R.S. Balmer

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.

Punch-through in short-channel AlGaN/GaN HFETs

Short-channel punch-through effects are demonstrated in 0.17 /spl mu/m gate length AlGaN/GaN single heterojunction field-effect transistors. These take the form of a high output conductance and the strong dependence of pinch-off voltage on drain voltage. It is shown by simulation that they can be explained by poor confinement of charge at the AlGaN/GaN interface resulting in current flow within the bulk of the GaN layer. This is caused by there being a concentration of only /spl sim/1.5/spl times/10/sup 16/ cm/sup -3/ deep levels in the insulating GaN buffer layer. It is found that a net acceptor density of around 10/sup 17/ cm/sup -3/ is required to ensure suppression of short-channel effects.

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.

Micro-Raman temperature measurements for electric field assessment in active AlGaN-GaN HFETs

Temperature profiles in the source/drain (S/D) opening of a single finger AlGaN-GaN heterostructure field-effect transistor were studied at increasing S/D voltages by micro-Raman spectroscopy with <1 /spl mu/m spatial resolution. These profiles imply high field regions near the gate edge of length /spl sim/0.4 /spl mu/m for S/D voltages between 45 and 75 V. Electric field strengths of /spl sim/1.2 and /spl sim/1.9 MV/cm are estimated for 45 and 75 V S/D voltage. The experimental results are in excellent agreement with 2-D Monte Carlo simulations.

Thermal mapping of defects in AlGaN∕GaN heterostructure field-effect transistors using micro-Raman spectroscopy

We illustrate the use of micro-Raman mapping to study the local effect of defects on device temperature in active AlGaN∕GaN heterostructure field-effect transistors. Significant temperature rises in active devices, 50–100% above average device temperatures, were identified in the vicinity of defects. Measured temperature distributions were compared to finite difference simulations. Reduced thermal conductivity in the defect vicinity was found to be responsible for the local temperature rises in these devices, combined with possible changes in the current flow distribution.

Insights into electroluminescent emission from AlGaN∕GaN field effect transistors using micro-Raman thermal analysis

We present an analysis of AlGaN∕GaN heterostructure field effect transistor’s electric field and current distributions by electroluminescent emission and micro-Raman thermal analysis techniques. Raman and electroluminescence are complementary since they probe lattice and electron energy distributions, respectively. Electroluminescent (EL) emission and Joule self-heating are normally confined close to the drain edge of the gate in the high field region. But in some nonideal devices, the EL and self-heating peak are shifted towards the drain. There is evidence for an inhomogeneous current distribution and possibly nonradiative energy dissipation.

Transport behavior of holes in boron delta-doped diamond structures

Boron delta-doped diamond structures have been synthesized using microwave plasma chemical vapor deposition and fabricated into FET and gated Hall bar devices for assessment of the electrical characteristics. A detailed study of variable temperature Hall, conductivity, and field-effect mobility measurements was completed. This was supported by Schrodinger-Poisson and relaxation time calculations based upon application of Fermis golden rule. A two carrier-type model was developed with an activation energy of ∼0.2 eV between the delta layer lowest subband with mobility ∼1 cm2/Vs and the bulk valence band with high mobility. This new understanding of the transport of holes in such boron delta-doped structures has shown that although Hall mobility as high as 900 cm2/Vs was measured at room temperature, this dramatically overstates the actual useful performance of the device.

Integrated Raman - IR Thermography on AlGaN/GaN Transistors

We report on the integrated Raman-IR thermography analysis of AlGaN/GaN heterostructure field effect transistors (HFETs). IR thermography provides fast temperature overviews while micro-Raman enables accurate peak temperature determination with high spatial resolution in selected device areas. Excellent agreement between Raman determined device temperatures and finite difference simulations was achieved in the active device region. IR provided temperature determination over large device areas, and complemented the Raman thermal data. In IR thermography, lateral averaging of temperature in the small active device region resulted in an underestimation of device peak temperature and an overestimation of temperature profile linewidth. Dependent on device layout, depth averaging of device temperature occurs in IR thermography. The developed novel approach opens unique possibilities for the thermal screening of MMICs and reliability optimization

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