The electroreflectance lineshapes of 111/V semiconductors are modelled using the intermediate
field theory and a multilayer reflection routine to include electric field variation. This
introduces additional structure into the lineshape, which has previously been assumed to be from
impurity or exciton effects. By using a thermal broadening parameter of F0=O.OO9eV we show that
spectrum size as well as shape can be fitted successfully to experimental data so that depletion
layer voltages can be measured.