Thomas Foulkes with advisers Prof. Nenad Miljkovic and Prof. Robert Pilawa-Podgurski

Designing and optimizing high frequency, ultra-efficient converters requires detailed knowledge of the behavior and parasitic parameters for both active and passive components. Recently, wide-bandgap transistors have enabled simultaneous expansion in both switching frequency and efficiency due to higher maximum operating junction temperature limits, lower dc on-state resistance, and reduced parasitic inductances and capacitances.

The early acceptance of gallium-nitride (GaN) transistors was plagued by detrimental dynamic on-state resistance effects. This non-linear, second-order phenomenon for GaN devices is characterized by an increase in on-state resistance with rising voltage and temperature stress. While device manufacturers have made significant improvements compared to early-generation devices, experimental evidence for a survey of commercial GaN transistors shows that a significant build-up in on-state resistance with voltage and temperature stress still exists. This new method for measuring dynamic on-state resistance has promise for shedding light on the dynamic on-state resistance limitations of GaN devices due to the independent control of drain current, voltage stress, pulse-width for device conduction time, and package temperature. Based on a survey of low-voltage GaN transistors, metrics were proposed to quantify the dynamic on-state resistance performance of a specific device and facilitate a fair comparison between different GaN device technologies, as shown in Figure 1. This research is supported by an NSF Graduate Research Fellowship, NASA, and POETS.

Figure 1: In contrast to Si MOSFETs (a), measurements for the EPC2034 GaN device (b) underscore significant dynamic on-state resistance between the reference case (12 V, 12 A) and stressed condition (190 V, 12 A) with varying package temperature. The 10 ms pulse length demonstrates the full decay of on-state resistance during stressed conditions back to the reference dc on-state resistance.