Light Emitting Diode

                             Light Emitting Diode     

What is Light Emitting Diode(LED)?

                A Light Emitting Diode (LED) is a two lead semiconductor light source that emits light when an electric current is applied in the forward direction of the device.
                Light emitting diode converts electrical energy in to light energy.

               When a fitting voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light is determined by the energy bandgap of the semiconductor. 
 
Structure: 
            The structure of a Light Emitting Diode is very different from that of a normal signal diode. The PN junction of an LED is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell or body which protects the LED from both vibration and shock.
             Some indication LEDs have a rectangular or cylindrical shaped construction that has a flat surface on top or their body is shaped into a bar or arrow. Also, nearly all LEDs have their cathode, ( K ) terminal identified by either a notch or flat spot on the body, or by one of the leads being shorter than the other, ( the Anode, A ).

Construction and working of LED:

                 The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb .The chip has two regions separated by a junction.
                 The P region is dominated by positive electric charges ,and the N region is dominated by negative electric charges.The junction acts as a barrier to the flow of electrons between the P and N regions.when sufficient voltage is applied to the semi-conductor chip across the leads of the LED,electrons can move easily in only one direction across the junction between the P and N regions.
                 In the P region there are many positive than negative charges.Each time an electron recombines with a positive charge,electric potential energy is converted into electromagnetic energy.
                For each recombination of a negative and a positive charge,a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material. 
 

Light Emitting Diode Colours:

Typical LED Characteristics
Semiconductor Material	Wavelength	Colour	VF @ 20mA
GaAs	850-940nm	Infra-Red	1.2v
GaAsP	630-660nm	Red	1.8v
GaAsP	605-620nm	Amber	2.0v
GaAsP:N	585-595nm	Yellow	2.2v
AlGaP	550-570nm	Green	3.5v
SiC	430-505nm	Blue	3.6v
GaInN	450nm	White	4.0v

LED V-I characteristics:
                     

Applications of LED:

• Sensor Applications
• Mobile Applications
• Sign Applications
• Automative Uses 
• LED Signals
• Illuminations
• Indicators 

 

                 

DIAC

DIAC

What is Diac?

                The Diac is a combination of two diodes. It is a two terminal,three layer,bidirectional semiconductor  device which can be switched from its OFF state to ON state for either polarity of applied voltage.
                  The two terminals are MT1 and MT2.
Circuit diagram:
 
The diac can be constructed in either pnp or npn form. In pnp form two leads are connected to p regions of silicon seperated by an n region.The structure of diac is similiar to that of a transistor.Howeverthere are some several important differences .
                        There is no terminal attached to the base layer.
                        The three regions are nearly identical in size.
                         The doping level at the two end P-layers is the same so that the device gives sym­metrical switching characteristics for ei­ther polarity of the applied voltage.
Operation:
           Diac circuits use the fact that a diac only conducts current only after a certain breakdown voltage has been exceeded. The actual breakdown voltage will depend upon the specification for the particular component type.
           When the diac breakdown voltage occurs, the resistance of the component decreases abruptly and this leads to a sharp decrease in the voltage drop across the diac, and a corresponding increase in current.   
            The diac will remain in its conducing state until the current flow through it drops below a particular value known as the holding current. When the current falls below the holding current, the diac switches back to its high resistance, or non-conducting state.
            Most diacs have a breakdown voltage of around 30 volts, although the exact specifications will depend upon the particular type of device. 
Characteristics of a Diac:
                    
 
Applications of Diac:
                       Diacs are used primarily for triggering of triacs in adjustable phase contril of a.c mains power.some of the circuit applications of diac are                                                           

•       Light dimming
•       Heat control
•       Universal motor speed control. 

DIODE

DIODE

What is Diode?

