FIG. 1 shows an example of an image sensor 10 with a charge multiplying CCD 40. The image sensor 10 consists of a pixel array 20 which may be of the well-known CCD types of full frame or interline. The pixel array 20 shifts row of charge into a horizontal serial CCD 30. The row of charge in the horizontal serial CCD 30 is shifted through a charge multiplying CCD 40 that amplifies the size of a charge packet by a factor 1 to 100. At the end of the charge multiplying CCD 40 is an output amplifier 50 that converts a charge packet into a measurable voltage.
FIG. 2 illustrates the operation of a prior art charge multiplying CCD. It consists of a repeating set of four control gates H1, H2, H3, and H4 separated from a silicon substrate 70 by a gate dielectric 60. The channel potential in the silicon substrate 70 is also drawn on FIG. 2. A large voltage is applied to gate H4 to produce a large channel potential. Gate H1 is at a low voltage to prevent two charge packets from mixing together. Gate H3 is held at a constant intermediate voltage while the voltage on gate H2 is decreased to a lower voltage. As the gate H2 voltage decreases the channel potential under gate H2 also decreases. This pushes the charge packet from under H2 through the gate H3 region where electrons enter the large channel potential region under gate H4. The large channel potential difference between gates H3 and H4 creates a large electric field in the silicon that accelerates the electrons to high enough energy to liberate additional electrons 80 from the silicon lattice. This is often called avalanche charge multiplication. This effect has a long history and details regarding its use in CCDs may be found in U.S. Pat. Nos. 4,912,536; 5,337,340; 5,656,835; 6,278,142; and 6,444,968; as well as US patent publications 2002/0126213A1; 2002/0191093A1; 2003/0035057A1; 2003/0042400A1; 2003/0223531A1; and 2004/0150737A1.
The common aspect of all the prior art is the requirement that a CCD be specially designed to implement a charge multiplying CCD. The present invention described hereinbelow shows how to operate an existing CCD as a charge multiplying CCD. The invention applies to commercially available CCD image sensors such as Eastman Kodak Company part numbers KAI-2020, KAI-2093, and KAI-4021. Use of existing image sensors as charge-multiplying CCDs costs less because there is no additional design or development required.