Thermal effects of gate connected field-plates and surface passivation on AlGaN/GaN HEMTs

Kara, Doğacan
AlGaN/GaN high electron mobility transistors (HEMTs) are widely preferred in automotive, space, and military applications due to their superior electrical and thermal properties. However, when operated in full capacity, their electrical reliability drops significantly due to electron collapse, device degradation, concentrated heating, and mechanical stresses. To increase the reliability and maximum performance of GaN HEMTs, field-plate and surface passivation technologies are used frequently. Although significant research has been done to understand the electrical effects of these structures, their true effect on thermal performance of devices is still missing in the literature. For this purpose, thermal simulations with and without gate field-plates having different thicknesses of SiO2 and Si3N4 surface passivation layers are performed. These simulations, performed using realistic Joule heating data obtained from device electrical simulations, proves that up to 6% reduction in hotspot temperature along with increased breakdown voltage can be obtained by using gate field-plate technology in GaN HEMTs operated around 4 W/mm. Since the percentage of temperature reduction is the same for devices operated at similar power densities, net temperature reduction will be higher in devices with more localized heating with higher maximum temperatures, as in the case for devices biased with more negative gate bias. Optimization studies performed as a part of this study suggests that while thick surface passivation (>200nm for Si3N4) eliminates the thermal advantages of field plate technology, thin passivation layers (<25 nm) decrease the breakdown voltage significantly and promote electron leakage. Similar results suggesting the importance of passivation thickness optimization are obtained for devices with thinner SiO2 passivation layers. Thus, significant thermal advantages are observed when gate field-plates are introduced to the device if field-plate length, passivation material and thickness are optimized based on the device operation condition.