12.16 : MOSFET の特性

The essential parameters of MOSFETs include the channel length, the channel width, the oxide thickness, the junction depth, and the substrate doping.

With zero gate voltage, the source-to-drain electrodes form two back-to-back p-n junctions, allowing only a reverse-leakage current from the source to the drain. This is known as the cutoff region.

A positive bias at the gate converts the MOS structure to form a surface inversion layer or n-channel between two n-plus regions.

With minor drain voltage, electrons flow from the source to the drain through the conducting channel, acting as a resistor, with its conductance adjustable via the gate voltage. This is the linear region where the drain current is proportional to the drain-source voltage.

Each MOSFET has a threshold voltage, the minimum gate-source voltage necessary to create a conducting path between the source and the drain.

As the drain voltage increases, it saturates, reducing the inversion layer thickness near the edge of the channel length to zero. This pinch-off point marks the start of the saturation region, where the drain current remains constant despite an increasing drain voltage.