Dissertation - FTP Directory Listing - University of Surrey

There are 3 main types of semiconductor; the first is an intrinsic semiconductor with no impuritiesthat at finite temperature has equal concentrations of electrons and holes. By adding impuritiesknown as dopants the electron hole concentrations can be dramatically altered. By adding donordopants (such as phosphorous) the electron concentration is increased giving an N typesemiconductor. Acceptor dopants (such as boron) increase hole concentration giving a P typesemiconductor. In a doped semiconductor the abundant form of charge carriers are called themajority charge carriers, but there still remains a small background level of minority charge carriers.As already mentioned electron hole pairs can also be generated by photons providing they haveenergy equal or greater than Eg. So there is a maximum detectable wavelength given by:hcλc=(2.1)EgWhere:λcis the cut off wavelengthh is Plancks constantc is the speed of lightFor silicon, the base material of image sensors, λ c is approximately 1.104µm, in the near infrared. Asthe photon wavelength approaches this cut off the probability it will be absorbed decreases rapidlyas the photon does not provide enough excess energy to conserve momentum in the process. Itmust then couple with a phonon in order to complete a direct transition between bands. Theprobability of absorption depends on wavelength and is described by the absorption coefficient αwhich is defined as the ratio of the decrease of light power ΔP/P when the light travels a distance Δzin the semiconductor. Where = 1 ∆∆ (2.2)4