CMOS Image Sensors with extended Spectral Range

摘要

It has to be seen as a lucky coincidence that the mainstream material used in the semiconductor industry is sensitive to radiation in the visible spectrum. Silicon based solid-state sensors domine the imaging sensor applications over many years. Initially Charge Coupled Devices CCD's where the sensor architecture by most used in imaging applications. More recent are commercial applications with Active Pixel Sensors APS, which can be implemented in standard and mainstream CMOS technologies. CMOS image sensors take an ever-growing market share due to capability of higher integration, lower power and higher resistance to over exposure. The physical principles behind the sensor in both cases are the same. Incident radiation excites electrons over the band gap and generates free charge carriers in the semiconductor. The electrical field generated in a depletion region in the semiconductor collects the free charge carriers. CMOS Image sensors use typically a reverse biased photodiode to generate the depletion region and to separate the free charge carriers. The 1.1 eV band gap of silicon permits absorption and detection of radiation in the visible and near IR region. The penetration depth of visible radiation is in the order of micrometers. The spectral response of silicon detectors is limited towards the infrared by the excessive increase of penetration depth as the photon energy approaches the band gap energy on the IR side, and by the recombination in surface states of charge carriers generated close to the surface of the silicon for radiation towards the UV.