Experimental study of negative differential conductivity in GaAs:Cr

We have studied the temperature and illumination dependencies of the current versus voltage characteristics $I(V)$ of a Cr-doped GaAs sample. An S-shaped negative differential conductivity with a threshold at around 1 kV cm^-1 was associated with a trap with activation energy of 90 meV. The low-conductivity state in $I(V)$ characteristics is strongly influenced by illumination and temperature. We also present evidence that the electrical conduction above the threshold electric field occurs by a mechanism that involves free electrons despite the fact that the low-conductivity state is p type. This supports the assumption that the 90 meV trap is a donor.