The Impact of Noncontinuum Thermal Transport on the Temperature of AlGaN/GaN HFETs

The effects of power density and heat generation zone size on the hotspot temperature of AlGaN/GaN HFET devices were predicted using an electrothermal modeling approach. The thermal response was modeled using a multiscale model that accounted for ballistic-diffusive phonon transport effects in the heat generation zone near the gate and diffusive transport effects outside of this zone. The Joule heating distribution was calculated using a hydrodynamic model in Sentaurus Device. The hotspot temperatures at different biasing conditions were determined using the multiscale thermal model and compared with a fully diffusive transport model. The results show that the hotspot temperature is higher when ballistic-diffusive transport effects are considered and this difference increases with increasing power density in the AlGaN/GaN HFETs.

I. Introduction

Heterostructure field effect transistors (HFETs) based on AlGaN/GaN represent an important technology for the advancement of both rf communications and power electronic devices. These devices posses high frequency switching capability, high current and carrier mobility in the 2-D electron gas, and large breakdown fields allowing for high voltage operation [1], [2]. Although these unique characteristics make this electronic material system attractive, the reliability of these devices remain a major concern [3], [4]. While several failure mechanisms in these devices have been investigated, it is clear that increasing temperature decreases the lifetime of these devices and thus, thermal management is a major concern [5], [6]. Localized self-heating in AlGaN/GaN HFETs is often most intense on the drain side edge of the gate due to the large electric fields that exist during operation [7]. As electrons pass through the channel, Fröhlich interactions cause the emission of optical phonons which result in the heating of the lattice [8]. While numerous experiments have been performed to measure the temperature distribution in operating AlGaN/GaN HFETs, they have limitations in directly observing the formation of these hotspots underneath the gate as the hotspots are often buried under the gate metallization and source connected field plates [7]–[9]. While temperature measurements are important, it is critical that thermal modeling is still performed to predict the hotspot temperature in these devices due to the current limitations in experimental capabilities. Therefore, accurate modeling of the hotspot temperature is important for the design of reliable devices.