A set of constitutive equations describing the plasticity of semiconductors is compared with the results of mechanical tests. Constant strain-rate compression tests are interrupted before the peak of the initial multiplication yield point, i.e. at the moment of intense dislocation multiplication, by transient tests (stress-relaxations and creep tests). Various laws for dislocation multiplication are used in constitutive modelling and their predictions are compared with experimental transient curves. A generalized law is proposed, which perfectly fits all the transient tests data, provided each sample is considered separately. It seems necessary to account for the properties of dislocation sources in the multiplication law, at least at the early stages of plasticity.