Photoconductive device

The deposition temperature range is 300-500°C, the partial pressure of the alkyl is 0.5-2.5 Torr, and that of THF is 20-80 Torr. ZnO has found applications in piezoelectric devices, transducers, coatings for photoconductive devices, and non-linear resistors (varistors), and overvoltage protectors. 

By the sol-gel-process, inorganic glassy and hybrid polymeric materials are accessible at comparatively low temperatures [1], Therefore, organic molecules or dyes can easily be incorporated into the oxide matrix. This combination is especially attractive for the development of the following devices optical filters, solid-state lasers, optical switches, nonlinear optical laser hosts, optical data storage media, and photoconductive devices and films [2]. 

When a semiconductor is bombarded with photons equal to or greater in energy than the band gap, an electron-hole pair is formed. The current that results is a direct function of the incident light intensity. Photoconductive devices consist of a p-n junction called a photodiode, or a p-i-n junction commonly used in a photodetector, an... 

The chances of synthesizing good dye photoconductors for use in photoconductive devices can be deduced from certain structural relationships (details are discussed in 10>. 

All the photoconductive devices need to be operated in conjunction with amplifiers, and ac amplification of a chopped signal is most satisfactory. The author also finds chopping the radiation and ac amplification followed by phase-sensitive detection to be the best way of amplifying photomultiplier signals, and it may be of interest to describe the system... 

There are a few complications to this simple picture. First it has been assumed the conduction band is normally empty of electrons. This is never entirely true, and for many semiconductor materials used in photoconductive devices with reasonably large values of Aj, it is not even approximately true. Where kT is an appreciable fraction of AEg there will be significant thermal promotion of electrons into the conduction band, and a consequently high detector dark current. For this reason, many photoconductive detectors are cooled, often using thermo-electric devices (for easy packaging) in order to reduce kT, and hence the dark current. 

AS2S3 has found use in the manufacture of glass, especially infrared transmitting glass, in pyrotechnics and, because it is a semiconductor, in photoconducting devices. 

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,... 

The synthesis of fullerene-containing polymers is noteworthy for several reasons. On the one hand, once Cgg is attached to a polymer, most of the fullerene properties are transferred to the polymer. Thus, for instance, electroactive and photoactive polymers or polymers with nonlinear optical properties can be prepared. On the other hand, hardly processible fullerenes embedded in highly soluble polymers may become more easily amenable to further treatments. The resulting materials might eventually be used for surface coating, photoconducting devices, or to create new molecular networks. 

Park, J.Y., et al. 1998. Doping effect of viologen on photoconductive device made of poly(p-phenylenevinylene). Appl Phys Lett 72 2871. 

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. 

Photodiodes are doped semiconductors that can be used as photovoltaic or photoconductive devices. When the p-n junction of the diode is irradiated, the photovoltage Vph is generated at the open output of the diode (Fig. 4.78a) within a restricted range it is proportional to the absorbed radiation power. Diodes used as photoconductive elements change their internal resistance upon irradiation and can therefore be used as photoresistors in combination with an external voltage source (Fig. 4.78b). 

An important performance parameter of an optical detector at a given wavelength is the external quantum efficiency It is defined as the ratio of the number of electrons generated to the number of incident photons before any photogain occurs. Here rj takes into account the surface reflection loss and other losses in the detector. In general, rj depends on the absorptivity of the materials and the dimensions of the absorption region. The photogain M can result from carrier injection in semiconductor materials, as in the case of photoconductive devices (see Sec. 9.4.3), or from impact ionization, as in the case of avalanche photodiodes (see Sec. 9.4.2). 

For many signal-processing applications, the ratio of the on-resistance to the off-resistance of the photoconductive device is an important parameter, although the off-resistance is primarily limited by the material resistivity in the dark. The on-resistance depends on the carrier mobihty and the excess carriers generated. 

Because of the low surface recombination velocity of InP-related materials, planar photoconductive devices on iron-doped semi-insulating InP have been extensively studied. This material has a room temperature resistivity of 10 -10 f2-cm and an electron mobility of 1500-4500 cm /V-s depending on the crystal quality and iron concentration. The carrier Hfetime ranges from less than 1 ps to 3 ns. 

A photoconductive device can function as a current switch. However, this requires a low on-resistance. For aUoyed contact devices, the current-voltage characteristics are linear at low voltage, and the current tends to saturate atincreased voltage. The current saturation is believed to be causedby electrons transferred from a high-mobility valley to a low-mobility valley, especially at the contact region where the electric field is relatively strong. 

A fluorescent dye, perylenetetracarboxydiimide, (DBu-PEDI, Fig. 34) is known to possess valuable functionalities as organic photoelectric mutual conversion devices (solar cell [98,99] and electroluminescence [100]), layered organic photoconductive devices for electrophotography [101,102], and dye for... 

A comparison between the calculated values of specific detectivity of an exclusion detector and the BLIP values for a field of view of 180° at the same cutoff wavelength (equal to performance of a photoconductive device at 77 K) shows that all of the calculated values are significandy below the BLIP limit. The detectivity increase in comparison to a photoconductor without nonequilibrium Auger suppression (conventional sub-BLIP photoconductor) is less three times. 

The time of transit between the contact pads of a photoconductive device is determined as the ratio between the inter-electrode distance L and the product of electric field and the mobility of the considered carrier type, T = UE i. If we divide the detector length into infinitesimally small elements dy, the differential transit time across each of these elements will be At = AylE(y) t. Thus the total transit time across the detector will be... 

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