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The requirement of electrical-thermal-stress (E-T-S) modeling of semiconductor devices as indicated by ITRS2006 demands the use of finite-element analysis (FEA) for device simulation. In this paper, we perform E-T-S coupled-field simulation from the first principle by considering the stress-induced modification of the Si band structures and employ FEA software COMSOL to apply the E-T-S model on lateral double-diffused MOS field-effect transistors (LDMOSFETs) as an example. From the comparison of the electrical characteristics computed using the models with and without the mechanical stress caused by material thermal mismatch in the device structures, we found that the effect of the mechanical stress is insignificant in the forward-bias condition but it does have impact in the reverse-bias condition, namely, the reverse leakage current is higher if the mechanical stress is considered. Simulations are performed on two types of LDMOSFETs, namely, silicon-on-insulator (SOI) and partial-SOI (PSOI) LDMOSFETs, because the temperature and mechanical stress distributions are very different in these LDMOSFETs even under the same operating conditions. From the comparison of the computational results of SOI and PSOI LDMOSFETs, the results indicate that PSOI power LDMOSFET structure enables efficient heat conduction from the hot junctions and offers higher breakdown voltage.