Abstract. The self-heating of a coupled thermo-electric circuit-semiconductor system is modeled and numerically simulated. The system consists of semiconductor devices, an electric network with resistors, capacitors, inductors, and voltage sources, and a thermal network. The flow of the charge carriers is described by the energy-transport equations coupled to a heat equation for the lattice temperature. The electric circuit is modeled by the network equations from modified nodal analysis coupled to a thermal network describing the evolution of the temperatures in the lumped and distributed circuit elements. The three subsystems are coupled through thermo-electric, electric circuit-device, and thermal network-device interface conditions. The resulting system of nonlinear partial differential-algebraic equations is discretized in time by the 2-stage backward difference formula and in space by a mixed finite-element method. Numerical simulations of a one-dimensional ballistic diode and a frequency multiplier circuit containing a pn-junction diode illustrate the heating of the semiconductor device and circuit resistors.Key words. Energy-transport equations, lattice heating, thermal network, circuit equations, mixed finite-element method, partial differential-algebraic equations.AMS subject classifications. 65L80, 65M60, 82D37.1. Introduction. Due to growing package densities, self-heating becomes more and more important in modern integrated circuits and power devices. Thermal effects may strongly influence the device behavior and even reduce its performance. In order to understand the influence of self-heating, power dissipation and temperature evolution have to be taken into account in the electric network models.In industrially used circuit simulators, complex semiconductor device models are usually substituted by circuits of basic network elements, resulting in simpler so-called compact models, and thermal effects are described heuristically by correction factors or simple heat models. In order to achieve accurate simulations of modern circuits, however, a very large number of circuit elements and a careful adjustment of a large number of parameters is needed. Therefore, it is preferable to model those devices which are critical for the parasitic effects by semiconductor transport equations and to include physical heat flow models.Beacuse of missing structural informations about the coupled device-circuit equations, first approaches to couple circuits and devices were based on a combination of device and circuit simulators as "black box" solvers [16] or on simple extensions of device simulators by more complex boundary conditions [26]. More recently, electric network models were coupled to semiconductor transport equations, such as driftdiffusion [34] or energy-transport models [11], leading to a coupled system of partial differential-algebraic equations. The work [11] includes temperature models for the