Probing the Channel Temperature in Poor Thermally Conductive Ultra-Wide Bandgap Semiconductor Devices

With the commercialization of wide bandgap (WBG) semiconductors for high frequency/power electronics, significant efforts have been invested in discovering the potential of ultra-wide bandgap (UWBG) semiconductors for the next generation of electronics. By reaching higher bandgaps (> 3.4 eV), UWBG devices will be able to operate at higher temperatures as well achieve lower on-resistance with faster switching. Recent efforts have particularly been focused on high Al content AlGaN channel devices (greater than 50% to obtain a bandgap > 4.5 eV). By epitaxially growing a barrier with a different Al% content, AlGaN channel high-electron-mobility transistors (HEMTs) are being explored to meet the market demands regarding high voltage operation (>20kV), high frequency switching (≈ 30-100 GHz), and high temperature applications for extreme environments (>100 °C baseplate). Through design and failure analysis, the main limiting factor related to the device power density, has been attributed to excessive localized Joule heating that arises from the poor thermal conductivity of UWBG such as AlGaN (10-20 W/mK). The channel temperatures have been demonstrated to reach 200 °C under biasing conditions equating to 5 W/mm. To mitigate self-heating and overcome this thermal challenge, research efforts must be focused on understanding the thermal transport in these heterostructures as well as other UWBG semiconductors. This talk will focus on implementing alternative approaches to established thermal metrology techniques that can assist in quantifying the thermal dynamics of AlGaN and Gallium Oxide based transistors.
High throughput techniques, such as Transient Thermoreflectance Imaging (TTI), use light emitting diodes (LEDs) to provide thermal images of transistors with high spatial (≈ 200 nm) and temporal resolution (≈ 50 ns). The full device image, however, can only be obtained by using illumination wavelengths near the bandgap of the semiconductor that will accurately probe the channel surface temperature. When moving to wider bandgap materials, the LED wavelength requirements start to pose a challenge to classical TTI setups. This talk will explore the development of customized DUV enhanced microscopes for thermal imaging of UWBG devices that can specifically accommodate deep UV wavelength optics, CCDs as well as LEDs. Additionally, recent reports have demonstrated the feasibility of sub-band gap thermoreflectance imaging for Gallium Oxide transistors. A hyperspectral approach will be implemented to extract accurate channel temperatures in Gallium Oxide field effect transistors (FETs). As verification, the electrical Gate Resistance Thermometry (GRT) method will be used to cross check the gate metal temperature via a four-point Kelvin measurement. Furthermore, the uncertainties associated with GRT will be discussed and solutions to minimize the error due to high leakage currents and non-zero gate biases will be presented. The presentation will conclude with passive thermal management solutions for UWBG devices. This will involve optimizing the device geometry, implementing low resistance contacts for minimized ohmic heating and utilizing high thermally conductive substrates such as AlN.