Self-Heating Effects in Multi-Finger AlGaN/GaN HFETs
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Self-Heating Effects in Multi-Finger AlGaN/GaN HFETs M. Kuball1, S. Rajasingam1, A. Sarua1, M.J. Uren2, T. Martin2, R.S. Balmer2, and K. P. Hilton2, 1 University of Bristol, H.H. Wills Physics Laboratory, Bristol BS8 1TL, United Kingdom 2 QinetiQ Ltd., St. Andrew’s Road, Malvern, Worcs WR14 3PS, United Kingdom ABSTRACT We report on the in-situ measurement of temperature, i.e., self-heating effects, in multifinger AlGaN/GaN HFETs grown on SiC substrates. Optical micro-spectroscopy was used to measure temperature with 1µm spatial resolution. Thermal resistance (temperature rise per W/mm) was measured as a function of device pitch and gate finger width. There is significant thermal cross talk in multi-finger AlGaN/GaN HFETs and this needs to be seriously considered for device performance and ultimately device reliability. A comparison with theoretical modeling is presented. Uncertainties in modeling parameters currently make modeling less reliable than experimental temperature assessment of devices.
INTRODUCTION Wide transistor widths arranged in compact multi-finger layout are required for highpower high-frequency AlGaN/GaN heterostructure field effect transistors (HFETs). In these multi-finger designs, self-heating induced “thermal” cross-talk between individual gate fingers becomes very important, affecting channel temperatures, and ultimately device performance and device reliability. Spatial resolutions on the order of 1µm are required to accurately measure the temperature in AlGaN/GaN HFETs with their narrow source-drain openings. Such spatial resolutions can be achieved by using micro-Raman spectroscopy [1]. Infrared techniques often employed to measure the temperature of an active device are not adequate for AlGaN/GaN HFETs due to their limited spatial resolution. We report here on the use of micro-Raman spectroscopy to measure accurately temperature in multi-finger AlGaN/GaN HFETs with 1µm spatial resolution as a function of device pitch and gate finger width. Comparison of the experimental results with theoretical modeling is presented.
EXPERIMENTAL DETAILS HFETs were fabricated from heterostructures consisting of 28nm of Al0.23Ga0.77N on 1.2µm thick GaN grown by metalorganic vapor phase epitaxy (MOVPE) on insulating SiC (0001) substrates. Conventional mesa-isolated device technology with Ti/Al/Ti/Au ohmic contacts, Ni/Au Schottky gates, and silicon nitride surface passivation were employed. Four multi-finger HFET layouts were studied, designed for operation at S-band (2-4GHz) with 4 or 8 fingers, with a finger spacing (device pitch) of 50 or 25µm, respectively, each finger either 250 or 500µm wide. The devices had 5µm wide source-drain gaps with 0.8µm electron beam drawn gates. The inset of Figure 1 shows a photograph of an 8 × 250µm device with 25µm finger pitch. Three (for 250µm-wide devices) or six (for 500µm-wide devices) equidistantly spaced broad gold airbridges have been used to interconnect the source contacts.
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