Materials such as silicon and germanium are known as SEMI¬CONDUCTORS. The electrons in these materials are fairly tightly bound to the atoms of which they are composed. Because there are few free electrons in these materials, only a little current flows when an electric field is applied across a specimen of the material. However, photons have enough energy to free electrons from their bonds to the atoms of the material. The electrons so created enable an increased electric current to flow in the specimen. For this reason, these materials are called PHOTOCONDUCTORS and they are often used in a device called a PHOTODIODE. By monitoring the current flowing in a photodiode we can measure the intensity of the light that is falling on it. It is possible to construct one-dimensional arrays of photodiodes but the necessity of getting two wires to each photodiode makes it impossible to construct two-dimensional arrays sufficiently compact to have adequate resolution for astronomical applications.
A different approach is used for two-dimensional detectors Rather than attempt to measure the continuous current flowing through each diode of the array, the number of electrons in each element is allowed to build up during the exposure. At the end of the exposure, the total charge on each diode is passed along from one diode to the next until it reaches the part of the array where the charge is actually measured. Devices of this type are called CHARGE-COUPLED DEVICES and have the advantage that they can be made by conventional integrated circuit manufacturing processes. This means that they are inexpensive. Photoconductors generally have a fairly good detective quantum efficiency – it can be as high as 75 per cent. However charge-coupled devices are less efficient than this because each time the charge packet is passed from diode to diode, some of it is lost or left behind. The charge-coupled devices are applied astronomically to record a two-dimensional image in place of a photographic plate. In this case, however, the image is recorded electronically rather than as the blackening of a photo¬graphic plate.
Television camera tubes also work in some cases by using photo-conductors. One kind of device uses an array of silicon diodes (called the target), just like that of the charge-coupled device. With these, the charge is allowed to build up on the diode and the total current is established at the end of the exposure by scanning the array, element by element, until all the accumulated charges have been collected. The television camera tube produces a beam of electrons which can be pointed at any part of the target by means of magnetic coils surrounding the camera tube. The electron beam acts like a fine wire which is connected to the diodes at which it points. The current which flows in the electron beam is then a measure of the amount of light that has fallen on the target during the exposure.