The theory is based upon the hypothesis that free charge carriers—electrons and holes—and phonons exert pressures inside a solid. Gradients of such pressures exert motive forces on the carriers. On this basis, the hole current densityIp, in the absence of a magnetic field, is assumed to beIp=&sgr;pE−&mgr;pgrad Pp−&mgr;p[open phi]gradP[open phi],where &sgr;p, &mgr;p, andPpare, respectively, the conductivity, mobility, and pressure of holes; &mgr;p&phgr;is the interaction mobility between holes and phonons;P&phgr;is phonon pressure; andEis the electrostatic field. A similar expression is obtained for electrons by exchanging the subscriptpforn. (The two mobilities associated with electrons, however, are negative.)The theory is applied to the nondegenerate semiconductor, with the assumption that the equation of the ideal gas law applies. (Thus,Pp=pkT, Pn=nkT, wherekis the Boltzmann constant,Tis temperature Kelvin, andpandnare concentrations of holes and electrons, respectively.) It is also assumed— for small currents—that deviation from the equilibrium pressures can be neglected.Assumptions concerning the phonon effect are quite general; the contribution from this source to the hole current densityIpis given byIp[open phi]=−&sgr;p(kT/e)&dgr;p grad ln T,whereeis magnitude of electronic charge. The dimensionless quantity &dgr;p, the phonon‐dragging coefficient for holes (a temperature‐ and material‐dependent parameter), is not amenable to calculation by the theory, in its present form, and must be determined experimentally. Again, a similar expression exists for electrons.