1. Field of the Invention
The present invention relates to the field of semiconductor devices. More particularly, the present invention provides a charge-coupled device with automatic gain compression.
2. Description of Related Art
It is well known in the art of semiconductor devices that MOS devices without a source region may be useful in performing certain electrical functions. In general, these devices have been termed charge transfer devices (CTD's) and charge injection devices (CID's). Two types of charge transfer devices are bucket brigade devices and charge-coupled devices (CCD's).
In a charge-coupled device, mobile charge packets (electrons or holes) are introduced into a silicon substrate such as a p- or an n+substrate. The charge packets may be introduced electrically, or with photon bombardment, such as would be the case in a charge-coupled imager (CCI). After an integration time, the mobile charge packets are moved along the surface of the substrate by applying clocked voltages to electrodes located on the surface of the substrate. The mobile charge packets can eventually be used in, for example, linear or area imaging. Charge-coupled devices are described in Meindl, Microelectronics: A Scientific American Book, pp. 22-23.
Problems arise in charge-coupled devices when substantial amounts of charge accumulate in a small region of the substrate. When substantial amounts of charge accumulate, it is possible that a collection region has insufficient capacity to accommodate the number of carriers which are generated, i.e., the collection region becomes saturated. Saturation occurs if, for example, an extremely high-intensity light is directed at a charge-coupled imager.
To overcome the above described problem, a variety of solutions have been proposed. For example, in U.S. Pat. No. 3,916,429 various values of reverse bias are applied to gating diodes to make the depletion depth large The problem with this solution is that the amount of gain compression that can be obtained is limited because impractically large potential differences must be applied to achieve effective gain control. Further, for some wavelengths of light (e.g., blue light), the technique is ineffective because charge is collected near the surface in any event.
Other more sophisticated versions of the above solution have also been proposed. For example, Bluzer, U.S. Pat. No. 4,636,980, discloses a device in which gain is varied exponentially in real time using a programmable gain control. Again, gain control is obtained via an applied bias voltage.
Weimer, U.S. Pat. No. 4,206,371 discloses a readout system for a CCD. An electrode is placed over, for example, a P-- region and a P--- region. A P-region serves as a barrier between the P-- region and the P--- region. Charge is moved within the substrate by applying varying voltages to the electrode. When the electrode potential is high, charge is retained in the P-- region. As the electrode potential is lowered, charge is generally moved from the P-- area over the barrier region to the P--- region. Eventually, essentially all of the charge is removed from the P--region to the P--- region. No method or apparatus of providing gain in the device is shown or suggested.