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This paper describes a comparative study of Ni/Au and Pd/Au ohmic contacts to p-GaN. Effects of annealing temperature and acceptor concentration on ohmic characteristics were investigated. It was found that both ohmic contacts exhibited a contact resistivity of less than 4×10−3 Ωcm2 after 600 °C annealing. Moreover, fairly good ohmic behaviors were observed for Pd/Au even without annealing.
This paper describes suppression of current collapse by field-plate (FP) in AlGaN/GaN MOS-HEMTs. Current collapse reduction was confirmed for a multi-gate fingered MOS-HEMT with a total gate width of 10 mm. Moreover, we have fabricated a 5-A class MOS-HEMT with both gate-FP and source-FP (dual-FP), showing more complete suppression in current collapse as compared to that with only gate-FP.
In this work, we report on high positive threshold voltage and respectable drain current density obtained from our AlGaN/GaN MIS-HEMTs with recessed-gate structure. After subjecting the device to reverse bias annealing, on top of improved subthreshold swing and leakage characteristics, its threshold voltage shifted from −0.2 V to +3.5 V. We have also confirmed that these desirable device characteristics...
The purpose of this work is to investigate the effect of ohmic electrode processing on the breakdown voltage of AlGaN/GaN HEMTs. The impact of ultrasonic cleaning condition during the lift-off process on metal edge definition was investigated. It was verified that the shape of the ohmic electrode was indeed crucial for ensuring high breakdown voltage characteristics.
This paper describes breakdown characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) with multi-finger gate patterns. We studied the spatial profile of electroluminescence (EL) from AlGaN/GaN HEMTs under high drain and near pinch-off gate bias. As a result, different EL emission profiles and breakdown characteristics were observed depending on the drain electrode pattern of the devices.
In this paper, the effect of thermal expansion in ohmic metals during ohmic annealing was studied to understand the mechanism of n-type ohmic formation in AlGaN/GaN heterostructures. We measured ohmic contact resistance as a function of annealing temperature for metal stacks composed of Cu/Al/Mo/Au, Ni/Al/Mo/Au, and Ti/Al/Mo/Au. Specific contact resistivity values of lower than 10−5 Ωcm2 were, for...
This paper describes the effect of thickening Au-plated ohmic electrodes in AlGaN/GaN HEMTs on the drain current and on-resistance. By increasing the thickness of Au-plated ohmic electrodes up to 5 µm, the fabricated AlGaN/GaN HEMT with a total gate width from 2 to 10 mm exhibited an increase in the maximum drain current by about 50 % and a reduction in the on-resistance by more than 40 %.
In this paper we have investigated the relationship between off-state breakdown voltage and gate-to-drain distance (Lgd) for AlGaN/GaN HEMTs fabricated on a free-standing GaN substrate. The off-state breakdown voltage exhibited a linear increase up to Lgd of around 80 µm but saturated at about 4000 V when Lgd > 80 µm. Therefore, we proposed that when Lgd < 80 µm, the breakdown voltage of HEMTs...
This paper describes significantly suppressed current collapse in AlGaN/GaN HEMTs with a 3-dimensional field plate (3DFP) structure. 3DFP consists of a gate field plate deposited on a passivated SiN film and equally-spaced multiple grooves formed by dry-etching of AlGaN/GaN layers, and hence the effective field plate area has been efficiently increased. Optimizing the groove spacing of 3DFP, the device...
This paper describes electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) with a GaN cap layer. The maximum drain current was kept constant at around 0.4 A/mm for a GaN cap layer thickness up to 5 nm. It was found that the degree of current collapse was gradually improved when the GaN cap thickness was increased from 0 (without cap) to 10 nm.
This paper describes results of DC characterization in AlGaN/GaN HEMTs passivated with sputter-deposited SiO2, SiN or their multilayered dielectric films. The device with SiO2 passivation exhibited low on-resistance and high breakdown voltage compared to the SiN-passivated device. However, the gate leakage current of the SiO2-passivated device was higher by one order of magnitude than that of the...
This paper describes comparative study of current collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) passivated with sputter-deposited SiO2 and SiNx dielectrics. The pulsed I-V measurements are employed to characterize switching response of the devices. It is found that the degree of current collapse for the SiNx-passivated device is superior to that for the SiO2-passivated device.
We describe the dependence of annealing temperature on the ohmic contact resistance for n-type GaN layers having ohmic metal stacks of V/Al/Mo/Au and Ti/Al/Mo/Au. Measurement results indicated that the contact resistance for V/Al/Mo/Au with a doping level of 2×1018 cm−3 was 0.35 Ωmm after annealing at 600 °C, while that for Ti/Al/Mo/Au did not show good ohmic behaviors after annealing below 650 °C...
This paper describes electrical characteristics of lateral tunnel diodes fabricated on an AlGaN/GaN heterostructure. Our lateral tunnel diode consists of a recessed Schottky source, in contact with the two-dimensional electron gas, and a non-recessed ohmic drain. We have fabricated three different device structures to study the current path. By analyzing conduction current components, we have achieved...
In this paper, we describe the dependence of ohmic contact resistance on annealing temperature in AlGaN/GaN heterostructures with ohmic metal stacks of Ti/Al/Ni/Au and Ti/Al/Mo/Au. A minimum ohmic contact resistance of 0.28 Ωmm was attained for Ti/Al/Mo/Au by annealing at 850 °C, while that of 0.44 Ωmm was obtained for Ti/Al/Ni/Au by annealing at 900 °C. The surface morphology after annealing was...
This paper describes DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on a free-standing GaN substrate. The three terminal breakdown voltage of the fabricated HEMT with a gate length of 3 µm exhibited a linear increase with increasing the gate-to-drain distance (Lgd), reaching more than 1200 V at Lgd=25 µm. It was found that the ON/OFF ratio in the drain current...
The government of Japan assumes that a large portion of renewable energy in the near future will be provided by photovoltaic generation systems installed at end consumers. Thus, smart grids in Japan will be established mainly at the distribution and the end-consumer level of power systems. Considering this situation, the Smart Grid Working Group, created inside the Cooperative Study Group on Applications...
This paper describes fabrication and characterization of AlGaN/GaN HEMTs for large current operation. By depositing a 1000nm-thick Au metal on Ti/Al/Mo/Au source and drain ohmic contacts, a drain current of more than 4 amperes has been achieved with a total gate width of 10 mm. We discuss the dependence of saturation drain current on gate width in terms of Au thickness and source/drain ohmic electrode...
We have fabricated AlGaN/GaN high electron mobility transistors (HEMTs) with a multi-field-plate structure. An enhanced breakdown voltage was achieved by introducing a multi-field-plate to a conventional HEMT structure. This is the first report demonstrating enhanced breakdown characteristics by applying a bias voltage to the field-plate.
This paper describes calculation of band structures in AlGaN based on an empirical pseudopotential method. The pseudopotential parameters were adjusted so that the Al composition dependent bandgap energy exhibited reasonable agreements with the experimental data. The full-band model of AlGaN was applied to the electron transport calculation in AlGaN/GaN structures. Current-voltage characteristics...
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