Assuming a triangular shape for the electric field in the base-collector space-charge region of an n-p-n Si BJT enables the electron mean energy to be calculated analytically from a simplified energy-balance equation. On this basis a nonlocal impact-ionization model, suitable for computer-aided circuit simulation, has been obtained and used to calculate the output characteristics at constant emitter-base voltage (grounded base) of advanced devices. Provided the experimental bias-dependent value of the base parasitic resistance is accounted for in the device model, the base-collector voltage at which impact-ionization-induced snap-back occurs can be accurately predicted.