J. TORKEL WALLMARK

A New Semiconductor Photocell Using Lateral Photoeffect

    Summary - The effect of illumination of a semiconductor junction is, as is well-known, a photovoltage between the two sides of the junction. In this article it will be shown that a nonuniform illuimination gives a lateral photovoltage parallel to the junction in addition to the (transverse) photovoltage mentioned above.  
    A photocell will be described that uses the lateral effect and can detect the position of a light spot to less than 100 Å. By utilizing an associated lens or aperture, one can measure an angular motion smaller than 0.1 second of arc. The output voltage of the cell is a linear function of the position of the light spot, with zero output for the light spot in the center, reversing in sign when the light spot changes from one side to the other of the center position. The linearity is better than 1.5 per cent over a distance of 0.030 inch. The equivalent noise resistance of the cell is equal to its output resistance, approximately 100 ohms. The sensitivity of the cell is approximately 200 microamperes per lumen and its frequency response is about the same as that of junction transistors.
    The response curve can be shifted by the application of a voltage between the base contacts. This is an electronic equivalent of a mechanical translation of the cell. It is also possible to do the equivalent of "chopping" the light by applying a modulating voltage to the alloyed dot. 

INTRODUCTION

    The effect of illumination of a semiconductor junction is, as has long been known, a photovoltage between the two sides of the junction. If the illumination is nonuniform, an additional effect arises which has not been recognized earlier and which will be the subject of this article, namely the development of a photovoltage parallel to the junction in addition to the photovoltage between the two sides of the junction mentioned above. To distinguish between the two effects, the latter photovoltage will be called transverse (with respect to the junction) whereas the new effect will be called lateral photovoltage (lateral with respect to the junction).
    This lateral photovoltage has been utilized in a new type of photocell whose most interesting characteristic is a photosensitivity that varies from a positive to a negative value over its surface. This means that a point source of light, imaged on the photocell by a lens, will produce a signal that varies with the angle between the direction to the light and the symmetry axis of the photocell-lens combination. When the direction to the light coincides with the symmetry axis of the cell, the signal is zero. When the light direction deviates from the symmetry axis in one direction a signal of one polarity is obtained and when it deviates in the opposite direction a signal of opposite polarity is obtained. The cell can therefore measure the direction to a light source by a null method and consequently with the high accuracy of such methods.
    The photocell is very simple and may be constructed in a manner well known from transistor technology. It consists typically of a germanium wafer on which a junction has been applied by alloying an indium dot onto the wafer. Two ohmic contacts, one at each end of the junction, are used for picking up the lateral photovoltage. It may be remarked that no contact is needed to the indium dot, which may be left electrically floating. A further valuable feature of the cell is that mechanical rotation of the cell to aim its optical axis in different directions can be replaced by a method of electronic sweeping.
    A desirable detail of the electronic sweeping is a method of electronically chopping the light signal, which has merits of its own in some applications.