As CMOS image sensors have migrated from low-end applications to multimegapixel cameras, emphasis has shifted from integrating digital circuits to the fundamental design of the pixel itself.

During the operating, or integration, mode, visible photons generate electrons in the pixel cell of a CMOS image sensor. The pixel collects the electrons and then translates the accumulated charge to voltage signals that serve as an output. Expressing this process quantitatively, pixel responsivity is the relation between the electrical signal and its exposure to light (V/lux×sec). “Full-well capacity” is the number of electrons that the pixel can collect. Dynamic range is roughly the number of resolvable gray levels, which you calculate by dividing the maximum signal by the minimum detectable signal and usually report in decibels.

Thermally generated electrons are obviously enemies of image sensors, because the pixel collects them along with optically generated electrons; they steal well capacity and add noise. Thus, the rate of the electrons’ thermal generation within the pixel, or “dark current,” is a major performance measurement that you generally report in electrons per second. Pixel development requires simultaneous optimization of all of the above parameters, along with a few additional ones-a major challenge within the world of pixel design.

Inside each pixel are a light-sensitive diode and at least three transistors. As resolution increases, transistors must be very small to allow as much area as possible where the diode can collect light. That necessity requires advanced CMOS processes. But in processes of 180 nm or less, transistors need strong well implants and shallow source-and-drain implants that cause higher junction leakage and eventually increase the dark current in the CMOS pixel array. Also, STI (shallow-trench isolation) between the transistors causes stress and increases the number of defects within the silicon, which again gives rise to higher dark current. To understand the complexity of this problem, explore the operation of a pixel and then look at optimizations.
Operating principles

Figure 1 shows a pixel with three transistors and a photo diode. A photon hits the silicon, generating an electron and a hole. This electron-hole pair travels together in the silicon until it encounters an electric field, in which the electron moves toward the higher voltage, and the hole migrates to ground. Within the CMOS pixel, a photo diode-usually an N-type implant in a P-type region-generates the separating electric field.

The photo electrons accumulate in the diode and cause a drop in voltage that is proportional to their number. The number of photo electrons is, in turn, proportional to the photon flux. Thus, the voltage drop in the diode is proportional to the photon flux. To increase this signal pixel, designers employ quantum efficiency-increasing the number of electrons for a given photon flux. They also try to increase the voltage drop on the diode for a given charge. This value is the “pixel-conversion gain,” and it is inversely proportional to the photo diode’s capacitance.

You optimize the quantum efficiency of a photo diode by engineering it so that its electric field penetrates deeply into the silicon. This step is important for photons in the green-to-red spectrum, because photons with longer wavelengths tend to penetrate deeper into the silicon. It helps to replace the standard starting material with low-doped, P-type epitaxial material grown on P+ + substrate.

In the first generation of the three-transistor pixel, designers made the diode by implanting the regular PMOS N well into the P– – substrate (Figure 2a). Another common approach was to use the N+ + of the NMOS-transistor source-and-drain implant. Although these options are fully compatible with the regular CMOS process, the resulting diode exhibits poor performance. For example, using a large N-well diode, which you need for good collection of photo electrons, causes the pixel capacitance to be excessive. Thus, the overall efficiency of translating photon flux to voltage drops becomes lower than necessary.