Optoelectronic devices are those whose characteristics are changed or controlled by light or those that create andlor modify light. In recent years, important technological advances in the design and fabrication of solid-state optoelectronic devices have been responsible for what are now widespread applications in areas such as visual displays, alarm systems, fiber-optic communications, optical readers, lasers, and solar energy converters.
Light is a form of electromagnetic radiation and, as such, is characterized by its frequency or, equivalently, by its wavelength. The fundamental relationship between frequency and wavelength is
Visible light occurs at frequencies in the range from about 4.3 x 1014 Hz (7000 A) to 7.5 X 1014 Hz (4000 A). The lowest frequency appears red and the highest appears violet. Frequencies less than that of red, down to 1012 Hz, are called infrared, and those greater than that of violet, up to 5 X 1017 Hz, are called ultraviolet. White light is a mixture, or composite, of all frequencies in the visible range.
A light spectrum is a plot of light energy versus frequency or wavelength. An emission spectrum is a spectrum of the light produced, or emitted, by a light source, such as an incandescent bulb or a light-emitting diode (LED). A spectral response shows how one characteristic of a light-sensitive device varies with the frequency or wavelength of the light to which it is exposed. Spectral response is similar in concept to the frequency response of an electronic system. For example, the spectral response of the human eye shows that it is most sensitive to yellow light, at wavelengths around 5700 A, and least sensitive to the frequencies at opposite ends of the visible band, i.e., red and violet.
Luminous flux is the rate at which visible light energy is produced by’ a light source, or at which it is received on a surface, and is measured in lumens (1m), where
Thus, a surface receiving visible light energy at the rate of 1.496 X 10-3 Jls is said to have a luminous flux of one lumen. Luminous intensity is luminous flux per unit area. For example, if the luminous flux on a 0.2-m2 surface is 1.5 X 10′ lrn, then the intensity is 1.5 X 1031111/(0.2 rn”) = 75 X 10.1lm/rn”. One lumen per square foot equals the older unit, one footcandlc. Manufacturers of light-sensitive electronic devices also use the units milliwatts pCI’ square centimeter to specify light intensity in their product literature. Care must be exercised in the interpretation of lightintensity figures, since measured values depend on the spectral response of the instrument used to measure them.
Example 18-9
Find the horsepower equivalent of 1 X 106 lm/rn? (the typical intensity of visible sunlight at noon on a 1O-m2 surface).
solution
P = (l06Im/m2)(1O m”) == 1011m
(107Im)(1.496 x 10-3 WI 1m) = 1.496 x 104 W
(1.496 X 104 W)(7~:~ ) = 20hp
Photo conductive Cells
A photo conductive cell is a passive device composed of semiconductor material whose surface -is exposed to light and whose electrical resistance decreases with increasing light intensity. Recall that electrons in a semiconductor become free electrons if t1H!Y acquire enough energy to break the bonds that hold them in a
covalent structure. In conventional diodes and transistors, electrons are freed by heat energy. In a photoconductive cell, free electrons are similarly created by light .cnergy; the greater the light intensity, the greater the numher of free electrons. Since the conductivity of the material increases as the number of free electrons increases, the resistance of the material decreases with increasing liglll intensity. Photo conductive cells are also called photo resistioe cells, or photo resistors. They are the simplest and least expensive of components belonging to the general class of photo conductive devices: those whose conductance changes with light intensity.
Figure 18-23 shows the typical geometry of a photoconductive cell and its schematic symbol. The light-sensitive semiconductor material is arranged in a zigzag strip whose ends are attached to external pins: A glass or transparent plastic cover is attached for protection. The light-sensitive material used in photo conductive cells is either cadmium sulfide (CdS) or cadmium selenide (CdSe). The resistance of each of these materials changes rather slowly with changes in light intensity, the CdSe cell having a response time of about 10 ms, and that of CdS being about 100 ms,
However. the slower CdS cell .is much less sensitive to temperature changes than is the CdSe cell. The spectral response of CdS, i.e .. its sensitivity as a function of the wavelength of incident light, is similar to that of the human eye, so it responds to visible light. The response of CdSe is greater at red and infrared frequencies.
Figure 18-24 shows a plot of resistance versus’ light intensity for a typical photo conductive cell. Note that the plot is linear on logarithmic axes. The plot demonstrates that the resistance of a typical cell can change over a very wide range, from about 100 kH,to a few hundred ohms. The large resistance of the cell when it is not illuminated is called its dark resistance.
Photo conductive cells are used in light meters, lighting controls, automatic door openers, and intrusion detectors. In the latter applications the change in resistance that occurs when incident light is interrupted causes a circuit in which the cell is connected to energize a switch or relay that in turn generates an audible or visual signal. The next example demonstrates such an application.
Example 18-10
When the photo conductive cell in Figure 11\-25 is illuminated ‘by a light beam, it has a resistance of 20 kil. The dark resistance is 100 kil. Show that the relay is de-energized when the cell is illuminated and energized when the light beam is interrupted.
solution
When the cell is illuminated and has resistance 20 kil, we find VI using superposition:
Since VI is negative, the output of the comparator is negative and the SCR .is off. No current flows in the relay coil, so it is de-energized .
The output of the comparator is therefore positive; the SCR turns on, and the relay is energized. To de-energize the relay, the reset pushbutton .is depressed, thus. shorting the SCR and dropping its current below the holding current.
Note that a latching device such as an SCR is required in this application if the circuit is to respond to momentary interruptions of the light beam, Also, cell response time may be critical in detecting short-duration interruptions. The circuitcan be made more sensitive by increasing the values of the positive and negative voltages applied to the comparator, but photodiodes, discussed in the next section, have much faster response times and arc therefore more appropriate for detecting very short duration interruptions of light
