GaAs p-i-n structures with very abrupt doping transitions and different lengths of the quasi-intrinsic zone have been fabricated by molecular beam epitaxy technology. For structures with ultra-thin intrinsic zones, tunnel currents were determined experimentally up to electric fields of 1.9 MV/cm, in good agreement with a modified Kane model. The difference of calculated tunnel current and measured total current in structures which also perform impact ionization is used to fit a Monte Carlo simulation program at high electric fields. Resulting idealized homogeneous-field ionization rates are given which are experimentally verified up to fields of 1.3 MV/cm. © 1997 American Institute of Physics.