Photoconductive sensors.

Photoconduction has been observed, studied, and applied for more than 100 years. In the year 1873, W. Smith noticed that the resistance of a selenium resistor depended on illumination by light. Since that time, photoconduction has been an important tool used to evaluate materials properties, to study semiconductor device characteristics, and to convert optical into electric signals. The Radio Corporation of America (RCA) was a leader in the study and development of photoconductivity and of photoconductive devices. Richard H. Bube of RCA Laboratories wrote the classic book Photoconductivity in Solids in 1960. Today, photoconducting devices are used to generate very fast electric pulses using laser pulses with subpicosecond rise and fall times. For optoelectronic communications, photoconducting devices, allow operation in the gigabit per second range. Photoconductive devices normally have two terminals. Illumination of a photoconductive device changes its resistance. Conventional techniques are used to measure the resistance of the photoconductor. Frequently, small changes in conductivity need to be observed in the study of material or device characteristics. Also, in the measurement of light intensities of faint objects, one encounters small photoconductive signals.

Only solid photoconductors, such as Si, PbS, PbSe, and HgCdTe, will be treated here. Photoconduction has been observed in amorphous, polycrystalline, and single-crystalline materials. During the last decade, major improvements in materials growth have occurred which directly translate in better device performance such as sensitivity and stability. Growth techniques such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) allow the growth of single-crystal layers with an accuracy of the lattice constant. Artificially structured materials can be fabricated with these growth techniques for use in novel photoconducting devices. Absorption of light in semiconductors can free charge carriers that contribute to the conduction process.

Excitation process is a band-to-band transition. The photon energy for this excitation has to exceed the bandgap of the semiconductor. The absorption constant is larger for this process than for any of the other processes. Typical semiconductors used for electronic applications have bandgaps in excess of 1 eV, corresponding to light in the near-infrared region. Special semiconductors have been developed with narrower bandgaps to provide absorption in the mid- and long-wavelength infrared regions. Indium antimonide (InSb) and mercury-cadmium-telluride (HgCdTe) semiconductors provide photosensitivity in the 4- and 10-mm wavelength range, respectively. The photogenerated carriers increase the electron and hole densities in the conduction and valence bands, respectively, which leads to an increase in conductivity.

4.Add information:

There are such Detector Performance Parameters as:

- Responsivity

- Johnson Noise

What is their role?

5.What do these chemical abbreviations mean?

a) HgCdTe

b) InSb

c) PbS

d) CdS

e) PbSe

Date: 2016-04-22; view: 853