• Diode is specialized electronic component with two electrodes called the anode and the cathode.Diodes are basic unidirectional semiconductor devices  that  the current flow in one direction only and blocks the current flow in the opposite direction 
• It is active component
• It is constructed of a P and an N junction connected together.   
• The most common type of diode is a ‘silicon diode.’ It is enclosed in a glass cylinder with the dark band marking the cathode terminal. This line points towards the positive of a circuit. The opposite terminal is called the anode.
• Generally, diodes do not conduct until the voltage reaches approximately .6 volts, this is called the ‘threshold point’. If the current becomes too high the diode may crack or melt.

Operation of Diode:
       semiconductor diode connects to the external source in two ways.The process is known as biasing.

•   Forward biasing:positive terminal to the anode and negative terminal to the cathode.
•   Reverse biasing: positive terminal to the cathode and negative terminal to the anode. 

Forward biasing:
                 When the diode is forward biased, due to the negative terminal on the n-side, electrons from the n-side are pushed towards the p-region. Similarly due to positive voltage on the p-side of the diode, Holes from the p-region are pushed towards n-side.
               Due to this the electrons will start converting the positive ions in the p-region into neutral atoms and holes will start converting the negative ions in the n-region to neutral atoms. Hence width of the depletion region starts reducing due to reduction in the barrier potential.
              This keeps happening and at a certain point the depletion region collapses and there is no opposition to the flow of current. Hence large number of electrons and holes will cross the junction and make the electric current to flow from anode to cathode.
               Hence, forward biased electrical resistance of diode is very small and hence there is a small voltage drop  across it. Its value for silicon diode is about 0.7 V and germanium for 0.3V.

 
Reverse biasing:
             When the diode is reverse biased the hole from the p-side will get attracted towards the negative terminal of the supply and electrons from the n-side are attracted towards the positive terminal. 
             Hence the process of widening of the depletion region takes place and hence more and more opposition to the flow of  electric current takes place.
             Hence, ideally the reverse biased resistance of the diode is infinite and no electric current flows from the diode when it is reversed biased. 

Characteristics of diode:
 
Application of Diode:

• Rectifiers
• Signal limiters
• Voltage regulators
• Switches
• Signal modulators
• Signal mixers
• Signal demodulator, and Oscillators.

                
                  
             

    

Basics of semiconductor

 Basics of semiconductor

what is semiconductor?

      A semiconductor is a material that has intermediate conductivity between a conductor and an insulator. It means that it has unique physical properties somewhere in between a conductor like aluminum and an insulator like glass.As its name suggests,a semiconductor features resistivity levels on a range between those of a  conductor and an insulator. 
       semiconductor resistivity usually falls in between 10^-4 and 10⁴ ohms per centimeter

       Low electrical conductivity at room temperature,electrical conductivity increases with temperature.

Types of semiconductor: 

          Semiconductors are mainly classified into two categories.

Intrinsic semiconductor: An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes).

Extrinsic semiconductor: An extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity. 

 Doping process produces two groups of semiconductors: the negative charge conductor (n-type) and the positive charge conductor (p-type).The most common n-type dopants for silicon are phosphorus and arsenic, while the most common p-type dopant for silicon is boron.

 Semiconductors are available as either elements or compounds. Silicon and Germanium are the most common elemental semiconductors. Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. 
List of semiconductor devices: 
     Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors.
       Semiconductor devices can be categorized depends upon the terminals
                 Two terminal devices:
                                        DIAC
                                        Diode
                                        Light-emitting diode  
                                        Photo cell
                                        Solar cell
                 Three terminal devices:
                                        TRIAC
                                         Bipolar transistor 
                                         Darlington transistor
                                         Field-effect transistor
                                         Silicon-controlled rectifier
                                         Unijunction transistor
                                         Insulated-gate bipolar transistor
                  Four terminal devices:

                                          Hall effect sensor

                 Multi terminal devices:   

                                          Integrated circuit(ICs)
                                          Charge-coupled device
                                          Microprocessor
                                          Random-access memory(RAM)
                                          Read-only memory(ROM)