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SEMICONDUCTORS

SEMICONDUCTORS. Optoelectronics. SEMICONDUCTORS. Light is a term used to identify electromagnetic radiation which is visible to the human eye. The light spectrum ranges from 300GHz to 300,000,000GHz and falls between RF and X-rays.

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SEMICONDUCTORS

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  1. SEMICONDUCTORS Optoelectronics

  2. SEMICONDUCTORS • Light is a term used to identify electromagnetic radiation which is visible to the human eye. • The light spectrum ranges from 300GHz to 300,000,000GHz and falls between RF and X-rays. • Light travels at 186,000 miles per second or 30,000,000,000 centimeters per second.

  3. SEMICONDUCTORS • Below is a chart that shows the visible light spectrum and the relationship between wave length and frequency.

  4. SEMICONDUCTORS • Light energy produced by a given source is referred to as luminous energy and is represented by the symbol Qv. • The amount of luminous energy produced by a source per unit of time is called luminous flux or luminous power. • Luminous power is represented by the symbol v and is measured in lumens.

  5. SEMICONDUCTORS • The lumen is the basic unit of measure in the photometric system and can be compared to a watt. • It takes 680 lumens to equal 1 watt which technically only represent green light in response to the human eye. • Luminous intensity Iv is a measurement of a candela, which is equal to 1 lumen.

  6. SEMICONDUCTORS • Illumination Ev (1 lumen striking a surface per unit area) is called a lux, 1 lux equals 1 lumen per square meter. • Luminous exitanceMv is measured in lumens per square meter and is the luminance flux emitted from a surface area. • Luminance Iv is luminance intensity per unit area leaving, passing through or arriving at a surface in a specific direction.

  7. SEMICONDUCTORS • A photoconductive cell is a light sensitive resistor whose internal resistance changes as the light as the light shining on it changes in intensity. • These cells are usually made from cadmium sulfide (Cd s) or cadmium selenide (Cd Se) and are doped with copper or chlorine.

  8. SEMICONDUCTORS • The diagrams show the construction of a photoconductive cell and the circuit symbols.

  9. SEMICONDUCTORS • Photoconductive cells are more sensitive to light than other types of light sensitive devices. • The resistance of a typical cell can range from several hundred mega ohms when there is no light striking its surface and as low as several hundred ohms when the illumination is over 100 lux ( 9 foot candles). • Photo cells have fairly high operating voltages, 100, 200 and 300 volts DC with low power consumptions, 30 to 300mW.

  10. SEMICONDUCTORS • Photoconductive cells are used as intrusion detection and automatic garage door openers. • Photovoltaic cells convert light energy into electrical energy, the voltage increases with light intensity. • The photovoltaic cell is a PN junction device made from semiconductor materials like silicon or selenium.

  11. SEMICONDUCTORS • A photovoltaic cell is forward biased like a PN junction diode and photons that have enough energy will impart its energy onto an atom.

  12. SEMICONDUCTORS • This causes an electron to be knocked out of its valence shell and become a free electron which will then fill a hole and this will cause electron flow at the depletion region just like a diode.

  13. SEMICONDUCTORS • All the photons striking the cell do not create electron hole pairs making photovoltaic cells inefficient, typical efficiencies are 3 to 15%. • A typical cell produces about .45 volts and 50mA. PHOTOVOLTAIC CELL SYMBOL

  14. SEMICONDUCTORS • A photodiode is very similar to a photovoltaic cell and is constructed in a similar fashion, in fact it can operate as a photovoltaic cell. • The photodiode is a reverse biased device and has an intrinsic layer (very few if none impurities) which gives it a high resistance (low conductivity). • This creates a much larger depletion region and increases the production of the electron hole pairs.

  15. SEMICONDUCTORS • This makes the photodiode more efficient over a wider range of light frequencies which gives them the advantage of being able to respond to changes in light intensity very quickly • The major disadvantage of the photodiode is that it produces a relatively low current.

  16. SEMICONDUCTORS • The phototransistor is also a PN junction device and just like a BJT it has two junctions and is used the same way as a photodiode. • The phototransistor is also constructed the same way as a BJT using the diffusion process on an N type silicon substrate. PHOTOTRANSISTOR SYMBOL

  17. SEMICONDUCTORS • The base lead in a phototransistor is rarely used, if it is used the base adjusts the phototransistor’s operating point. • The phototransistor like the photodiode provides an output current that is controlled by the intensity of the light striking its surface. • The phototransistor can produce much higher currents than the photodiode because of its built in amplifying ability.

  18. SEMICONDUCTORS • The phototransistor doesn’t respond as quickly to changes in light intensity as the photodiode. • Phototransistors are used in smoke and flame detectors and photographic exposure controls.

  19. SEMICONDUCTORS • A phototransistor connected to a ordinary transistor allows the phototransistor to control the operation of the BJT and is called a photo-darlington circuit.

  20. SEMICONDUCTORS • The photo-darlington arrangement offers increased sensitivity and produces a high output current, but sacrifices response time. • The two transistors can be formed and packaged together as one circuit. PHOTO-DARLINGTON COMPONENT

  21. SEMICONDUCTORS • LEDs are also optoelectronic devices, you learned about them in an earlier lesson, now we’re going to determine the proper resistor to use. • The short lead of an LED is the cathode and this is the negative side.

  22. SEMICONDUCTORS • LEDs require a resistor in series to limit the amount of current supplied to the LED. • Typical voltage drops across an LED range from 1.6 volts to 3.3 volts depending on the color of the LED • An average voltage drop is 1.8 volts for most LEDs and they operate on approximately 20mA to 50mA.

  23. SEMICONDUCTORS • For more specific circuit design consider voltage drops for the following: • 1.7 volts for non-high-brightness red. • 1.9 volts for high-brightness, high-efficiency and low-current red. • 2 volts for orange and yellow. • 2.1 volts for green. • Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types.

  24. SEMICONDUCTORS • To limit the forward current (IF), we subtract the voltage drop of the LED (Vd ) from the source voltage (E) and divide by the forward current (IF ), then subtract the internal resistance of the LED.

  25. SEMICONDUCTORS • A typical LED internal resistance is 5Ω. • As an example a voltage source of 6 volts supplied to an LED specified to 20mA with a 1.8 volt drop and an internal resistance of 5Ω will need a 205Ω resistor.

  26. SEMICONDUCTORS • The results are determined as follows:

  27. SEMICONDUCTORS • Diodes should not be connected in parallel, it will work however it is not reliable and diodes will conduct more current as they warm up which will decrease their life span. • Only one resistor is needed for a string of LEDs connected in series and the total voltage drop of all LEDs in series should not exceed 80% of the supply or source voltage.

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