(Invited) Coupled Experimental and Numerical Investigation of High-Voltage GaN Hemts
Authors: Cui Yubo , El Helou Assaad , Raad Peter E. 
Published: February 2020

The wide band gap compound semiconductor Gallium Nitride and its ternary alloys have found commercial acceptance in the fabrication of high electron-mobility transistor (HEMT) rf devices. In addition, high power applications have been very attractive due to their high breakdown voltage and thus high output power. Higher power limit improvements can be reached by several means, including the integration of field-plates above the conduction channel, which tend to improve the uniformity of the electric-field distribution and reduce its peaks. These field-plates can be connected to the source or to the gate. The reduction of electrical and thermal “hotspots,” where severe degradation is thought to initiate, increases the allowable power levels for HEMT devices by as many as five folds.
In this study, GaN HEMT devices with source-connected field-plates are characterized thermally to quantify their performance. Deep submicron resolution thermoreflectance (TR) temperature imaging is used to measure the temperature profile of the exposed surface features of the device, which are mainly the source and drain metal contacts as well as the GaN channel region. The TR thermography method is based on detecting changes in surface reflectivity (ΔR) that result from changes in local surface temperature (ΔT). The relationship between the changes in reflectivity and temperature is linear to first-order effects, and is typically referred to as the coefficient of thermoreflectance (CTR). As expected, this coefficient depends on the material and the wavelength of light, but also on any transparent layers (e.g., passivation) in the optical path between the first reflective surface and the sensor where the light is collected. Therefore, the success of a TR measurement requires that the surface of interest exhibit both a high reflective index and be thermally-reflective, i.e., have a good value of CTR. By using a near ultra-violet source to illuminate the device under test (DUT), the GaN is rendered opaque, thus ensuring that the obtained temperature fields are those of the GaN surface and not an integrated average over multiple semi-transparent layers. However, the presence of the field-plate obstructs imaging access to the gate where the peak temperatures are expected. Therefore, this investigation couples the measurements to 3D computations in order to produce temperature fields that would otherwise be inaccessible to direct measurement.
Both quasi-steady-state and transient temperature fields of the GaN HEMTs are obtained at different activation power levels and an experimental assessment is made with regard to the electrical activation limits and the thermal self-heating. In addition to the measurements, thermal computations are performed using a self-adaptive, multi-scale, numerical simulation engine. The 3D transient thermal model is then driven by the experimentally-obtained temperature maps to provide a more detailed and device-specific interpretation of the coupled electro-thermal effect of the field-plates on reducing the temperature and increasing the break-down voltage. Moreover, the full 3D thermal model also helps provide physically-accurate temperature fields of regions of critical channel regions, where the heat sources are located, and where localized hotspots that are hidden underneath field-plates are inaccessible to optical measurement approaches. The attached figure contains two temperature fields, numerical on the left and experimental on the right. These results are representative of the type of useful data that can be obtained from the computational and measurement techniques. The presentation will additionally include the results of the coupling of the two approaches and the insights that can be gained, especially for those features that are optically inaccessible.

Figure 1