In this paper, for the first time, we report a nonmonotonic dependence of the snapback voltage (Vsb) on the buried oxide thickness (tBOX) in silicon-on-insulator laterally double-diffused MOS (SOI-LDMOS) transistors. Step-by-step analysis of this effect is carried out by decoupling the self-heating and impact-ionization effects that cause the turning ON of the parasitic bipolar junction transistor (BJT) and subsequently the snapback effect. It is observed that for LDMOS transistors with low tBOX, Vsb increases with increase in tBOX due to reduction in drain current density as well as reduced impact ionization at higher lattice temperature. On the other hand, for high tBOX, Vsb reduces with the increase in tBOX due to early switching ON of the parasitic BJT at higher temperature. Therefore, it is possible to find an optimum value of tBOX to obtain the highest Vsb for an SOI-LDMOS transistor. An interesting observation is that with proper choice of tBOX, the safe operating area in SOI-LDMOS can be more than that of the corresponding bulk-LDMOS. A physics-based compact model is developed and implemented in Verilog-A. When compared with the Technology Computer Aided Design simulated results, our model exhibits high level of accuracy. © 1963-2012 IEEE.