Impurity freezeout has a substantial effect on the threshold and subthreshold characteristics of large depletion mode devices. Recent experiments on short and narrow channel depletion mode devices demonstrate that geometry effects are independent of temperature and comparable to those of enhancement mode devices. Reduction of the electrical device size does not alter the degree of impurity freezeout. To predict the behavior of small channel enhancement and depletion mode devices, freezeout effects and geometry effects can be directly superimposed. A simple qualitative model is used to describe the conditions under which the superposition is valid. © 1981.