Photoconductor quantum efficiency

Calculate the current induced in a photodiode with an inhinsic quantum efficiency of 0.90 when it is illuminated at room temperature with a 0.35 mW light beam whose wavelength is 1140 nm. The photodiode is working in the photoconductor regime, and in the absence of illumination no electrical current is generated by this photodiode. What happens if the photodiode is cooled down to 5 °C ... 

Photovoltaic and photoconductive phenomena for various types of CT complexes between saturated polymers and dopant molecules, heterojunctions between polymers and organic and inorganic photoconductors were also investigated in the last few years [86-92]. The quantum efficiency of the energy conversion of 10-3% was obtained for such systems and output power density of 3 x 102 mV cm-2. The mobilities of the heterogeneous polymer systems with despersed inorganic photoconductors reach the value — 10 3-10-4 m2 V 1s 1. 

Currently, a photoconductor does respond to the number of photons that produce an electronic excitation in the detector. When defining qv as the photon quantum efficiency at frequency v and vq as the frequency of interest, it can be shown (see [15] for the derivation) that if the photodetector temperature is much less than the temperature T of the surroundings, the radiation background NEP for a photoconductor is given by ... 

Semiconductor detectors use the intemal photo effect". That means that the photons generate electron-hole pairs inside the semiconductor. Theoretically the internal photoeffect works with a quantum efficiency of 1. In practice the quantum efficiency of a good silicon photodiode reaches 0.8 around 800 nm. In photodiodes and photoconductors an electrical field separates the electrons and holes, so that a photocurrent flows through the device when it is illuminated. Of course, the photocurrent caused by a single electron-hole pair is far too small to be recorded directly. Single photons can therefore be detected only if the semiconductor detec-... 

The quantum efficiency for carrier generation in organics is strongly field-dependent and increases with the applied field. A theory developed by Onsager (30) for the dissociation of ion pairs in weak electrolytes under an applied field has been found to describe reasonably well the temperature and field dependence of the photogeneration efficiency in most of the organic photoconductors (3J). 

Condition 4 provides the highest possible BLIP detectivity by requiring that the quantum efficiency approach its maximum value of unity. This condition is easily met by relatively thin photovoltaic and intrinsic photoconductive detectors. However, it is a major problem for extrinsic Si photoconductors, because limited maximum values of dopant concentrations and absorption cross sections give rather low absorption coefficients, requiring undesirably thick detectors for high quantum efficiencies. 

The use of an extrinsic photoconductor with direct injection has the advantage that dc gain can enhance I and therefore g, but gain saturation due to sweepout will limit ac gain to 1/2 at frequencies near f [6.30]. The capacitance C of an extrinsic photoconductor can be an order of magnitude lower than for a photodiode, which will lead to a higher /. With an extrinsic photodetector, crosstalk problems must be considered for the detector thicknesses necessary to provide reasonable quantum efficiency, and even for the 3-5 pm window operating temperatures will tend to be below 50 K. 

The photoconductor, as shown in Fig. 7, depends upon the creation of holes or electrons in a uniform bulk semiconductor material, and the responsivity, temporal response, and wavelength cutoff are unique to the individual semiconductor. An intrinsic photoconductor utilizes across-the-gap photoionization or hole-electron pair creation. An extrinsic photoconductor depends upon the ionization of impurities in the material and in this case only one carrier, either hole or electron, is active. The same is true for a quantum-well photoconductor, in which electrons or holes can be photoexcited from a small potential well in the narrower band-gap regions of the semiconductor. The quantum efficiency for the structure in the figure is determined by the absorption coefficient, o, and may be written 2isrj = (l — / )[ — where R is the reflection coefficient at the top surface. Carriers produced by the radiation, P, flow in the electric field and contribute to this current flow for a time, r, the recombination time. The value of the current is... 

An expression for the quantum efficiency of photoconductors, taking into accoimt reflection coefficients from both the firont and the back side of detector, as well as surface recombination rates on both sides (interference effects are neglected), is [10]... 

The most important characteristic parameters of each photoconductor are the quantum efficiency of photogeneration 0 and the carrier mobililv //. In most examined unsensitized polymeric photoconductors 0 < 10 , even at high electric fields. 

Jones (1957) Quantum Efficiency of Photoconductors by R. Clark Jones Proc. IRIS 2,13. Kittel and Kroemer (1980) Thermal Physics by Charles Kittel and Herbert Kroemer, Wh. H. Freeman, San Francisco. 

Polyacetylene itself has not found application in light-emitting diodes. rrans-Polyacetylene has a very low quantum yield for emission, being instead an efficient photoconductor. See Chapter 6 in [14]. 

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