A theory is developed for the voltage arising from a gradient in a high electric field, i.e., a field sufficient to change the average energy of the carriers, and applied to calculations for the warm‐carrier region in a semiconductor. For a situation in which there is a region of constant fieldEnext to a region, consisting of the same material, in which the field decreases to zero, it is found that, forEin the warm‐electron range, the field‐gradient voltage is proportional toE2. This dependence is found experimentally for bothnandpgermanium at room temperature. Because a region of inhomogeneous material at a contact also makes some contribution to the measured voltage, it is not possible to make a quantitative comparison between experiment and theory. The contribution of the contact is found to depend on the nature of the transition region between semiconductor and metal and, in the hot‐electron region, upon the magnitude of the electric field.