Infrared detectors in general

Frequencies associated with infrared radiation, that is, from approximately 3 x 10 to 3 X 10 Hz, cannot be detected directly by radio frequency techniques. One exception exists in the heterodyne method discussed in Section 5.9, but, in general, infrared frequencies are too high for a direct application of microwave technology. On the other hand, the frequencies and, therefore, the energies of infrared photons are too low to liberate electrons from surfaces by the photoelectric effect. Consequently, standard photomultipliers and all devices based on photoelectric phenomena are also unsuitable as detectors above about 1 m. For all practical purposes only two classes of phenomena provide infrared detection mechanisms. 

The other class of detectors is based on the effect infrared photons exert directly on electrons in semiconducting materials such detectors are called photon or quantum detectors, a somewhat unfortunate name because thermal detectors absorb photons or quanta as well. The energy of an absorbed infrared photon may not be high enough to cause emission of an electron by the photoelectric effect, but it is sometimes sufficient to lift an electron from a valence band into a conduction band, thereby altering the macroscopic properties of the material. The change in the electrical resistance (in photoconductors) or in the electrical potential (in photovoltaic elements) may then be sensed electrically. 

One characteristic parameter of a detector is the responsivity, expressed in volt watt or in ampere watt , depending on open or short-circuit operation. To avoid repetition of equations for both cases we limit the discussion to open circuit terminology, but we keep operation into a low impedance amplifier in mind. The responsivity of a detector is defined as the change in voltage, A V, measured across the output leads, while the detector is exposed to a change in radiative power, AW, 

Besides the responsivity, the noise properties of a detector are of fundamental importance. The noise is measured in volts (rms) within a specified electrical bandwidth. Since noise in one frequency interval is statistically independent of that in others, the noise power increases linearly with bandwidth and the noise voltage with the square root of the electrical passband. Therefore, the noise characteristic of a detector is expressed in units of V Hz 5. More instructive than the noise voltage per se is the noise normalized to the responsivity. Neither a high responsivity detector with excessive noise nor a low noise element that lacks responsivity is of interest. The noise normalized to the responsivity [V Hz 2 /V W ] is expressed in W Hz 2, and is called the Noise-Equivalent-Power (NEP) per root hertz. In the literature the term NEP (which has units of power, e.g., W) is often applied to the NEP per root hertz (which has units of W 2). This inconsistency is deeply embedded in the literature and we also use the term NEP for both, but we state units where needed to avoid confusion. As the responsivity, the NEP may be a function of wavenumber, the term spectral NEP, NEP or NEP is then appropriate. The NEP [watt] can also be understood as the signal power for a signal-to-noise ratio of unity. The NEP [watt] is generally a small number, the smaller the value the better the detector. The inverse of the NEP [watt] is called the detectivity, D (Jones, 1952), 

For certain types of detectors, D is inversely proportional to the root of the detector area and the term D is more characteristic of this class of detectors it allows comparison of detector quality independently of size (Jones, 1959). Expressing it in units of root hertz, 

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