Tham khảo tài liệu 'crc press mechatronics handbook 2002 by laxxuss episode 2 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Evaporated metal electrodes Photoresistor Incident radiation Bias _ voltage R ĩơutput L ịvoltage Photoconducting material FIGURE A simple light detector circuit employing a photoresistor is shown. An increase in light illumination causes the resistance of the photoresistor to decrease and the output voltage to increase. The comb-like pattern typically employed in photoresistors gives a relatively large active area of photoconducting material and a small electrode spacing resulting in high sensitivity. spectral response which peaks at a wavelength about hc Eg. Photoresistors and junction detectors are discussed in more detail in the following sections. Photoresistors The electrical conductivity of a semiconductor is the sum of two terms 5 one contributed by electrons and the other by holes as follows ơ neụ. Each term is proportional to n p the number of electrons holes per unit volume in the conduction valence band the electron hole mobility pn pp and the magnitude of the charge of the electron e. The increase in conductivity caused by the absorption of photons increasing n and p is the basis for the operation of the photoresistive detector. This consists of a slab of semiconductor material on the faces of which electrodes are deposited to allow the resistance to be monitored as illustrated in Fig. . The photon-induced current is proportional to the length of the electrodes and inversely proportional to their separation hence the typical comb-like electrode geometry of photoresistors shown in Fig. . Because the resistance RC is inversely proportional to conductivity the variation of RC with incident power PD is very nonlinear and is often expressed in the form log10 Rc a - blog Pd where a and b are constants. Cadmium sulfide is commonly used as a detector of visible radiation because it is low cost and its response is similar to that of the human eye. Other photoconductive materials include lead sulfide with a useful response from
