The electric field and the hole and electron concentrations are found for reverse biased junctions in which one side is either intrinsic (I) or so weakly doped that the space charge of the carriers cannot be neglected. The analysis takes account of space charge, drift, diffusion and non linear recombination. A number of figures illustrate the penetration of the electric field into a PIN structure with increasing bias for various lengths of the I region. For the junction between a highly doped and a weakly doped region, the reverse current increases as the square root of the voltage at high voltages; and the space charge in the weakly doped region approaches a constant value that depends on the fixed charge and the intrinsic carrier concentration. The mathematics is greatly simplified by expressing the equations in terms of the electric field and the sum of the hole and electron densities.