A reasonably good physical picture is to imagine the transistor as a hose. The gate is a valve, and basically works by smushing the channel closed. In standard

A reasonably good physical picture is to imagine the transistor as a hose. The gate is a valve, and basically works by smushing the channel closed. In standard planar designs, the hose is smushed from the too, and eventually, you cannot close off that flow. A finfet let's you smush that channel.closed from three sides. And a nanowire or sheet let's you close it from all sides. You can reasonably ask: why not make it really small and make sure it closes. The reason is that you need current to drive the computation process. So anyway, you balance these factors at each node, which is what BSIM lets you do.