A vertical amorphous silicon thin‐film transistor that has a very short channel length that is determined by deposition, not lithography, is described. These transistors have a field‐effect mobility of approximately 0.5 cm2/V s, an effective channel length of 1.5 &mgr;m, and a dynamic range of over five orders of magnitude. A method for suppressing excessive leakage currents and improving the saturation of the output characteristics by a novel current‐blocking technique is shown. A two‐dimensional computer program is used to analyze these devices and guide their design and optimization. Unlike a conventional thin‐film transistor, the current path is primarily parallel to the electric field created by an insulated gate electrode. These vertical transistors are easy to fabricate, compatible with large‐area processing techniques, and have suitable terminal characteristics for use in practical circuits.