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This paper analyzes the operation of PowerMOSFETs in the 42-V-PowerNet and shows that very stressful conditions are encountered, which can lead to severe reliability problems. To enable thorough investigations by circuit simulations an accurate physics-based compact model of the devices is proposed: it includes all important electrothermal effects relevant to the description of the observed failure mechanisms. By means of an advanced thermal-modeling approach, multichip assemblies can be accurately described, including mutual heating effects between neighboring devices. Some properly chosen examples demonstrate the validity of the model and its usefulness for reliability investigations. Index Terms--Electrothermal effects, modeling, power metal-oxide semiconductor field-effect transistors (MOSFETs), reliability.

The ability of monitoring the chip temperatures of power semiconductor modules at all times under various realistic working conditions is the basis for investigating the limits of the maximum permissible load. A novel transient thermal model for the fast calculation of temperature fields and hot spot temperature evolution presented recently is extended to include time-dependent boundary conditions for variations of ambient temperature and surface heat flows. For this a Green's function representation of the temperature field is used. Also, general initial temperature conditions are included. The method is exemplified by application to a dc/ac converter module for automotive hybrid drives. The thermal model, which can be represented by a thermal equivalent circuit, then is combined with an electrical PSpice-metal-oxide semiconductor field-effect transistor (MOSFET) model to allow for the fully self-consistent electrothermal circuit simulation of 42-V/14-V dc/dc-converter modules. 670 converter periods with altogether 8000 MOSFET switching cycles in the six-chip module can be simulated within 1-h computing time on a Pentium PC. Various simulation results are presented, which demonstrate the feasibility of the simulation method and allow for the optimization of converter losses. Short circuit modes of converter operation are investigated with a high temperature increase also revealing the thermal interaction between different chips. Index Terms--DC- and ac-converter for automotive, electrothermal simulation, multichip modules, PSpice metal-oxide semiconductor field-effect transistor (MOSFET) model, thermal equivalent circuit, time-dependent boundary conditions.

The on-state current voltage characteristic of thyristors is investigated by numerical simulation. For sufficiently high p-base concentration - as already known - an abrupt increase in on-state voltage is observed above a critical current density. Driving the device to higher currents results in a reduction of on-state voltage. Similar results are obtained for thyristor profiles with very shallow emitters. An analytic model that explains the described phenomena from first principles and leads to a simple criterion for current limiting in terms of Gummel numbers and carrier mobilities is presented.