  Electron beam induced current (EBIC) is a powerful technique which measures
the charge collection efficiency of photovoltaics with sub-micron spatial
resolution. The exciting electron beam results in a high generation rate
density of electron-hole pairs, which may drive the system into nonlinear
regimes. An analytic model is presented which describes the EBIC response when
the {\it total} electron-hole pair generation rate exceeds the rate at which
carriers are extracted by the photovoltaic cell, and charge accumulation and
screening occur. The model provides a simple estimate of the onset of the high
injection regime in terms of the material resistivity and thickness, and
provides a straightforward way to predict the EBIC lineshape in the high
injection regime. The model is verified by comparing its predictions to
numerical simulations in 1 and 2 dimensions. Features of the experimental data,
such as the magnitude and position of maximum collection efficiency versus
electron beam current, are consistent with the 3 dimensional model.
