Martin Kuball

Measurement of temperature in active high-power AlGaN/GaN HFETs using Raman spectroscopy

We report on the noninvasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN HFETs grown on sapphire and SiC substrates. Micro-Raman spectroscopy was used to produce temperature maps with /spl ap/1 /spl mu/m spatial resolution and a temperature accuracy of better than 10/spl deg/C. Significant temperature rises up to 180/spl deg/C were measured in the device gate-drain opening. Results from a three-dimensional (3-D) heat dissipation model are in reasonably good agreement with the experimental data. Comparison of devices fabricated on sapphire and SiC substrates indicated that the SiC substrate devices had /spl sim/5 times lower thermal resistance.

Integrated micro-Raman/infrared thermography probe for monitoring of self-heating in AlGaN/GaN transistor structures

Self-heating in AlGaN/GaN device structures was probed using integrated micro-Raman/Infrared (IR) thermography. IR imaging provided large-area-overview temperature maps of powered devices. Micro-Raman spectroscopy was used to obtain high-spatial-resolution temperature profiles over the active area of the devices. Depth scans were performed to obtain temperature in the heat-sinking SiC substrate. Limitations in temperature and spatial resolution, and relative advantages of both techniques are discussed. Results are compared to three-dimensional finite-difference simulations

Measurement of temperature distribution in multifinger AlGaN/GaN heterostructure field-effect transistors using micro-Raman spectroscopy

The temperature distribution in multifinger high-power AlGaN/GaN heterostructure field-effect transistors grown on SiC substrates was studied. Micro-Raman spectroscopy was used to measure channel temperature with 1 μm spatial resolution, not possible using infrared techniques. Thermal resistance values were determined for four different device layouts with varying number of fingers, finger width, and spacing. The experimental thermal resistance was in fair agreement to that predicted by three-dimensional finite difference heat dissipation simulations. Uncertainties in thermal properties of this device system made simulation less reliable than experiment.

Thermal Boundary Resistance Between GaN and Substrate in AlGaN/GaN Electronic Devices

The influence of a thermal boundary resistance (TBR) on temperature distribution in ungated AlGaN/GaN field-effect devices was investigated using 3-D micro-Raman thermography. The temperature distribution in operating AlGaN/GaN devices on SiC, sapphire, and Si substrates was used to determine values for the TBR by comparing experimental results to finite-difference thermal simulations. While the measured TBR of about 3.3 x 10^-8 W^-1 ldr m² ldr K for devices on SiC and Si substrates has a sizeable effect on the self-heating in devices, the TBR of up to 1.2 x 10^-8 W^-1 ldr m² ldr K plays an insignificant role in devices on sapphire substrates due to the low thermal conductivity of the substrate. The determined effective TBR was found to increase with temperature at the GaN/SiC interface from 3.3 x 10^-8 W^-1 ldr m² ldr K at 150degC to 6.5 x 3.3 x 10^-8 W^-1 ldr m² ldr K at 275degC, respectively. The contribution of a low-thermal-conductivity GaN layer at the GaN/substrate interface toward the effective TBR in devices and its temperature dependence are also discussed.

A study of the phase diagram of (K,Na,Li)NbO3 determined by dielectric and piezoelectric measurements, and Raman spectroscopy

A composition-temperature phase diagram of the system (1−x)(K0.5Na0.5)NbO3–xLiNbO3 is presented for 0⩽x⩽0.1. Using dielectric and piezoelectric resonance measurements, and Raman spectroscopy, ceramic samples containing 2%–10% LiNbO3 were studied over a temperature range of 7–770K showing a complex sequence of phase transitions. Analysis of the different Raman, piezoelectric, and dielectric data shows distinct transitions from cubic to tetragonal to orthorhombic to rhombohedral phase for x=0.02–0.05. The symmetries of the phases were assigned using analogy to phase diagram of (K0.5Na0.5)NbO3 single crystals and ceramics (x=0). At x>0.07 only one transition between ferroelectric phases occurs where tetragonal phase transforms to another phase, possibly of rhombohedral, orthorhombic, or monoclinic symmetry. In the region between x=0.05 and x=0.08, the phase transition sequence is more complex. Below 100K this phase with unidentified symmetry creates a vertical boundary with the rhombohedral phase present nea...

