Abstract
Simple expressions of threshold and subthreshold characteristics for a very small buried-channel MOSFET is derived from a model of majority-carrier distribution along the channel. The carrier distribution is determined from the Poisson equation for a high-low junction. The basic formula for the subthreshold characteristic is derived from the majority-carrier drift-current equation. The theory is compared with the measured threshold voltages and the measured inverse semilogarithmic slopes of subthreshold current. The theoretical curves are in a reasonable agreement with experimental results. It is shown for a buried-channel MOSFET having a channel length less than 1 μm that the threshold and subthreshold characteristics change abruptly as the channel length is reduced because the majority-carrier concentration increases through the carrier diffusion from the source and drain terminals. The theoretical estimation shows that buried-channel MOSFETs will have the less short-channel effect than surface-channel MOSFETs for a small drain voltage. The theory also predicts that the buried-channel MOSFET can be scaled down in the same way as the surface-channel MOSFET.
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