Photogeneration – Physics Underlying Image Sensors

Microelectronic image sensors used in digital still cameras, such as CCD and CMOS, rely on electron generation by incoming photons to detect light. We want to give a deeper insight to the physics underlying this phenomenon.

Photons Collide against the Image Sensor

Incident photons can break the covalent bonds holding electrons at atomic sites in the lattice, provided that the photon energy is sufficient. This is what happens when we press the shutter release button of our camera. Light of the scene we are shooting strikes the image sensor. Image sensors are made of silicon, as all other integrated circuits. Once the covalent bond has been broken, the freed electron is able to move through the semiconductor crystal. This process is called "photogeneration". In terms of the energy-band structure, this is equivalent to exciting electrons from the valence band into the conduction band.

Sensors Are Sensitive to Infrared Radiation

For the incident photon to be able to do this, it must possess an energy equal or greater than the bandgap energy, that is the energy gap between the valence and the conduction bands. The band gap in silicon with no voltage applied and at ambient temperature is 1.124eV. This corresponds to the far infrared portion of the electromagnetic spectrum, at a wavelength of 1.10 microns. So now we know that sensors used in digital still cameras are sensitive to infrared radiation. As a photographer does not usually want to capture this part of the spectrum, a lens is necessary in order to filter out infrared radiation before the light reaches the sensor. All cameras are equipped with such a filter. Those digital cameras, permitting infrared photography, just have the option to internally remove the filter away.

Absorption Coefficient

The radiation incident on the semiconductor surface is absorbed as it penetrates into the crystal lattice. The equation describing this process is
I(x) = Io exp(-ax)
where "Io" is the energy reaching the surface of the semiconductor (the sensor), "x" is the depth in the semiconductor and "a" is a coefficient called "absorption coefficient". As the exponential expression always implies, the absorption is very strong, so that photons are readily absorbed as they enter into the sensor. The absorption coefficient is a strongly decreasing function of photon wavelength. As an order of magnitude, high-energy ultraviolet radiation penetrates about 10nm into silicon before decaying appreciably, while infrared light penetrates about 100 microns, i.e. 10000 times deeper. Absorption of photons with energies higher than the band gap is almost entirely due to the generation of electrons.