Buffer Design to Minimize Current Collapse in GaN/AlGaN HFETs

The bulk trap-induced component of current collapse (CC) in GaN/AlGaN heterojunction field-effect transistors is studied in drift diffusion simulations, distinguishing between acceptor traps situated in the top and the bottom half of the bandgap, with Fe and C used as specific examples. It is shown that Fe doping results in an inherent but relatively minor contribution to dispersion under pulse conditions. This simulation is in reasonable quantitative agreement with double pulse experiments. Simulations using deep-level intrinsic growth defects produced a similar result. By contrast, carbon can induce a strong CC which is dependent on doping density. The difference is attributed to whether the trap levels, whether intrinsic or extrinsic dopants, result in a resistive n-type buffer or a p-type floating buffer with bias-dependent depletion regions. This insight provides a key design concept for compensation schemes needed to ensure semi-insulating buffer doping for either RF or power applications.

Self-heating effects at high pump currents in deep ultraviolet light-emitting diodes at 324 nm

We present a detailed high-pump-current study of self-heating effects in ultraviolet light-emitting diodes (LEDs) grown on sapphire. For deep ultraviolet LEDs on sapphire, our results establish self-heating to be a primary cause of premature power saturation under dc pumping. Even the flip-chip packaged devices undergo a steady-state temperature rise to about 70 °C at a dc pump current of only 50 mA (at 8 V) resulting in a significant decrease in LED output. Temperature rise values estimated from peak emission wavelength shifts and from micro-Raman mapping of the active devices were in good agreement.

Integrated Optical and Electrical Analysis: Identifying Location and Properties of Traps in AlGaN/GaN HEMTs During Electrical Stress

A new methodology is developed to determine spatial location and properties of traps generated by electrical stressing of AlGaN/GaN high-electron mobility transistors, based on integrated optical and electrical analysis. Mild off-state stress increases irreversibly the number of traps located in the near-surface AlGaN region at the gate edge. A deep level with 0.45-eV activation energy in fresh devices changes its nature to interacting defect after the off-state stress, accompanied by an activation energy change. These results are consistent with trap generation in the near-surface AlGaN region at the gate edge related to high electric field and gate leakage current, as stressing does not result in the generation of cracks in the AlGaN layer.

Benchmarking of Thermal Boundary Resistance in AlGaN/GaN HEMTs on SiC Substrates: Implications of the Nucleation Layer Microstructure

A thermal boundary resistance (TBR) is associated with the presence of an AlN nucleation layer (NL) in AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on SiC substrates, raising device temperature beyond what is expected from the simple thermal conductivities of the main device layers. TBR was found to differ by up to a factor of four between different device suppliers, all using standard metal-organic chemical vapor deposition (MOCVD) growth techniques, related to the detailed NL microstructure. Optimizing the NL crystalline structure in MOCVD could therefore significantly improve heat extraction from AlGaN/GaN HEMTs into the SiC substrate, potentially reducing peak channel temperature rise by up to 40%, significantly benefiting device reliability.

Piezoelectric strain in AlGaN/GaN heterostructure field-effect transistors under bias

Micro-Raman spectroscopy was used to study piezoelectric strain in AlGaN∕GaN heterostructure field-effect transistors under bias. The measurements were made through the transparent SiC substrate. Strain in the GaN layer varied over the device area and was dependent on bias voltage, and affected, in particular, the gate-drain gap and area underneath the drain contact. The observed strain in GaN was shown to be related to the electric field component normal to the surface. Finite element simulations of electric field distribution show good qualitative agreement with the experimental data. Effects of strain on Raman temperature measurements in transistors are also discussed.