Takao Noda

Field-Plate Structure Dependence of Current Collapse Phenomena in High-Voltage GaN-HEMTs

Four types of the field-plate (FP) structure were fabricated to discuss the relation between the current collapse phenomena and the electric-field peak in high-voltage GaN-HEMTs. The on -resistance increase caused by current collapse phenomena is dramatically reduced by the single-gate-FP and dual-FP structures compared with the source-FP structure, because the gate-edge electric field was reduced by the gate-FP electrode. The dual-FP structure was slightly more effective to suppress the collapse phenomena than the single-gate-FP structure, because the two-step FP structure relaxes the electric-field concentration at the FP edge. These results show that the gate-edge peak strongly affects the on -resistance modulation. Although the FP edge peak also causes the collapse phenomena, its influence is weak.

LPMOCVD growth of C doped GaAs layers and AlGaAs/GaAs heterojunction bipolar transistors

Low-pressure MOCVD growth of GaAs using trimethylgallium (TMG) and arsine was investigated under conditions of low growth temperatures and low V/III ratios. The growth rate was limited not by the supply rate of TMG but by surface kinetics. Incorporation of carbon atoms was enhanced by decreasing the growth temperature and V/III ratio. GaAs layers with hole concentrations as high as 6×1019 cm-3 were obtained with a V/III ratio near unity. AlGaAs/GaAs heterojunction bipolar transistors with carbon doped GaAs bases were fabricated using this novel growth technique. The diffusion of carbon atoms was so small that a spacer layer, in the base, as narrow as 10Awas found to be sufficiently effective to establish the emitter-base junction.

InGaAs/InAlAs HEMT with a strained InGaP Schottky contact layer

The authors have fabricated an InGaAs/InAlAs HEMT structure with a strained InGaP Schottky contact layer to achieve selective wet gate recess etching and to improve reliability for thermal stress. Strained In/sub 0.75/Ga/sub 0.25/P grown on InAlAs has been revealed to have sufficient Schottky barrier height for use as a gate contact. Threshold voltage standard deviation has been reduced to one fifth that of a conventional InGaAs/InAlAs HEMT, as a result of successful selective recess etching. After thermal treatment at 300 degrees C for 5 min, the drain current and transconductance did not change, while those of the conventional HEMT decreased by more than 10%.<<ETX>>

Spontaneous Emission Enhancement in Pillar-Type Microcavities

A reduction in photoluminescence decay time of pillar-type microcavities was observed with decreasing the detuning between the resonance wavelength of the cavities and the luminescence peak wavelength of the active layers. The radiative recombination time was extracted from the measured photoluminescence decay time by a diffusion equation analysis. The obtained spontaneous emission enhancement factor was between 1.6 and 1.8. This is the first observation of a spontaneous emission enhancement in three-dimensionally confined semiconductor microcavities.

Effect of Buffer Layer Structure on Drain Leakage Current and Current Collapse Phenomena in High-Voltage GaN-HEMTs

High-voltage (> 400 V) GaN high-electron mobility transistors were fabricated using two types of heterostructures with different buffer layer structures. The buffer layer structure affected the crystal defect density in grown AlGaN/GaN heterostructure. The static on-resistance under low applied voltage was independent of the buffer layer structure because it has no influence on the 2-D electron-gas density. On the other hand, the drain leakage current through the grown layers and the dynamic on-resistance increase caused by the current collapse phenomena depended on the buffer layer structure. The leakage current was reduced by the AlN/n-GaN/AlN layers because of the potential barrier at the AlN/n-GaN interface and no-depletion of the n-GaN layer. In addition, the experimental results showed that the dynamic on-resistance was increased with the edge dislocation density and was not influenced by the screw dislocation density. From these results, it can be expected that edge dislocation is related to the electron trapping center, which must be reduced to suppress the current collapse phenomena.

Electrical properties and deep levels of InAlAs layers grown by metalorganic chemical vapor deposition

Deep donors whose thermal activation energy is about 250 meV are detected in undoped InAlAs layers grown by MOCVD. These deep donors determine the background carrier concentrations and cause a discrepancy between the donor concentrations measured by capacitance-voltage measurements and free carrier concentrations measured by Hall measurements. Oxygen as high as 1019 cm-3 is detected in these InAlAs layers. Oxygen atoms are thought to be the probable candidates of the deep donors. Oxygen in InAlAs layers is also thought to form deep levels, whose activation energies are 0.5 eV (level A), 0.3 eV (level B), 0.07 eV (level C), and 0.45 eV (level D). The reduction of oxygen in InAlAs layers results in low concentrations of both deep donors and deep levels.

Barrier height lowering of Schottky contacts on AlInAs layers grown by metal‐organic chemical‐vapor deposition

Schottky characteristics of undoped AlInAs grown by metal‐organic chemical‐vapor deposition have been investigated. I‐V characteristics and their dependence on donor concentrations in AlInAs layers were explained well by assuming the existence of an interfacial layer. The presence of oxygen atoms at the metal‐AlInAs interface was revealed by Auger electron spectroscopy measurements. The oxide formation on AlInAs surface may be an inevitable result because of the high composition of aluminum atoms in AlInAs. Reduction in donor concentration in AlInAs layers is effective in minimizing the influence of the interfacial layer on barrier height lowering. Reduction in gate leakage current was successfully demonstrated for fabricated GaInAs/AlInAs high‐electron‐mobility transistors by reducing donor concentration in AlInAs Schottky layers.

Surface related degradation of InP-based HEMTs during thermal stress

Abstract Degradation of unpassivated InP HEMTs related to the surface of InAlAs was studied. In ungated HEMTs, the drain current decreased after annealing at temperatures as low as 250°C. RBS measurements revealed that lattice disorder was induced on the surface on InAlAs during thermal treatment, which was considered to be due to either the introduction of vacancies or In segregation. Surface-related degradation was effectively suppressed by passivating devices with SiN layers.

Novel HEMT structures using a strained InGaP Schottky layer

The authors propose a new structure for InGaAs/InAlAs high electron mobility transistors (HEMTs) using a strained InGaP Schottky contact layer to achieve selective wet gate recess etching and to improve reliability for thermal stress. Strained In/sub 0.75/Ga/sub 0.25/P grown on InAlAs has been revealed to have sufficient Schottky barrier height for use as a gate contact. InGaAs/InAlAs HEMTs using an In/sub 0.75/Ga/sub 0.25/P layer have been grown by metal-organic chemical vapor deposition and 1/spl mu/m-gate devices have been fabricated. High selectivity in gate recess etching and higher reliability during thermal treatment have been demonstrated.<<ETX>>

Influence of V/III Molar Ratio on Deep Traps in Metalorganic Chemical Vapor Deposition Grown InAlAs Layers

The V/III molar ratio dependence of deep traps in InAlAs layers grown by metalorganic chemical vapor deposition is studied. Three kinds of deep traps–A (0.7 eV), B (0.5 eV), and C (0.07 eV)–are observed and their concentrations decrease drastically with increasing V/III molar ratio. The concentrations of these traps are also affected by the purity of the trimethylaluminum source. Trap A concentration and donor concentration show strong correlation with the oxygen concentration. This indicates that oxygen is the most probable candidate not only for trap A but also for deep donors.