Optical reading of quantum well device

Abstract

Optical apparatus is disclosed wherein narrow line width light from a source is directed through the substrate of a semiconductor structure and reflected from the gate electrode of a field effect transistor element fabricated on the surface of the semiconductor structure. A quantum well layer serves as the current channel for the field effect transistor, and charge carries from a doped semiconductor layer provide high mobility carriers in the quantum well layer. Changes in the potential between the gate and source electrodes of the field effect transistor causes the normal pinchoff of carriers in the quantum well layer thereby causing changes in the absorption characteristic presented by the quantum well layer. By directing light from the source at the gate electrode through the substrate of the semiconductor structure, a photodetector can be positioned so as to detect a change in light which has passed twice through the quantum well layer, thereby detecting a change in the electrical state of the field effect transistor.