| Microelectronic image sensors used in digital still | | | | sensors used in digital still cameras are sensitive to |
| cameras, such as CCD and CMOS, rely on electron | | | | infrared radiation. As a photographer does not usually |
| generation by incoming photons to detect light. We | | | | want to capture this part of the spectrum, a lens is |
| want to give a deeper insight to the physics | | | | necessary in order to filter out infrared radiation |
| underlying this phenomenon. | | | | before the light reaches the sensor. All cameras are |
| Photons Collide against the Image Sensor | | | | equipped with such a filter. Those digital cameras, |
| Incident photons can break the covalent bonds | | | | permitting infrared photography, just have the option |
| holding electrons at atomic sites in the lattice, | | | | to internally remove the filter away. |
| provided that the photon energy is sufficient. This is | | | | Absorption Coefficient |
| what happens when we press the shutter release | | | | The radiation incident on the semiconductor surface is |
| button of our camera. Light of the scene we are | | | | absorbed as it penetrates into the crystal lattice. The |
| shooting strikes the image sensor. Image sensors are | | | | equation describing this process is |
| made of silicon, as all other integrated circuits. Once | | | | I(x) = Io exp(-ax)where "Io" is the energy reaching |
| the covalent bond has been broken, the freed | | | | the surface of the semiconductor (the sensor), "x" is |
| electron is able to move through the semiconductor | | | | the depth in the semiconductor and "a" is a |
| crystal. This process is called "photogeneration". In | | | | coefficient called "absorption coefficient". As the |
| terms of the energy-band structure, this is equivalent | | | | exponential expression always implies, the absorption |
| to exciting electrons from the valence band into the | | | | is very strong, so that photons are readily absorbed |
| conduction band. | | | | as they enter into the sensor. The absorption |
| Sensors Are Sensitive to Infrared Radiation | | | | coefficient is a strongly decreasing function of photon |
| For the incident photon to be able to do this, it must | | | | wavelength. As an order of magnitude, high-energy |
| possess an energy equal or greater than the | | | | ultraviolet radiation penetrates about 10nm into silicon |
| bandgap energy, that is the energy gap between the | | | | before decaying appreciably, while infrared light |
| valence and the conduction bands. The band gap in | | | | penetrates about 100 microns, i.e. 10000 times |
| silicon with no voltage applied and at ambient | | | | deeper. Absorption of photons with energies higher |
| temperature is 1.124eV. This corresponds to the far | | | | than the band gap is almost entirely due to the |
| infrared portion of the electromagnetic spectrum, at | | | | generation of electrons. |
| a wavelength of 1.10 microns. So now we know that | | | | |