Negative differential resistivity in field-effect transistors

In some GaAs MESFETs the plot of drain current aginst drain bias shows a negative slope in the saturation region. In the paper, the author demonstrates that this is due to the transport properties of GaAs. Three MESFETs of the same geometry are simulated, but with different doping using the Monte-Carlo particle model. Two of the transistors have a p-type layer of different strength between the epilayer and the substrate, the third one is without the p-type layer. The doping of the epilayer is the same for all three transistors. It was found that the stronger the p-type layer, the more it was able to contain the carrier in the epilayer; and the electric field at the source side of the gate region was correspondingly stronger. An increase in this field causes more carriers to be excited into the upper conduction bands. where their velocity decreases due to their larger effective mass. The net result is that the current decreases with increased drain bias All MESFETs tend towards negative differential resistivity, but, when the carriers can enter the substrate, this effect is counteracted by the substrate currents; often to such a degree that a positive differential resistivity will result. The transistor with the smallest substrate currents has the smallest noise figure, constant transconductance down to pinch-off and the smallest high-frequency feedback: and is, therefore, a promising candidate for microwave applications.