changes in the surface electrical characteristic of p-GaN/AlGaN material were observed after each annealing step. It is indicated that the surface electrical characteristic of p-GaN/AlGaN material plays an important role in the current transport through Pd/Au pads. A possible reason for those changes is impurity contamination, most likely oxygen and carbon contamination introduced from processing. The microstructure of Pd on p-GaN/AlGaN was investigated by glancing-incidence x-ray diffraction (GXRD). The results of GXRD show that the AlPd2 and GaPd2 phases were formed at the interface after annealing, and the formation of these phases could be effective for forming ohmic contacts on p-GaN/AlGaN.

Structural Optimization of 4H-SiC BJT for Ultraviolet Detection with High Optical Gain

The 4H-SiC n-p-n BJT for ultraviolet detection with high optical gain is proposed and optimized in this paper. The effect of structural parameters of 4H-SiC phototransistor on the performance of the detectors is simulated and the effect mechanism is analyzed. The simulation results show that the 4H-SiC phototransistors detect UV light with a response wavelength below 380 nm. It is suggested that the base parameters are important to the responsivity of the 4H-SiC BJT. With optimized parameters the 4H-SiC UV phototransistor exhibits peak responsivity as high as 4617 A/W corresponding to a quantum gain of 2.2×105 under the bias voltage of 5 V.

An Investigation of Capacitance-Voltage Hysteresis in Al2O3/SiC MIS Capacitors

Charge trapping behavior in Al2O3/SiC MOS structures was investigated by C-V hysteresis measurements in combination with XPS analysis. According to the quadratic fit of C-V hysteresis vs. tox curves, the density of the injected charges in the bulk Al2O3 films are the same under different maximum electric field, while the density of sheet charges increase with the increase of maximum electric field. Thus, a simple sheet charge model has been used to evaluate the actual effect of the electron injection phenomenon. The charge trapping levels can be as high as 1013 cm-2, indicating the importance of C-V hysteresis in Al2O3/SiC structures. All the trapping charges are found to be located at a distance ranging from 3 to 4 nm from the interface. Furthermore, no detectable interface oxide between Al2O3 and SiC has been found through our XPS measurements. We conclude that the origin of charge trapping sites in Al2O3/SiC structures is the native defects in ALD Al2O3 layer and predominantly the border traps in the Al2O3 near the oxide/semiconductor interface.

Improving Interface Quality of 4H-SiC MOS Devices with High Temperature Oxidation Process in Mass Produce Furnace

A high-temperature process is used to enhance the COx desorption rate to reduce trap density in SiC/SiO2 interface for SiC MOSFETs. Interface state density as measured by Terman method and C-ψs method for the oxidation processes at a high temperature of 1350°C show significant improvement compared to traditional Si thermal oxidation temperature of 1200°C. The higher oxidation temperature led to a much faster growth rate and some observable hysteresis in the CV curves, which could be due to electron trap and can be resolved by NOx post oxidation anneal (POA).

Study of Temperature-Dependent Mechanisms and Characteristics of 4H-SiC Junction Barrier Schottky Rectifiers

Temperature-dependent mechanisms and characteristics of 4H-SiC JBS rectifiers were described by theoretical and experimental results. The forward on-resistance of 4H-SiC JBS rectifier consists of several components, the drift region resistance is most sensitive to temperature than others. Comparing theoretical results with experimental data indicates that the leakage current is mainly affected by the thermionic emission, the image force barrier height lowering and tunneling. At different temperatures and reverse bias, the contribution of barrier lowering and the tunneling to leakage current is not the same. The temperature of critical point decreases with the increasing of the concentration of ND or the reverse bias voltage VR. Samples with the doping concentration of ND=6.5E15cm-3 and ND=1E16cm-3 were manufactured in the same process. The forward I–V-T and reverse I–V-T characteristics of the JBS samples were measured at different temperatures (300K to 523K), and temperature-dependent ideality factor, barrier height and resistance were also analyzed, which are in good agreement with simulation results.

Design and Optimization of AlGaN Solar-Blind Double Heterojunction Ultraviolet Phototransistor

The n-p-i-n AlGaN solar-blind ultraviolet double heterojunction phototransistor (DHPT) with internal gain is proposed and optimized in this paper. The dependences of spectral responsivity and quantum gain on structure parameters of the AlGaN DHPT are simulated in detail. Then, the polarization effect of AlGaN heterojunction on the performance of AlGaN DHPT is also investigated. Results show that positive polarization charge would enhance the photoresponse of the device, whereas the negative polarization charge would reduce the photoresponse significantly. The reasons for the polarization effect on performance of AlGaN DHPT are discussed.

Design and simulation of 4H-SiC MESFET ultraviolet photodetector with gain

The 4H-SiC ultraviolet detector of the MESFET structure with gain is proposed and simulated in this paper. The Schottky gate of MESFET is transparent or semi-transparent to allow more of the incident UV light to be absorbed in the device. The effect of the doping and thickness of the channel layer on the photocurrent of the 4H-SiC MESFET UV detector is simulated and the effect mechanism is analyzed. The simulation results show that the 4H-SiC MESFET exhibits photocurrent below 380 nm. And only when the channel of the 4H-SiC MESFET is in the open state there will be a gain in the detector. Shorter gate length is beneficial to improve the responsivity and the gain of the 4H-SiC MESFET UV detector. When the gate length is set to 10 μm with the channel thickness of 0.3 μm and channel doping of 1×1017 cm-3, the peak responsivity and the gain are calculated to be 12.9 A/W and 55.6 respectively.

Investigation of the Interface Quality and Reliability of 4H-SiC MOS Structure with NO and Forming Gas Annealing Treatment

Effects of NO and forming gas annealing treatment on the interface quality and reliability of 4H-SiC MOS were systematically studied by low temperature conductance measurements in combination with time-zero dielectric breakdown and time-dependent dielectric breakdown methods. The interface trap density (Dit) showed no obvious reduction after forming gas annealing, and the values of Dit decreased significantly after combined NO and forming gas annealing treatment. The F-N barrier height, electric field to breakdown (Ebd) and charge to breakdown (Qbd) of the MOS structure increased from 2.42 eV, 10 MV/cm, 1mC/cm2 to 2.62 eV, 10.7 MV/cm, 78mC/cm2 after forming gas annealing. The values of F-N barrier height, Ebd and Qbd for MOS capacitors with combined NO and forming gas annealing treatment are 2.69 eV, 10.2 MV/cm, and 24mC/cm2. These results suggest that forming gas annealing is more effective in reliability improvement. While when considering the interface trap density, it seems that combined NO and forming gas annealing treatment is a better choice.