Monolithic Cmos Sensors

Monolithic CMOS sensors are image sensor chips that combine the photodetectors and the readout circuitry on the same piece of silicon. Compared to hybrid FPAs, monolithic arrays are less expensive to manufacture, but the sensitivity is limited to the visible and near infrared wavelength range. Over the course of the last decade, monolithic CMOS technology has made significant improvements. In recent years, a large number of consumer products, including cell phones, digital cameras, and camcorders, have been replacing CCDs with CMOS sensors for the sake of smaller, cheaper, and lower-power systems. A monolithic CMOS sensor is also called an APS.

Although a wide variety of pixel designs exist, most sensors use one of the two circuits shown in Fig. 7. The circuit on the left side consists of three transistors (3T): a reset field-effect transistor (FET), a selection switch, and a source follower for driving the signal onto the column bus. The right side of Fig. 7 displays a pixel with four transistors (4T) and a pinned photodiode. Three of the FETs have the same function as in the 3T pixel. The fourth transistor works as a transfer gate that moves charge from the photodiode to a floating diffusion. Usually, both pixels operate in rolling shutter mode. The 4T pixel is capable of performing CDS to eliminate the reset noise (kTC noise) and the pixel offsets. The 3T pixel can only be used with noncorrelated double sampling (DS), which is sufficient to reduce the pixel-to-pixel offsets but does not eliminate the temporal noise. However, temporal noise can be addressed by other methods like soft reset or tapered reset.

Figure 7. Diagram of two common monolithic CMOS pixel circuits: (left) three transistors with regular photodiode and (right) four transistors with pinned photodiode.

In a monolithic CMOS sensor, the photodiodes share the pixel area with the transistors. For that reason, the fill factor is always less than 100%. In addition, most CMOS imagers are front side illuminated. This limits the sensitivity in the red because of a relatively shallow absorption material. A typical quantum efficiency (QE) plot for a monolithic CMOS sensor with microlenses can be seen in Fig. 8 [3]. The curve corresponds to a 3T pixel with roughly 50% fill factor. For comparison, the same figure shows two QE plots of silicon PIN detectors. Because they are hybrid FPAs with a dedicated detector layer, the thicker material and 100% fill factor result in much improved red response.

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