Temperature Dependence of GaN HEMT Small Signal Parameters

This study presents the temperature dependence of small signal parameters of GaN/SiC HEMTs across the 0–150°C range. The changes with temperature for transconductance ( ), output impedance ( and ), feedback capacitance ( ), input capacitance ( ), and gate resistance ( ) are measured. The variations with temperature are established for , , , , , and in the GaN technology. This information is useful for MMIC designs. 1. Introduction Devices based on wide bandgap materials (such as GaN, SiC) promise much higher power densities and potential for higher temperature operation than GaAs, Si, and SiGe devices [1–3]. The reliability and performance of HEMTs and MMICs depend critically on the device operating channel temperature [4, 5]. Previous studies [6–11] have focused on various effects with temperature. However, the referenced temperature was the chuck (or base plate) temperature. This study presents characterization and comparison of two current GaN/SiC devices from different foundries across temperature where the temperature is reference to the channel reference. 2. Measured Results To quantize the effect of temperature on the performance of GaN/SiC device, two state-of-the-art AlGaN/GaN HEMT devices were characterized at ？25, 25, 75, and 125°C base plate (on-wafer chuck). At each temperature, S-parameters are measured at = 20？V and a fixed drain current (equal to 25% of the room temperature ) and the small signal extracted. The dissipated DC power is fixed, and hence the channel temperature to the chuck temperature is constant. For example, in the first device the temperature difference between the channel and the chuck was 26°C (calculated from finite element simulation of the structure), temperature contours shown in Figure 1. In both devices, the gate length ( ) for the HEMT was about 0.25？μm and the gate width was 2 × 100？μm. A standard equivalent circuit is used to match the measurements, see Figure 2. The model used includes a source inductance and resistance to model the via holes to ground. In the current case, a via hole structure was measured independently in order to find and . Additionally, the input and output feeding structures (Figure 3), were constructed on full wave analysis simulator (EM Sight from Microwave Office Suite) and simulated. The structures were used to de-embed the S-parameters. This is a critical step to separate the intrinsic device behavior from the extrinsic-layout-dependent behavior. In the optimization, the S-parameters are normalized to give equal-weight real and imaginary parts as well to all the parameters (S11,