Jessica O. Maclean

Time-Resolved Temperature Measurement of AlGaN/GaN Electronic Devices Using Micro-Raman Spectroscopy

We report on the development of time-resolved Raman thermography to measure transient temperatures in semiconductor devices with submicrometer spatial resolution. This new technique is illustrated for AlGaN/GaN HFETs and ungated devices grown on SiC and sapphire substrates. A temporal resolution of 200 ns is demonstrated. Temperature changes rapidly within sub-200 ns after switching the devices on or off, followed by a slower change in device temperature with a time constant of ~10 and ~140 mus for AlGaN/GaN devices grown on SiC and sapphire substrates, respectively. Heat diffusion into the device substrate is also demonstrated

Analysis of DC–RF Dispersion in AlGaN/GaN HFETs Using RF Waveform Engineering

This paper describes how dc-radio-frequency (RF) dispersion manifests itself in AlGaN/GaN heterojunction field-effect transistors when the devices are driven into different RF load impedances. The localized nature of the dispersion in the I-V plane, which is confined to the ldquokneerdquo region, is observed in both RF waveform and pulsed I-V measurements. The effect is fully reproduced using 2-D physical modeling. The difference in dispersive behaviors has been attributed to the geometry of a trap-induced virtual-gate region and the resulting carrier velocity saturation being overcome by punchthrough effects under high electric fields.

Reducing Thermal Resistance of AlGaN/GaN Electronic Devices Using Novel Nucleation Layers

Currently, up to 50% of the channel temperature in AlGaN/GaN electronic devices is due to the thermal-boundary resistance (TBR) associated with the nucleation layer (NL) needed between GaN and SiC substrates for high-quality heteroepitaxy. Using 3-D time-resolved Raman thermography, it is shown that modifying the NL used for GaN on SiC epitaxy from the metal-organic chemical vapor deposition (MOCVD)-grown standard AlN-NL to a hot-wall MOCVD-grown AlN-NL reduces NL TBR by 25%, resulting in ~10% reduction of the operating temperature of AlGaN/GaN HEMTs. Considering the exponential relationship between device lifetime and temperature, lower TBR NLs open new opportunities for improving the reliability of AlGaN/GaN devices.

Nanosecond Timescale Thermal Dynamics of AlGaN/GaN Electronic Devices

Time-resolved Raman thermography, with a temporal resolution of , was used to study the thermal dynamics of AlGaN/GaN electronic devices (high-electron mobility transistors and ungated devices). Heat diffusion from the device active region into the substrate and within the devices was studied. Delays in the thermal response with respect to the electrical pulse were determined at different locations in the devices. Quasi-adiabatic heating of the AlGaN/GaN devices is illustrated within the first of device operation. The temperature of devices on SiC was found to reach of the dc temperature when operated with -long electrical pulses.

Control of Short-Channel Effects in GaN/AlGaN HFETs

GaN/AlGaN HEMTs can suffer from short channel effects as a result of insufficient buffer doping. The paper show that controlled iron doping of the GaN buffer during MOVPE growth can suppress all short-channel effects in 0.25mum gate length devices. The authors show that optimised iron doping has no effect on the RF output power or on the knee walkout (current-slump), but significantly improves the power added efficiency

Nitrogen incorporation into GaAs(N), Al0.3Ga0.7As(N) and In0.15Ga0.85As(N) by chemical beam epitaxy (CBE) using 1,1-dimethylhydrazine

Nitrogen incorporation in the growth of epitaxial GaNxAs(1−x) by chemical beam epitaxy (CBE) using 1,1-dimethylhydrazine (DMHy) was compared, for the first time, when using each of 2 sources of arsenic. Each of cracked arsine (AsH3) and uncracked tris(dimethylamino) arsine were used with triethylgallium as Group III source, at a growth temperature of 490(±5)°C. Different incorporations were found at high flow pressures of DMHy in the two cases. Nitrogen compositions of fully-strained GaNxAs(1−x) epilayers from 0.01% to 4.2% were measured by X-ray diffractometry (XRD) on the assumption that Vegards law applies. Higher nitrogen compositions, up to 7(±2)%, were measured by cross-sectional high resolution electron microscopy (HREM) to yield structural information and a measurement of the lattice parameter. Collation of the secondary ion mass spectrometry (SIMS), XRD and HREM data showed that there is good agreement between the nitrogen composition inferred from the lattice parameter and the chemical content, suggesting that the nitrogen is incorporated substitutionally in GaNxAs(1−x) grown under these conditions by CBE. Using trimethylindium and ethyldimethylaminealane with AsH3 and TEGa, the nitrogen incorporations into both 15% InGaAs and, for the first time, into 30% AlGaAs were also measured. Distinctly different nitrogen incorporation efficiencies were observed for the three alloys, particularly at high nitrogen percentages. Unintentional hydrogen incorporation was measured by SIMS for layers grown with AsH3 and found to be at around 10% of the nitrogen level for all three alloys studied.

High-performance InSb based quantum well field effect transistors for low-power dissipation applications

Indium antimonide (InSb) has the highest electron mobility and saturation velocity of any conventional semiconductor, giving potential for a range of analogue and digital ultra-high speed, low power dissipation applications. N-channel quantum well FETs have been fabricated with current gain cut-off frequency (fT) of more than 250 GHz and power gain cut-off frequency (fmax) of 500 GHz. Outline designs confirm the potential for multi-stage low noise amplifiers operating at more than 200 GHz, for applications such as integrated passive millimetre wave imaging.

Detailed Analysis of DC-RF Dispersion in AlGaN/GaN HFETs using Waveform Measurements

Detailed time-domain IV waveforms at RF frequencies are employed for characterisation of AlGaN/GaN HFETs in order to steer and advance device development. The IV time-domain data is used to isolate the separate effects of pinch-off and knee-walkout behaviour in limiting device performance. Furthermore, the waveform measurements which are obtained with a previously unseen level of detail, allowed the direct extraction of optimum device operating conditions

X-Band GaN SPDT MMIC with over 25 Watt Linear Power Handling

Single pole double throw (SPDT) switches are becoming more and more key components in phased-array radar transmit/receive modules. An SPDT switch must be able to handle the output power of a high power amplifier and must provide enough isolation to protect the low noise amplifier in the receive chain when the T/R module is transmitting. Therefore gallium nitride technology seems to become a key technology for high power SPDT switch design. The technology shows good performance on microwave frequencies and is able to handle high power. An X-band SPDT switch, with a linear power handling of over 25 W, has been designed, measured and evaluated. The circuit is designed in the coplanar waveguide AlGaN/GaN technology established at QinetiQ.

Quantitative comparison of trimethylindium sources and assessment of their suitability for low threshold 980 nm InGaAs/GaAs lasers grown by chemical beam epitaxy

Metalorganic compounds used as precursors for epitaxial growth by metalorganic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE), may be contaminated with oxygen-containing impurities. These impurities are a particular problem in the precursor purification process when they, or their adducts, are of similar volatility to the precursor. We report that improvements in precursor purity, in this case trimethylindium (TMIn), may be quantitatively assessed through growth at low temperatures by CBE and subsequent two-stage testing. Indium-containing III–V semiconductor test structures were characterized first using secondary ion mass spectrometry (SIMS) and second by photoluminescence (PL) lifetime measurements which sensitively probe the presence of nonradiative centers. A factor of 4 improvement in PL lifetime was found for Epipure™ TMIn as compared with conventional adduct-purified TMIn. The Epipure™ grade of TMIn was used for 980 nm lasers. State-of-the-art threshold currents (162 A cm−2) and low i...