Photoconductive Diodes

When a photodiode is illuminated, its electric resistance decreases from a dark value to a value Rj under illumination. In the circuit, shown in Fig.4.77, the change of the output voltage is given by 

The time constant of the photoconductive diode is determined by r RC, where C is the capacitance of the diode plus the input capacitance of the 

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The time constant of the photoconductive diode is determined by r > RC, where C is the capacitance of the diode plus the input capacitance of the circuit. Its lower limit is set by the diffusion time of the electrons on their way from the p-n junction where they are generated to the electrodes. Detectors from PbS, for example, have typical time constants of 0.1-1 ms, while InSb... 

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

The lead compounds PbS, PbSe, PbTe are narrow-gap semiconductors that have been widely investigated for infrared detectors, diode lasers, and thermo-photovoltaic energy converters. Their photoconductive effect has been utilized in photoelectric cells, e.g., PbS in photographic exposure meters. Integrated photonic devices have been fabricated by their heteroepitaxial growth on Si or III-V semiconductors. 

Due to the small size and applied bias voltage (when used in photoconductive mode) the response time of each diode element in the PDA can be very fast indeed, so total integration times can also be short, hi principle, data rates of 200 spectra per second are achievable, but in reality spectral co-adding is a more sensible objective, to improve overall SNR. 

The photoconductivity of polysilanes was described in Section 5.8, and their electroluminescence is covered in Section 5.9.2. These properties make polysilanes possible components of polymer light-emitting diodes, either as charge transport layers or as the actual emissive materials.146 A drawback of the polysilanes is their photodegradation under ultraviolet irradiation, a problem which must be overcome if polysilanes are to become commercially useful. 

PtSi arrays are sensitive over the 1.0-5.0 p,m wavelength range. These are silicon-based arrays in which the platinum-silicon junction in each pixel forms a Schottky barrier diode. Large, exceptionally uniform arrays can be made from this material, but the quantum efficiency is < 1% (D = 3 x 1010 cm I Iz0 5 W-1), making such arrays unsuitable for low light level applications. Photoconductive lead sulfide (PbS) arrays with 320 x 240 pixels are also available for operation in the 1.0-3.0 un range, with reported D values up to 3 x 1011 cm Hz0 5 W 1 if cooled to 200 K. 

The measurement system employed to observe PCA signals is shown in Fig. 2. The N2 laser with a pulse width of Ins and a spot size of 3x5 mm irradiated the surface of the silicon wafer sample to generate excess carriers. The sample placed apart om the aperture of the WG-10 waveguide by several millimeter was irradiated by the mm-wave of 100 GHz. PCA signals as photoconductivity variation detected with the diode from the reflected mm-wave were observed directly using the fast digitalizing oscilloscope (Lecroy, 6120) with 5G-sampling and 350 MHz -bandwidth. 

There are numerous techniques to measure the recombination lifetime. Some of the better known are photoconductive decay (13). diode reverse recovery (14). diode open circuit voltage decay (15). surface photovoltage (JL ) and forward-biased pn junction I-V characteristic (17. I will describe one particular photoconductive decay method, because it is a relatively new, non-contact method that requires no junctions. This makes it very suitable for a large number of measurements as for a process sequence characterization tool. 

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