Current Instability and Domain Propagation Due to Bragg Scattering

We propose a new mechanism for a current instability in single-valley semiconductors based on the momentum loss of hot carriers by Bragg reflection. We construct a simple model which gives rise to bulk negative differential conductivity of the uniform current state above a critical field $E_c$ associated with a soft dielectric relaxation mode, and to traveling dipole-domain solutions. The possibility for this mechanism to occur in realistic situations is discussed.