Transport in Relaxation Semiconductors

Dielectric relaxation time greater than diffusion-length lifetime ${\tau }_0$ defines the relaxation semiconductor, typical examples of which include high-resistivity wide-energy-gap crystals as well as amorphous materials. Exploratory analysis which extends earlier linear theory is given for carrier transport in the nonlinear large-signal case. A principal result is "recombinative space-charge injection": A stable space-charge region of majority-carrier depletion with zero net local recombination can be realized through injection of minority carriers. The analysis and independent considerations show that trapping and recombination are enhanced, centers being unscreened and Coulomb-attractive ones having extremely large "space-filling" cross sections. Measurements made for crucial check of the theory with crystals of highresistivity $n$-type GaAs confirm a predicted range of sublinear forward current. With fitting of data, ${\tau }_0$ is found to be subnanosecond and determined by recombination in very deep acceptor centers. Many amorphous semiconductors may be expected to have a Fermi level pinned at the position for minimum ($p$-type) conductivity simply through the recombination statistics, and so have a negative Hall coefficient. The electronic switching and charge storage in certain heterojunctions and the threshold switching in amorphous materials are considered from the viewpoint of the relaxation case, a unifying principle.