Photoconductive detectors noise

Hg Cd Te is an example of a ternary detector, in which the value of x controls the cutoff wavelength. Photoconductive detectors are generally simpler to couple to low noise amplifiers photodiodes generally have lower power consumption because these have no external bias, and better high frequency performance (15,16). 

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. 

At a difference with thermal detectors, the background noise of photoconducting detectors is frequency-dependent. If it is assumed that the photoconductor is used to detect radiation at a frequency just above its cut-off frequency z/c, the detectors with a cut-off in the near IR display a much smaller background noise than those with a cut-off at lower energies. This is because in the near IR, the black body emissivity contribution at room temperature and below is very small. 

Besides the BRN, there are additional sources of noise due to the physical nature and operation method of the detectors. Most bolometers and photoconducting detectors are basically resistors, and they display at their terminals voltage fluctuations due to the random motion of the electric charges within this resistor. The corresponding noise is called Johnson noise or thermal noise. The voltage fluctuations Vn at the terminal of a resistor R at temperature T for an electrical band width A/ is ... 

Figure 16.27 in practice approximates the error only for instruments with Johnson or thermal noise-limited detectors, such as photoconductive detectors like CdS or PbS detectors (400 to 3500 nm) or thermocouples, bolometers, and Golay detectors in the infrared region. Johnson noise is produced by random thermal motion in resistance circuit elements. 

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])...

Generation-recombination (gr) noise and Johnson noise are the fundamental mechanisms in photoconductive detectors. The total noise current is given by... 

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

Heterodyne detectors in the microwave and millimeter regions (hv< kT) include square-law mixers such as the crystal diode detector [7.93], the InSb photoconductive detector [7.94-96], the Golay cell [7.95], the pyroelectric detector [7.95], the metal-oxide-metal diode, and the bolometer [7.87]. The latter three types of detectors have also been used successfully in the middle infrared (at 10.6 pm) [7.97-100]. For this type of detector Johnson noise generally predominates, and the input SNR is given by [7.100]... 

For our present consideration g-r noise is the most important noise mechanism. The g-r noise spectmm is flat (white) up until the cutoff frequency, approximately given as the reciprocal value of free carriers lifetime. For the component of this noise not connected with illumination in a case of an ideal photoconductive detector one may write [7]... 

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. 

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,... 

Consider first the simple extrinsic photoconductor. Here the sample is a semiconductor containing a single impurity level, the source of the free electrons (or holes) present in the sample. Thus the fluctuation in the number of the free carriers arises from the fluctuation in the generation and recombination rates through that level. If it is assumed that the temperature is so low that very few of the extrinsic centers are thermally ionized (which is valid for most extrinsic cooled photoconductive infrared detectors), then the short circuit g—r noise current and the open circuit g — r noise voltage which appear only in the presence of a bias current Ig, are given by... 

On the other hand, photoconductive devices in principle respond only to the number of photoexcited carriers, regardless of where they are generated within the material. Thus they will receive equal contributions of background noise from both hemispheres if both are at the same temperature. This will cause another reduction of Df and D (7 ) by the square root of two. Photoemissive detectors having translucent photocathodes will be sensitive also to radiation from both hemispheres. Those with opaque photocathodes will not. 

---