PHT.301 Physics of Semiconductor Devices

Subthreshold current

A subthreshold current flows from source to drain in a MOSFET in the cut-off regime when the transistor is nominally turned off. In this regime, the gate voltage is below the threshold voltage so no inversion channel is formed. Assuming that the source and the body contacts are shorted together, the source-body junction is unbiased and the drain-body junction is reverse biased. At the source-body junction, diffusion causes minority carriers to be injected into the body. This diffusion current is opposed by a drift current which is driven by the built-in electric field. Some of the injected minority carriers diffuse to the drain-body junction before they recombine. Since the drain-body junction is reverse biased, the minority carriers are sweep into the drain similar to what happens at the collector of a bipolar transistor. The subthreshold current is largest when the gate voltage is just below the threshold voltage and region under the gate is depleted. This regime is called weak inversion and the concentration of minority carriers exceeds the concentration of majority carriers. As the gate voltage moves away from the threshold voltage, the concentration of minority carriers decreases and the concentration of majority carriers increases as the semiconductor approaches accumulation. The higher concentration of majority carriers decreases the minority carrier lifetime and more minority carriers recombine before they are swept into the drain. The subthreshold current has the form,