Photodiodes

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). 

For their use as radiation detectors the spectral dependence of their absorption coefficient is of fundamental importance. In an undoped semiconductor the absorption of one photon hv causes an excitation of an electron from the valence band into the conduction band (Fig. 4.79a). With the energy gap AEg = Ec — Ey between the valence and conduction band, only photons with hv A Eg are absorbed. The intrinsic absorption coefficient 

When a photodiode is illuminated, its electrical resistance decreases from a dark value / d to a value R under illumination. In the circuit shown in 

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 

While photoconductors are passive elements that need an external power supply, photovoltaic diodes are active elements that generate their own photovoltage upon illumination, although they are often used with an external bias voltage. The principle of the photogenerated voltage is shown in Fig. 4.84. 

In doped semiconductors photon-induced electron transitions can occur between the donor levels and the conduction band, or between the valence band and the acceptor levels (Fig. 4.90). Since the energy gaps AE = or AE = 

Ey — E are much smaller than the gap E — Ey, doped semiconductors absorb even at smaller photon energies hv and can therefore be employed for the detection of longer wavelengths in the midinfrared. In order to minimize thermal excitation of electrons, these detectors must be operated at low temperatures. For A 10 p,m 

AEg = Ec Ey between the valence and conduction band, only photons with hv A Eg are absorbed. The intrinsic absorption coefficient 

With a small resistor R, a high-frequency response can be achieved which is limited only by the drift time of the carriers through the boundary layer of the p-n junction. Using diodes with large bias voltages and a 50 load resistor matched to the connecting cable, rise times in the subnanosecond range can be obtained. 

For photon energies hv close to the band gap, the absorption coefficient decreases [see (4.139)]. The penetration depth of the radiation, and with it the volume from which carriers have to be collected, becomes large. De- 

Very fast response times can be reached by using the photoeffect at the metal-semiconductor boundary known as a Schottky barrier [4.86]. Because of the different work functions and (j) of the metal and the semiconductor, electrons can tunnel from the material with low [) to that with high (Fig,4.85) causing a space-charge layer and a potential barrier 

(a) Work functions of a metal and f g of a semiconductor and electron affinity x- Ec is the energy at the bottom of the conduction band and Ep is the Fermi-energy. (b) Schottky barrier at the contact layer between metal and n-type semiconductor, (c) Generation of a photocurrent 

For measurements of optical frequencies, ultrafast metal-metal diodes have 

As discussed above, the critical factor influencing the properties of heterostructures is the similarity of the lattice parameters. In this respect, 6H-SiC seems to be a good candidate because it has a relatively small lattice mismatch with ZnO ( 3.5%, 

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Figure Bl.19.23. Principle of simultaneous measurement of nomial and lateral (torsional) forces. The intensity difference of the upper and lower segments of the photodiode is proportional to the z-bending of the cantilever. The intensity difference between the right and left segments is proportional to the torsion, t, of the force sensor. (Taken from [110], figure 2.)...

The gates referred to above can be created in various ways. For example, suppose that the probe beam goes tlirough the sample, but only half of its physical width (in the sample) is crossed with the pump beam. Now, if we have two photodiodes, one can measure the intensify of the perturbed part of the probe beam, whilst the second measures the unperturbed part as a result of creating spatial gates, the two recorded output signals can be used to measure the... 

A linear array of photodiodes providing the ability to detect simultaneously radiation at several wavelengths. 

Listed below is a two-part series on the application of photodiode arrays in UV/Vis spectroscopy. 

Spectroscopy Partl, AnaZ. Chem. 1985, 57, 1057A-1073A. Jones, D. G. Photodiode Array Detectors in UV-Vis... 

Remcho, V. T. McNair, H. M. Rasmussen, H. T. HPLC Method Development with the Photodiode Array Detector, /. Chem. Educ. 1992, 69, A117-A119. 

A mixture of methyl paraben, ethyl paraben, propyl paraben, diethyl phthalate, and butyl paraben is separated by HPLC. This experiment emphasizes the development of a mobile-phase composition capable of separating the mixture. A photodiode array detector demonstrates the coelution of the two compounds. 

Visible Tungsten filament xenon arc Glass Prism grating interferometer Photomultiplier photodiode photographic plate... 

Available halogen Avalanche photodiodes Avan Avatec... 

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