Photodiodes dark current

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...

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 experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

In one of the most common types of photodiodes used for time-resolved work, the p-i-n photodiode (see Figure 12.24), the depletion layer thickness (i for intrinsic) is fabricated to obtain this optimum performance. Manufacturers usually give full specification sheets detailing, active area, time/frequency response, responsivity amps/watt (AAV) at a given wavelength, dark current, depletion layer capacitance, and bias volts such that with minimal external electronics devices can be made operative. 

We now turn to photoconductive and photodiode detectors, both of which are semiconductor devices. The difference is that in the photoconductive detector there is simply a slab of semiconductor material, normally intrinsic to minimize the detector dark current, though impurity doped materials (such as B doped Ge) can be used for special applications, whereas by contrast, the photodiode detector uses a doped semiconductor fabricated into a p-n junction. 

Figure 11.13 —Schematic am optical path showing the principle and simplified view of a diode array spectrophotometer. The shutter is the only mobile piece in the assembly, allowing subtraction of the background signal (dark current) without any light intensity striking the photodiodes. This inverted optical design allows the sample to be exposed to the exterior light. These instruments are widely used as detectors in liquid chromatography (cf. 3.7).

In order to suppress dark current, the read-out structure of JP-A-2272766 is formed on a semiconductor layer having a larger energy bandgap than the energy bandgap of a semiconductor layer in which photodiodes are formed. 

Because dark current limits the integration times obtainable at room temperature, Peltier cooling (to -150C) is used to reduce thermal population of the conduction band. In contrast to the silicon vidicon and the SIT [vide infra], where the presence of intense radiation may bloom out the entire sensor, blooming is greatly reduced with photodiode arrays even when intense lines saturate individual diodes. 

A spectrum in a specified ranalogue signals from eadi photodiode are digitised and transferred to a computer, where they e corrected for dark current response and transformed to absorbance. A number of digital techniques are available to increase sensitivity and to extend the use of rapid-scanning detectors to multicomponent analysis, reaction kinetics, tablet dissolution tests, process control, and detection in HPLC (A. F. Fell et al, Chrom-atographia, 1982, 16, 69-78). 

The dynamic range of PDA s is limited by the dark current in the photodiodes and by the maximum charge that can be stored in the cells. Dynamic ranges of up to... 

The response of photomultipliers, photodiodes and diode arrays is generally linear in light flux within a specified range. Within this range, the readings will be proportional to the transmittance T of the sample. Noise associated with the dark current is commonly negligible, because integration times are short. Under these circumstances, the relative... 

One of the parameters characterising the EELS detection system is its Detective Quantum Efficiency, DQE, defined as the ratio of the number of counts to the mean square fluctuation in them. A detection system is said to have unit DQE if it is shot noise limited, i.e. the mean square signal variation in a chaimel is equal to the number of counts within it. However, channel-to-channel gain variations in photodiode arrays, dark current, and detector noise... 

State-of-the-art p-n 6H-SiC photodiodes have exhibited quantum efficiencies higher than 80 % with responsivities of up to 175 mAW 1 with dark currents as low as 10 11 A cm"2 at -1V at 473 K. The dark current increased to approximately 10"9 to 10 A cm 2 at 623 K. These low values of the dark current are still higher than those in rectifier SiC diodes [10,11] pointing... 

State-of-the art Cree Research, Inc. UV SiC photodiodes are more sensitive than commercial Si photodiodes. The reverse dark current at room temperature in these devices is less than 1 pAcm 2 and a few nAcm 2 at 400 °C. 

---