Semiconductor diodes

OBJECTIVE Energy band and energy band gap Classification of materials on the basis of

energy band gap What is Semiconductor Types of semiconductor Extrinsic semiconductor Semiconductor junction Semiconductor device (P-N DIODE) Applications of diode

DIFFERENCE BETWEEN CONDUCTORS,INSULATORS,SEMICONDUCTORS

Energy band & energy band gap:- Each isolated atom has a discrete energy

level. But in general isolated atoms are not exist .they exist in the form of crystal. In that crystal there are nearby atoms ,which also have an energy level nearly equal to the previous energy level.

so these “closely spaced energy levels form a band of energy” called energy band.

valance band is located blow the conduction band eperated from it by a energy band gap.

• In conductors C.B. and V.B. are overlapped• In insulators energy band gap is 6eV• In semiconductors energy band gap is 1eV

CLASSIFICATION OF MATERIALS ON THE BASIS OF ENERGY BAND GAP

Coductors Insulators Semiconductors

SEMICONDUCTOR Semiconductor are those materials

which behaves like insulators at 0 degree Celsius and like conductor at room temperature.

They have properties between conductors and insulators.

TYPES OF SEMICONDUCTOR Intrinsic semiconductors:- Intrinsic semiconductors are pure semiconductors, no impurities are added in these conductors. So the no. of free electrons and holes are equal . Conductivity of these semiconductors is low because of electrons are in perfect covalent bonding. Extrinsic semiconductors

INTRINSIC (PURE) SILICONAt 0 Kelvin Silicon density is 5*10²³ particles/cm³Silicon has 4 valence electrons, it covalently bonds with four adjacent atoms in the crystal lattice

Higher temperatures create free charge carriers.A “hole” is created in the absence of an electron.At 23C there are 10¹º particles/cm³ of free carriers

EXTRINSIC SEMICONDUCTORSAn extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than theintrinsic (pure) semiconductor.

DOPING INVOLVES ADDING DOPANT ATOMS TO AN INTRINSIC SEMICONDUCTOR, WHICH CHANGES THE ELECTRON AND HOLE CARRIER CONCENTRATIONS OF THE SEMICONDUCTOR AT THERMAL EQUILIBRIUM. DOMINANT CARRIER CONCENTRATIONS IN AN EXTRINSIC SEMICONDUCTOR CLASSIFY IT AS EITHER AN N-TYPE OR P-TYPE SEMICONDUCTOR. THE ELECTRICAL PROPERTIES OF EXTRINSIC SEMICONDUCTORS MAKE THEM ESSENTIAL COMPONENTS OF MANY ELECTRONIC DEVICES.

TYPES OF EXTRINSIC SEMICONDUCTOR P type N type

P-TYPE N-TYPE When a doped

semiconductor contains excess holes it is called P-type semiconductor.

Doping is trivalent,B,Ga,In,Al

When a dped semiconductor contains excess electrons it is called N-type semiconductor.

Doping is pentavalent,As,Bi,Sb,P

DOPING

The N in N-type stands for negative.A column V ion is inserted.The extra valence electron is free to move about the lattice

There are two types of dopingN-type and P-type.

The P in P-type stands for positive.A column III ion is inserted.Electrons from the surrounding Silicon move to fill the “hole.”

CRYSTALLINE NATURE OF SILICON

Silicon as utilized in integrated circuits is crystalline in nature

As with all crystalline material, silicon consists of a repeating basic unit structure called a unit cell

For silicon, the unit cell consists of an atom surrounded by four equidistant nearest neighbors which lie at the corners of the tetrahedron

P-N JUNCTION Also known as a diode One of the basics of semiconductor

technology - Created by placing n-type and p-

type material in close contact Diffusion - mobile charges (holes)

in p-type combine with mobile charges (electrons) in n-type

P-N JUNCTION Region of charges left behind

(dopants fixed in crystal lattice) Group III in p-type (one less proton

than Si- negative charge) Group IV in n-type (one more proton

than Si - positive charge) Region is totally depleted of mobile

charges - “depletion region” Electric field forms due to fixed

charges in the depletion region Depletion region has high resistance

due to lack of mobile charges

THE P-N JUNCTION

THE JUNCTION

The “potential” or voltage across the silicon changes in the depletion region and goes from + in the n region to – in the p region

BIASING THE P-N DIODE

Forward BiasApplies - voltage to the n region and + voltage to the p regionCURRENT!

Reverse BiasApplies + voltage to n region and – voltage to p regionNO CURRENT

THINK OF THE DIODE AS A SWITCH

P-N JUNCTION – REVERSE BIAS positive voltage placed on n-type

material electrons in n-type move closer to

positive terminal, holes in p-type move closer to negative terminal

width of depletion region increases allowed current is essentially zero

(small “drift” current)

P-N JUNCTION – FORWARD BIAS positive voltage placed on p-type

material holes in p-type move away from

positive terminal, electrons in n-type move further from negative terminal

depletion region becomes smaller - resistance of device decreases

voltage increased until critical voltage is reached, depletion region disappears, current can flow freely

P-N JUNCTION - V-I CHARACTERISTICSVoltage-Current relationship for a p-n junction (diode)

CURRENT-VOLTAGE CHARACTERISTICS

THE IDEAL DIODE

Positive voltage yields finite currentNegative voltage yields zero current REAL DIODE

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whereI diode current with reverse bias

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THE IDEAL DIODE EQUATION

SEMICONDUCTOR DIODE - OPENED REGION The p-side is the cathode, the n-side is the

anode The dropped voltage, VD is measured from

the cathode to the anode

Opened: VD VF:VD = VF

ID = circuit limited, in our model the VD cannot exceed VF

SEMICONDUCTOR DIODE - CUT-OFF REGION Cut-off: 0 < VD < VF:

ID 0 mA

SEMICONDUCTOR DIODE - CLOSED REGION

Closed: VF < VD 0: VD is determined by the

circuit, ID = 0 mA Typical values of VF: 0.5 ¸ 0.7 V

ZENER EFFECT

Zener break down: VD <= VZ:VD = VZ, ID is determined by the circuit.

In case of standard diode the typical values of the break down voltage VZ of the Zener effect -20 ... -100 V

Zener diode Utilization of the Zener effect Typical break down values of VZ : -4.5 ... -

15 V

LED

Light emitting diode, made from GaAs

VF=1.6 V

IF >= 6 mA

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