Characteristics of junction field effect devices with small channel length-to-width ratios

Abstract

The characteristics of junction field effect devices with small channel length-to-width ratios are discussed. For such devices, the junctions deviate significantly from the parallel planar junction case normally considered in the analysis of field effect devices. The spreading of the junction depletion region beyond the ends of the gate junctions becomes important. A model is developed for calculating the shape of the junction depletion region including circular gate geometries near the ends of the channel. The model is essentially an extension of Shockley's original model to nonparallel gate junctions. Numerical results for several channel length-to-width ratios show slight deviations from Shockley's model in the terminal I–V characteristics for small length-to-width ratios but reduce to the results of Shockley's model for large length-to-width ratios. A significant feature of the model which differs from previous models is the absence of complete channel pinch-off in the current saturation region. The effect of carrier velocity saturation near the drain end of the channel is also discussed. For devices with small values of channel length-to-width ratio, carrier velocity saturation greatly reduces the saturation value of drain current. The importance of velocity saturation is found to depend upon a single parameter which is a function of the material constants and device dimensions.