Moeljanto W. Leksono

A GaN/4H-SiC heterojunction bipolar transistor with operation up to 300°C

We report on the fabrication and characterization of GaN/4H-SiC n-p-n heterojunction bipolar transistors (HBTs). The device structure consists of an n-SiC collector, p-SiC base, and selectively grown n-GaN emitter. The HBTs were grown using metalorganic chemical vapor deposition on SiC substrates. Selective GaN growth through a SiO 2 mask was used to avoid damage that would be caused by reactive ion etching. In this report, we demonstrate common base transistor operation with a modest dc current gain of 15 at room temperature and 3 at 300°C.

Cathodoluminescence study of erbium and oxygen coimplanted gallium nitride thin films on sapphire substrates

The cathodoluminescence(CL) of erbium and oxygen coimplanted GaN(GaN:Er:O) and sapphire (sapphire:Er:O) was studied as a function of temperature. Following annealing, the 1.54 μm intra‐4f‐shell emission line was observed in the temperature range of 6–380 K. As the temperature increased from 6 K to room temperature, the integrated intensity of the infrared peak decreased by less than 5% for GaN:Er:O, while it decreased by 18% for sapphire:Er:O. The observation of minimal thermal quenching by CL suggests that Er and O dopedGaN is a promising material for electrically pumped room‐temperature optical devices emitting at 1.54 μm.

Electrical characterization of GaN/SiC n-p heterojunction diodes

GaN/SiC heterojunction diodes have been fabricated and characterized. Epitaxial n-type GaN films were grown using metalorganic chemical vapor deposition (MOCVD) and electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE) on p-type Si-face 6H-SiC wafers. The I–V characteristics have diode ideality factors and saturation currents as low as 1.2 and 10−32 A/cm2, respectively. The built-in potential in the MOCVD- and ECR-MBE-grown n-p heterojunctions was determined from capacitance–voltage measurements at 2.90±0.08 eV and 2.82±0.08 eV, respectively. From the built-in potential the energy band offsets for GaN/SiC heterostructures are determined at ΔEC=0.11±0.10 eV and ΔEV=0.48±0.10 eV.

Optical characterization of GaN/SiC n-p heterojunctions and p-SiC

Optical characterization of GaN/SiC heterojunctions and p-SiC has been performed to explain the current–voltage (I–V) characteristics in GaN/SiC n-p heterojunction diodes. The I–V characteristics exhibit tunneling-assisted current with low forward “turn-on” voltages around 1.15 V as opposed to the expected drift/diffusion current with a turn on around 2.5 V. Electroluminescence (EL) measurements on these diodes revealed an infrared peak at 1.25 eV and a red peak at 1.75 eV. Photoluminescence (PL) measurements on p-SiC yielded peaks at 1.25 and 1.80 eV. Since the band gap of 6H–SiC is 3.03 eV, we attribute the EL and PL peaks to radiative transitions from the conduction band edge to a defect level and subsequently down to the valence band edge of p-SiC. This defect level is located 1.25 eV above the valence band edge.

High-Power High-Temperature Heterobipolar TransistorWith Gallium Nitride Emitter

A new heterobipolar transistor was made with the wide bandgap semicon-ductors gallium nitride (GaN) and silicon carbide (SiC). The heterojunction allows high injection efficiency, even at elevated temperatures. A record current gain of ten million was obtained at room temperature, decreasing to 100 at 535°C. An Arrhenius plot of current gain vs 1/T yields an activation energy of 0.43 eV that corresponds to the valence band barrier blocking the escape of holes from the base to the emitter. This activation energy is approximately equal to the difference of energy gaps between emitter and base. This Transistor can operate at high power without cooling. A power density of 30 KW/cm 2 was sustained.

500/spl deg/C operation of a GaN/SiC heterojunction bipolar transistor

Silicon Carbide has been proposed as a preferred material for high-power, high temperature semiconductor devices, primarily because of its large energy-bandgap and high thermal conductivity. Heterojunction bipolar transistors with a GaN wide bandgap emitter and SiC base and collector region have recently been demonstrated to have very high DC current gain (>100,000) and have been operated up to 260/spl deg/C. We present the first operation of a semiconductor bipolar transistor at a temperature of 500/spl deg/C with a current gain greater than 100. The GaN/SiC n-p-n HBTs common base I-V characteristics, current gain versus emitter current curves, and Gummel plots were obtained at temperatures ranging from 25/spl deg/C to 535/spl deg/C. The I-V characteristics showed little change over this temperature range, except for an increase in leakage current with increasing temperature. Only common base characteristics were obtained due to the high gain of the devices and the leakage current between base and collector. The high temperature of operation and the large gain even at elevated temperatures indicate the extraordinary potential of these devices for high-temperature and high-power operation.

Effects of X-ray and γ-ray Irradiation on GaN

As part of the feasibility study of using III-V nitride semiconductors for x-ray and γ-ray detection, the irradiation effects on GaN were investigated. GaN films with very different electrical resistivity and electron concentration were used in the study. The electron mobility, photoconductivity spectra, and photo-luminescence spectra were measured before and after irradiation. An enhanced ημτ product for undoped GaN films and an enhanced blue luminescence for a Zn-doped sample were observed after irradiation.

Blue-Green Electroluminescence of Free-Standing Diamond Thin Films

Blue-green electroluminescence has been observed in free-standing diamond films which were deposited by microwave plasma assisted CVD on silicon substrates. The electroluminescence device is driven by a 60 Hz power supply. The threshold voltage was about 112 V peak-to-peak. The electroluminescence spectrum at room temperature, showed a blue-green band with the peak centered at 485 nm suggesting band A type emission. Electroluminescence was also observed at 77 K.