Characterization and Compensation of Thermal Effects in GaN HEMT Technologies

Licentiate thesis, 2020

Further advancements with GaN based technologies relies on the ability to handle the heat flux, which consequently arises from the high power density. Advanced cooling techniques and thermal optimization of the technology are therefore prioritized research areas. Characterization techniques play a key role in the development of new cooling solutions, since these rely on accurate measurements of e.g. the temperature of the device. This thesis covers techniques to electrically characterize the lateral and vertical heat properties in GaN, and a temperature compensation technique for GaN MMICs.

The first part outlines a methodology to electrically extract the thermal resistance of a GaN resistor without risking distortion from field induced electron trapping effects, which are exhibited by GaN heterostructures. The technique uses differential resistance measurements to identify a suitable resistor geometry, which minimizes trapping effects while enhancing the self-heating. Such conditions are crucial for electrical methods since these exploit the self- heating for a thermal analysis.

Furthermore, a test structure and measurement method to electrically characterize the lateral heat spread was designed and evaluated. The structure is implemented with a thermal sensor, which utilizes the temperature-dependent IV characteristics of a GaN resistor, making it suitable for integration in GaN MMICs. The transient response can be obtained to extract the thermal time constants and propagation delay of the heat spread. At higher ambient temperatures, the propagation delay increases and the thermal coupling is increased. Lastly, a biasing technique to compensate for thermal degradation of the RF performance of an LNA was developed. By utilizing the gate- and drain voltage dependence of the RF performance, a constant gain against increasing temperature can e.g. be achieved.