A theoretical model including the electric field penetration analysis and its dependence on temperature were derived in order to study the threshold behavior of vertical-cavity surface-emitting lasers. The longitudinal mode of oscillation is found by adopting a conveniently chosen reference plane, and considering the complex reflection coefficients for traveling waves at this plane. The influence of thermal effects on laser wavelength, linear gain, carrier density, and threshold current density are evaluated. Agreement with experimental results from the open literature was achieved. (C) 1998 John Wiley & Sons, Inc.1729497