Photovoltaic detectors noise

The noise is expressed as noise density in units of V/(Hz), or integrated over a frequency range and given as volts rms. Typically, photoconductors are characterized by a g-r noise plateau from 10 to 10 Hz. Photovoltaic detectors exhibit similar behavior, but the 1/f knee may be less than 100 Hz and the high frequency noise roU off is deterrnined by the p—n junction impedance—capacitance product or the amplifier (AMP) circuit when operated in a transimpedance mode. Bolometers exhibit an additional noise, associated with thermal conductance. 

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

Typical detectivity values as a function of wavelength for PbS photoconductive and various photovoltaic detectors. is a figure of merit defined as A /NEP, where A is the detector area and NEP is the noise-equivalent power, the rms radiant power in watts of a sinusoidally modulated input incident on the detector that gives rise to an rms signal equal to the rms dark noise in a 1-Hz bandwidth. Data from Hughes Aircraft Company. 

Fig. 2.15. Photon noise limited Df at peak wavelength (assumed to be cutoff wavelength), for a photovoltaic detector for selected background temperatures Tg. Values for a cooled photoconductive detector are 0.71 times those shown. (Assumes 27t steradian field of view and >/ = 1) (after Jacobs and Sargent [2.160])...

The detectivity of a detector is limited by noise mechanisms. Shot noise is the fundamental mechanism in photovoltaic detectors. The shot noise current is given by [4.4]... 

We divide the detector stmcture into a large number of layers sufficiently thin to permit us to regard g-r rate and photoelectric gain as constant. To one such layers with a thickness dy and located at a position y we apply the standard expression for the g-r noise of photoconductors (1.88). For a photovoltaic detector (1.89) is used in the same manner. Thus, we obtain the increase of noise current due to the processes taking place in the layer itself... 

Detectors Detector choice is crucial in a RAIRS experiment. The sensitivity of a detector is characterized by the noise equivalent power (NEP WHz / ), defined as that amount of radiant power which must fall on the detector to give a root mean square (rms) electrical signal equal to the rms value of the detector noise. The lower the NEP, the more sensitive is the detector. Generally, pho-toconductive or photovoltaic detectors are utilized for RAIRS experiments, such as liquid-nitrogen-cooled InSb, PbSnTe, and PbSe with a typical spectral range of 5000—1400cm and NEP values of WHz / and the widely... 

This graph summarizes the wavelength response of some semiconductors used as detectors for infrared radiation. The quantity D (X) is the signal to noise ratio for an incident radiant power density of 1 W/cm and a bandwidth of 1 Hz (60° field of view). The Ge, InAs, and InSb detectors are photovoltaics, while the HgCdTe series are photoconductive devices. The cutoff wavelength of the latter can be varied by adjusting the relative amounts of Hg, Cd,... 

The expressions (4.10) and (4.12) for the noise currents of photovoltaic and photoconductive detectors are both of the form... 

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