in the p-region crossing into the n-region, combiningwith electrons and producing positively-charged impurity atoms [Book 4]. The net resultof these movements is the disappearance of conduction electrons and holes from thevicinity of the junction and the establishment there of a reverse electric field, which ispositive on the n-side and negative on the p-side. This reverse field plays a vital part in thefunctioning of the device. The area in which it is set up is called the "depletion area" or"barrier layer".When light falls on the front surface, photons with energy in excess of theenergy gap (1.1 eV in crystalline silicon) interact with valence electrons and lift them tothe conduction band. This movement leaves behind holes, so each photon is said togenerate an "electron-hole pair". In the crystalline silicon, electron-hole generation takes place throughout thethickness of the cell, in concentrations depending on the irradiance and the spectralcomposition of the light. Photon energy is inversely proportional to wavelength. Thehighly energetic photons in the ultra-violet and blue part of the spectrum are absorbedvery near the surface, while the less energetic longer wave photons in the red and infraredare absorbed deeper in the crystal and further from the junction. Most are absorbed withina thickness of 100 m. The electrons and holes diffuse through the crystal in an effort toproduce an even distribution. Some recombine after a lifetime of the order of onemillisecond, neutralizing their charges and giving up energy in the form of heat.Othersreach the junction before their lifetime has expired. There they are separated by the reversefield, the electrons being accelerated towards the negative contact and the holes towardsthe positive. If the cell is connected to a load, electrons will be pushed from the negativecontact through the load to the positive contact, where they will recombine with holes.This constitutes an electric current. In...
