1 Chapter 2 Electic-ight conversion. 2 p-n junction We insert atoms of another material (called dopants) into a semiconductor so that either a majority

1Prof. Yang, School of Optical-Electrical and Computer Engineering , USST

Chapter 2Chapter 2Electic-ight conversionElectic-ight conversion


1. Light –Emitting Diodes (LEDs)

p-n junction We insert atoms of another material (called dopants) into a semiconductor so that either a majority of electrons (negative charge carriers) or a majority of holes (positive charge carriers) will be created. The former semiconductor is called the n type and the latter is called the p type. We call these n type and p type doped, or extrinsic, not a pure, or intrinsic.


When an n-type semiconductor is brought into physical contact with a p type, a p-n junction is created. At the boundary of the junction, electrons from the n side diffuse to the p side and recombine with holes and, at the same time, holes from the p side diffuse to the n side and recombine with electrons. An electron-hole recombination releases a quantum of energy—a photon. In other words, to make a semiconductor radiate, it is necessary to sustain electron-hole recombinations.


LED: Principle of action

The forward-biasing voltage, V, causes electrons and holes to enter p-n junction and recombine Alternatively, we can say that the external energy provided by V excites electrons at the conduction band. From there, they fall to the valence band and recombine with holes. Whatever point of view you prefer, the net result is light radiation by a semiconductor diode.


Spontaneous radiation :Because the transition of electrons from many energy levels of conduction and valence bands contributes to the radiation produced, thus making the spectral width of such a source very wide.


(a) Electronic circuit; (b) An input-output characteristic.


Radiating wavelengths:A radiating wavelength is determined by the energy gap of a semiconductor


Rise/fail time is determined by an LED’s junction capacitance (C) ,which determine the frequency response or Bandwidth of LED.


Bit rate (BR) or data rate is the number of bits that can be transmitted per second over a channel, it is for digital transmission. Bandwidth (BW) is the frequency range within which a signal can be transmitted without significant deterioration .it is for analog transmission(Hz).Bit rate(BR) and bandwidth(BW) are the direct measure of information-carrying capacity of a communication channel.

Normally, BW=BR/2


The driving circuits of LED

Conditions: Forward-biasing, Current limitation


2. Semiconductor Laser


The conditions to emit the laser ：Forward-biasing, Current limitation , Population inversion,Stimulated emission,Positive feedbackStimulated radiation:

1.The external photon stimulates radiation with the same frequency (wavelength).

2.Current-to-light conversion is with high efficiency.

3.The stimulated light will be well directed.4.A stimulated photon and an external photon are synchronized, This means that both photons are in phase and so the stimulated radiation is coherent.


Special structures of LDFirst, the thickness of an active region in a laser

diode is very small, typically on 0.1 um . Second, a laser diode’s two end surfaces are cl

eaved to make them work as mirrors.


1.Monochromatic . The spectral width of the radiated light is very narrow. ( a nanometer)

2.Well directed. A laser diode radiates a narrow, well-directed beam

3.Highly intense and power-efficient. A laser diode can make current-to-light conversion 10 times more efficient than it is in the best LEDs.

4.Coherent . All oscillations are in phase. This property is important for detection of an signal.

The properties of Laser



Laser driver/modulation circuit


3. Superluminescent Diodes (SLDs) Conditions: Forward-biasing, Current

limitation , Population inversion




4. LCD --liquid crystal display A type of display used in digital watches and mA type of display used in digital watches and m

any computers or TV sets. LCD displays utilize tany computers or TV sets. LCD displays utilize two sheets of polarizing material with a wo sheets of polarizing material with a liquid cliquid crystal solutionrystal solution between them. An electric curre between them. An electric current passed through the liquid causes the crystalnt passed through the liquid causes the crystals to s to alignalign so that light cannot pass through the so that light cannot pass through them. Each crystal, therefore, is like am. Each crystal, therefore, is like a shutter shutter, eith, either allowing light to pass through or block the lier allowing light to pass through or block the light. It ght. It requires backlight. It only modulates transmitted or reflected light.(Passive emission)


Tridimensional GPS ---LCD


Photo frame supported bluetooth



5. OLED -- organic light-emitting diode The OLED consists of two charged electrodes

sandwiched on top of some organic light emitting material--carbon-based films. When voltage is applied to the OLED cell, the injected positive and negative charges recombine in the emissive layer and create electro luminescent light. Unlike LCDs, which require backlighting, OLED displays are emissive devices (active). OLED technology enables full color, full-motion flat panel displays with a level of brightness and sharpness not possible with other technologies.


The basic OLED cell structure


How to work? Electrical current is passed through selected pixels by applying a voltage to the corresponding rows and columns to realize the scanning of the video.


Blocky MP3 PlayerMP3—OLED screen


Lamp with flexible OLEDs


6.PLED– polymer light-emitting diode Polymers are substances formed by a che

mical reaction in which two or more molecules combine to form larger molecules. PLEDs are thin film displays that are created by sandwiching an undoped conjugated polymer between two proper electrodes. PLEDs enable full-spectrum color displays and are relatively inexpensive compared to other display technologies such as LCD or OLED and require little power to emit a substantial amount of light.


8.Plasma flat panel display It is a emissive display.The principle is just like fluorescent lamp. There are so many lamps to arrange like the flat panel display.

7.CRT--cathode-ray tubeIt is a emissive traditional display, but it is not good for the body’s health.Principle: Electrons are emitted by the cathode and accelerated by the high voltage. Very strong electrons bomb the fluorescent screen to emit the light.



9.New applicationS of LEDs






Radio










Test 211 、、 Please find LED’s new application,that mePlease find LED’s new application,that me

ans to use LED and some other elements to deans to use LED and some other elements to design a device which is novel.sign a device which is novel.

22 、、 Can LEDs of flowing circuit work wellCan LEDs of flowing circuit work well ？？ If noIf not , please make them Ok .t , please make them Ok .

Documents

1 Chapter 2 Electic-ight conversion. 2 p-n junction We insert atoms of another material (called dopants) into a semiconductor so that either a majority