Dark currents

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

The number of integrated carriers, iV, is QA-Iwhere q is the electron charge. Because dark current, is a combination of thermal excitation processes, neglecting avalanche and tunneling, ideal performance occurs when the photon-induced current density Jp is greater than Fluctuations of N are the... 

As time progresses, charge is generated by photon currents, semiconductor dark currents, diffusion, depletion, surface, avalanche, and tunneling. The time required to fill the well by dark currents is the storage time, given by the following. 

Under Httle or no illumination,/ must be minimized for optimum performance. The factor B is 1.0 for pure diffusion current and approaches 2.0 as depletion and surface-mode currents become important. Generally, high crystal quality for long minority carrier lifetime and low surface-state density reduce the dark current density which is the sum of the diffusion, depletion, tunneling, and surface currents. The ZM product is typically measured at zero bias and is expressed as RM. The ideal photodiode noise current can be expressed as follows ... 

HgCdTe photodiode performance for the most part depends on high quantum efficiency and low dark current density (83,84) as expressed by equations 23 and 25. Typical values of at 77 K ate shown as a function of cutoff wavelength in Figure 16 (70). HgCdTe diodes sensitive out to a wavelength of 10.5 p.m have shown ideal diffusion current limitation down to 50 K. Values of have exceeded 1 x 10 . Spectral sensitivities for... 

Fig. 16. Resistance area (R ) product for HgCdTe photodiodes cooled to 77 K. The soHd line represents the theoretical limit, the dashed lines (—) and (- -) high and low performance, respectively. Dark current caused by defects lowers R and detector sensitivity. In the high performance range dark...

Fig. 5. Dark(current—voltage) characteristics ofp—n ( ) andp—i—n (A) junctions at 295 K.

The latter mainly results from the thermal emission current. The dark current is apparent mainly in the long-wavelength range of the spectrum when the photocurrent is appropriately small [53, 54, 131]. It is relatively small for alloy cathodes (e.g. Sb-Cs cathodes), but not small enough to be negligible. 

J. C. Carrano, P. A. Grudowski, C. J. Biting, R. D. Dupuis, J. C. Campbell. Very low dark current metal-semiconductor-metal ultraviolet photodetectors fabricated on single-crystal GaN epitaxial layers. Appl Phys Lett 70 1992, 1997. 

The electrical current of a coplanar interdigilal gold/LPPP/gold device is space charge limited due to p-type charge earner traps localized in the bandgap [28]. This can be inferred from the field dependence of the dark current at room temperature. The thermally stimulated current spectrum exhibits two peaks, corresponding to two distinct trap levels ,1 and ,", which can be calculated from the rise in current, /, below the peak temperature ... 

Figure 15-12. Spectrally resolved pliotocurrent of a ITO/PPV/Mg photodiode at dilTcrcnt bias after correction for dark current, light source, and monochromator response, and normalization to the same peak value. The broken line is the normalized absorption spectrum of PPV (reproduced by permission of the Institute of Physics from Ref. 143)).

The splitting and recombination of the beam is accomplished by means of two rotating sector mirrors which are geared to the same electric motor so that they work in unison (Fig. 17.12). The microprocessor which is used to operate such an instrument will automatically correct for the dark current of the photocell, i.e. the small current which passes even when the cell is not exposed to radiation. 

Thermoelectrical cooling of the photomultiplier tube at about — 30°C reduces the dark noise current to a very low level. However, as the quantum efficiency of the S-20 type decreases as rapidly as the dark current in the red region, cooling brings only modest increases in the signal-to-noise ratio 23). 

Detect 100% of photons Photon detected as a delta function Large number of pixels Time tag for each photon Measure photon wavelength Measure photon polarization No detector noise fr Up to 99% detected fr One electron for each photon fr Over 377 million pixels 0 No - framing detectors 0 No - provided by optics 0 No - provided by optics 0 Readout noise and dark current... 

Detector noise - The two most signihcant noise sources of a detector are readout noise and dark current. 

Dark current comes from the thermal excitation of electrons in the detector material - thermally generated electrons can not be distinguished from photoelectrons. 

Photocells The basic construction of a photocell is illustrated in Figure 17. A photocurrent flows when the photocathode is illuminated, this is proportional to the intensity of illumination if the supply potential has been chosen to be higher than the saturation potential. A minimal potential is required between the photocathode and the anode in order to be able to collect the electrons that are emitted. The sensitivity is independent of frequency up to 10 Hz. The temperature sensitivity of evacuated photocells is very small. The dark current (see below) is ca. 10 " A[l]. 

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