Understanding transistor channel temperature in nonlinear microwave measurements and device operation

With the widespread use of high-voltage GaN devices and other high-power transistors, understanding the heating of a device during large-signal excitation and measurement is critical, both to ensure efficient operation and to prevent destruction of devices during measurement. Device self-heating during RF excitation is directly dependent upon the power-added efficiency of operation. All DC and RF power that is not converted to output RF power is dissipated in the device as heat. When properly applied, the traditional electrothermal model used in circuit simulators adequately calculates this self heating and yields some interesting results. In this paper, it is demonstrated that the load impedance providing maximum power-added efficiency also results in the lowest average dissipated power, and hence the lowest channel temperature, of the device. The variation in average dissipated power (and hence channel temperature) is also examined for different loading conditions; it is shown that some loading conditions produce a dissipated power that is actually higher than the DC power, a situation which often can lead to device failure during load-pull measurements.