Sign of the Hall Coefficient in a Relaxation-Case Semiconductor

In a relaxation-case semiconductor the dielectric relaxation time exceeds carrier diffusion-length lifetime, and the neutrality condition no longer holds. It is shown that the sign of the Hall coefficient in a p-type relaxation-case material is positive, if the hole density exceeds the electron density times the square of the mobility ratio. This relation is identical to the one for conventional lifetime-case materials, but has here been derived without assuming neutrality. Practical Hall-effect measurements with relaxation-case samples may, however, be seriously altered by space-charge effects. We describe one case where a negative Hall coefficient always results regardless of the initial doping; this situation may be related to the ``Hall-effect/Seebeck-effect anomaly'' in amorphous films.