Negative Differential Resistivity in GaN Metal-Semiconductor-Metal Photoconductors

We have observed a negative differential resistivity (NDR) in metal-semiconductor-metal (M-S-M) photoconductor made from unintentionally doped GaN grown by metalorganic chemical vapor deposition. The threshold field for the onset of NDR was found to be independent of the spacing of M-S-M fingers, and was measured to be 1.91×105 V/cm for GaN with an n-type carrier concentration of 1014 cm−3. We believe that the observed NDR is due to transferred electron effect in GaN. The threshold field value is very close to the value obtained from the theoretical simulation. This observation, to the best of our knowledge, is the first experimental evidence of transferred-electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn-effect devices using GaN materials.