Impact of AlGaN Back Barrier on the Thermal Resistance of RF HEMTs

Incorporation of low aluminum composition AlGaN buffer layers mitigates short channel effects in high-frequency GaN HEMTs. However, there is a trade-off between improved carrier confinement and increased thermal resistance of the AlGaN buffer. This work investigates the impact of the thermal conductivity of AlGaN back barriers on the overall thermal resistance of RF HEMTs. Nanosecond time-domain thermoreflectance is used to measure the thermal conductivity of GaN and AlxGa ${}_{\text {1-{x}}}$ N in the range of x =0.01 to 0.06. The room temperature thermal conductivity decreased from 39 W/m $\cdot $ K to 19 W/m $\cdot $ K for x =0.01 and 0.06 respectively, which is considerably lower than the 145 W/m $\cdot $ K measured for GaN. Notably, the Callaway model underpredicts the AlGaN thermal conductivity reduction in the low Al composition range. Transistor channel temperatures determined using micro-Raman thermography were consistent with 3-D finite element thermal simulation using the measured thermal conductivities. Changing from a GaN buffer to Al ${}_{{0}.{01}}$ Ga ${}_{{0}.{99}}$ N gives rise to a 46% increase in the overall thermal resistance. Increasing Al composition to x =0.06 increased thermal resistance by a further 35% beyond that of x =0.01. This result highlights the importance of considering the electrothermal trade-off when tuning Al composition in the lower range to optimize electrical performance, while minimizing the impact on the transistor thermal resistance.