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Lead zirconate titanate (PZT) thin films offer advantages in microelectromechanical systems (MEMSs) including large motion, lower drive voltage, and high energy densities. Depending on the application, different substrates are sometimes required. Self-heating occurs in the PZT MEMS due to the energy loss from domain wall motion, which can degrade the device performance and reliability. In this work, the self-heating of PZT thin films on Si and glass and a film released from a substrate were investigated to understand the effect of substrates on the device temperature rise. Nano-particle assisted Raman thermometry was employed to quantify the operational temperature rise of these PZT actuators. The results were validated using a finite element thermal model, where the volumetric heat generation was experimentally determined from the hysteresis loss. While the volumetric heat generation of the PZT films on different substrates was similar, the PZT films on the Si substrate showed a minimal temperature rise due to the effective heat dissipation through the high thermal conductivity substrate. The temperature rise on the released structure is 6.8× higher than that on the glass substrates due to the absence of vertical heat dissipation. The experimental and modeling results show that the thin layer of residual Si remaining after etching plays a crucial role in mitigating the effect of device self-heating. The outcomes of this study suggest that high thermal conductivity passive elastic layers can be used as an effective thermal management solution for PZT-based MEMS actuators. 

Gallium nitride (GaN) high electron mobility transistors (HEMTs) are key components of modern radio frequency (RF) power amplifiers. However, device self-heating negatively impacts both the performance and reliability of GaN HEMTs. Accordingly, laser-based pump-probe methods have been used to characterize the thermal resistance network of epitaxial material stacks that are used to fabricate HEMT structures. However, validation studies of these measurement results at the device level are lacking. In the present work, a GaN-on-SiC wafer was characterized using frequency-domain thermoreflectance and steady-state thermoreflectance techniques. The thermal conductivity of the GaN channel/buffer layer, SiC substrate, and the interfacial thermal boundary resistance at the GaN/SiC interface were determined. Results were validated by performing thermal imaging and modeling of a transmission line measurement (TLM) structure fabricated on the GaN-on-SiC wafer. 

Gallium nitride (GaN) high electron mobility transistors (HEMTs) are key components enabling today’s wireless communication systems. However, overheating concerns hinder today’s commercial GaN HEMTs from reaching their full potential. Therefore, it is necessary to characterize the respective thermally resistive components that comprise the device’s thermal resistance and determine their contributions to the channel temperature rise. In this work, the thermal conductivity of the GaN channel/buffer layer and the effective thermal boundary resistance (TBR) of the GaN/substrate interface of a GaN-on-SiC wafer were measured using a frequency-domain thermoreflectance technique. The results were validated by both experiments and modeling of a transmission line measurement (TLM) structure fabricated on the GaN-on-SiC wafer. The limiting GaN/substrate thermal boundary conductance (TBC) beyond which there is no influence on the device temperature rise was then quantified for different device configurations. It was determined that this limiting TBC is a function of the substrate material, the direction in which heat primarily flows, and the channel temperature. The outcomes of this work provide device engineers with guidance in the design of epitaxial GaN wafers that will help minimize the device’s thermal resistance.