Semiconductors and P-N junction Diode..

What is Semiconductor??? 

- It’s conductivity lies between that of conductors and insulators.

 - Conductivity increases with increase in temperature. 

- They have negative temperature coefficient(NTC) of resistance.

 Free electrons and holes are charge carriers

            E.g. Ge, Si, GaAs.

           Types of Semiconductors:

            1. Intrinsic Semiconductors

           2.  Extrinsic Semiconductors

            

           Intrinsic Semiconductors :

Extremely pure semiconductor crystal is called as intrinsic semiconductor.           

Extrinsic Semiconductors :

            In an intrinsic semiconductor, there aren’t enough free electrons and holes to produce sufficient current.

            The process of adding impurity atoms to a crystal to increase either the number of free electrons or holes is called ‘doping’.

            When a crystal of intrinsic semiconductor has been doped, it is called an extrinsic semiconductor.

            Two types-

            p-type

            n-type

           p-type Extrinsic Semiconductors :

            Impurity of trivalent material is added to pure semiconductor.

            E.g.-boron, aluminum, indium, gallium, Boron

            Only one or two impurity atoms are added to 10⁶ Si or Ge atoms.

           

           n-type Extrinsic Semiconductors :

            Impurity of penta-valent material is added to pure semiconductor.

            E.g.-Arsenic, antimony, phosphorous

            Pentavalent atoms are called donor atoms because they produce conduction-band

                (free) electrons. 

       

           Diodes :

            A diode is a two terminal device that can pass a large current in one direction and almost no current in other direction.

            A p-n junction is formed by introducing donor impurities into one side and accepter impurities into the other side of a single crystal of a semiconductor as silicon or germanium.

            A p-n junction diode is a rectifier diode, which is constructed by diffusing p-type and n-type of semiconductor materials together.

            

Formation of PN junction diode

Formation of PN junction Diode

            The surface that separates the p and n regions in the single crystal diode is called metallurgical junction.

            The junction where the ions are depleted of free electrons and holes is called as depletion region.

            At a certain point, the depletion layer acts like a barrier to further diffusion of free electrons across the junction, this is because of the creation of so many ions, which repel the electron back to its original position and further combination with hole is stopped.

            The width of the depletion layer keeps increasing with crossing electron and recombination with hole. But when the equilibrium is reached, the internal repulsion of the depletion layer stops further recombination of free electrons.

            The difference of potential across the depletion region is called the barrier potential.

            0.7 Volt for Si and 0.3 Volt for Ge diode.

            Biasing:

            The process of connecting battery or power supply to the pn junction diode is called as biasing.

            Two types:

            Forward Biasing

            Reverse Biasing

           Forward Biasing :

            When the positive terminal of the battery is connected to p side (anode) of diode and negative terminal is connected to n side (cathode) then the diode is said to be forward biased.

            - potential barrier Reduces (i.e. width of the depletion layer is small)

            - current flowing is large in amount.          

             

            

           Reverse biasing :

            When the negative terminal of the battery is connected to p side (anode) of diode and positive terminal is connected to n side (cathode) then the diode is said to be forward biased.

            Current caused by minority carriers is called reverse saturation current or leakage current.

            Depletion layer increases

            

            I-V Characteristics  of Forward bias diode:

          

           Working in forward bias condition

            The voltage at which the current starts to increase rapidly is called the knee of offset or cut-in voltage.

            This voltage is equal to the barrier potential.

            A current limiting resistor is always used in series with a diode. This limits the current to less than the maximum current rating of the diode.

            The diode offers very low resistance in the forward bias condition

            

           I-V Characteristics  of Reverse bias diode:

           

           Working in Reverse bias condition :

            After a particular voltage, the current increases rapidly with small increase in voltage, this voltage is called as the breakdown voltage.

            When reverse voltage is less than breakdown voltage, the diode offers a high resistance ad acts as an open switch.

           Diode VI characteristics

Diode I-V characteristics

V-I Characteristics of PN junction diode

 

Specification Parameters

            Specification Parameters :

For rectifier diode the maximum reverse biased voltage PIV , maximum  Current flowing through it and wattage are important parameters.

            The specification parameters for few rectifying diodes are listed below-

 

Applications :

It is used as a rectifier.

It is used as a Switch.

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