SEMICONDUCTOR

 CLASSIFICATION OF METALS, CONDUCTORS, AND SEMICONDUCTORS??

ON THE BASIS OF CONDUCTIVITY

ON THE BASIS OF THE RELATIVE VALUES OF ELECTRICAL CONDUCTIVITY OR RESISTIVITY THE SOLIDS ARE BOARD CLASSIFIED AS:

1. METALS

THEY POSSESS VERY LOW RESISTIVITY (OR HIGH CONDUCTIVITY)

2. SEMICONDUCTORS

THEY HAVE RESISTIVITY OR CONDUCTIVITY  INTERMEDIATE TO METALS AND INSULATORS

3. INSULATORS

THEY HAVE HIGH RESISTIVITY (OR LOW CONDUCTIVITY)

ELEMENTS OF SEMICONDUCTORS COULD BE 

1. ELEMENTAL SEMICONDUCTORS

EXAMPLE:- Si AND Ge

2. COMPOUND SEMICONDUCTORS COULD BE

EXAMPLE:- GaAs, InP ETC

SEMICONDUCTORS, METALS, AND INSULATORS CAN ALSO BE CLASSIFIED ACCORDING TO THE ENERGY BAND DIAGRAM.

SO WHAT IS ENERGY BAND DIAGRAM LET US CONSIDER THAT A SILICON ATOM (Si)

           ATOMIC NO- 14

    ELECTRONIC CONFIGURATION:-

        1S2, 2S2, 2P6, 3S2, 3P2           

NO OF SHELLS=3

NO OF ORBITS=5

NO OF ORBITALS=9

NO OF ELECTRONS=14

AS VALENCY OF SILICON IS 4. THEREFORE IT 4 INTERATOMIC FORCES TOWARD THE NUCLEUS.

IN A SOLID-STATE A SINGLE ATOM EXPERIENCES INTERATOMIC FORCES. TOWARDS THE NUCLEUS BY SURROUNDING ATOMS AND THIS CHANGES THE ENERGY LEVEL OF OUTERMOST ORBITS WHILE THE INNERMOST ENERGY STATE OF ORBITS REMAINS THE SAME.

THE ENERGY OF ORBITS IN ATOM IS MEASURED IN eV(ELECTRON VOLT) AND BAND FORMATION DIAGRAM FOR Si ATOM DUE TO INTERATOMIC FORCES. ENERGY BAND IS FORMED WHEN THE ENERGY OF OUTERMOST ORBIT OVERLAPS WITH EACH OTHER DUE TO INTERATOMIC FORCES FURTHER THIS ENERGY BANDS SPLITS INTO 2 NEW BAND AT THE LOWER BAND IS COMPLETELY FILLED WITH ELECTRON BUT THESE ELECTRONS ARE NOT FREE TO MOVE. THE UPPER BAND WHICH IS EMPTY INITIALLY MAY CONTAIN FREE ELECTRONS WHEN SUITABLE ENERGY IS SUPPLIED TO THE VALANCE BAND. THE GAP WHICH SEPARATES THESE BANDS IS KNOWN AS FORBIDDEN ENERGY GAP. 

CONDUCTORS

ARE THE STATE OF SOLID IN WHICH THE VALANCE BAND IS PARTIALLY OVERLAPPED ON THE CONDUCTION BAND DUE TO WHICH FREE ELECTRONS ARE ALWAYS AVAILABLE FOR THE CONDUCTION OF CURRENT.

INSULATORS

THEY ARE THE STATE OF SOLID IN WHICH ENERGY GAP IS VERY LARGE(3ev>EG). THIS GAP BY THE ENERGY PROVIDES 

SEMICONDUCTORS

THEY ARE THE STATE OF SOLID IN WHICH THERE IS AN INTERMEDIATE GAP BETWEEN VALANCE BAND AND CONDUCTION BAND WHICH CAN BE EASILY OVERCOME BY THE ELECTRON FROM VALANCE BAND TO CONDUCTION BAND SUITABLE ENERGY IS PROVIDE TO THEM.

INTRINSIC SEMICONDUCTOR

CURRENT CARRIERS IN SEMICONDUCTORS:- THERE ARE 2 CURRENT CARRIERS IN SEMICONDUCTORS:-

1. FREE ELECTRON IN CONDUCTION BAND

2. HOLES IN THE VALANCE BAND 

IN INTRINSIC TYPES FORMATION OF CURRENT TOTALLY DEPENDS ON EXTERNAL HEAT PROVIDED TO THE VALANCE BAND. DUE TO WHICH FEW-ELECTRON IS EXCITED TO VALANCE BAND TO CONDITION BAND THIS RESULT IN THE EQUAL NUMBER OF HOLES IN VALANCE BAND.

ne=nh

ne/nh=1

NOTE:-IN INTRINSIC TYPE NUMBER OF FREE ELECTRON IN THE CONDUCTION BAND ARE ALWAYS EQUAL TO NUMBER OF HOLES GENERATED IN THE VALANCE BAND.

THESE ARE THE PUREST FORM OF SEMICONDUCTORS AND THEIR CONDUCTIVITY IS VERY LESS.

EXTRINSIC SEMICONDUCTOR

DOPED SEMICONDUCTOR

IN THESE TYPES EXTERNAL IMPURITIES ARE ADDED LIKE FREE ELECTRONS IN THE CONDUCTION BAND AND HOLES IN THE VALANCE BAND TO INCREASE THE CONDUCTIVITY OF THE SUBSTANCE.

DOPING:-ADDING IMPURITY FOR DESIRABLE THING

TYPES:-

1. N-TYPE:-IN THIS TYPE TETRAVALENT SILICON ATOM IS DOPED BY PENTAVALENT PHOSPHORUS ATOM 

2. P-TYPE:- IN THIS TYPE TETRAVALENT SILICON ATOM

N-TYPE SEMICONDUCTOR

IN N-TYPE TETRAVALENT Si ATOM IN DIFFUSED STATE IS DOPED BY PENTAVALENT ATOM PHOSPHORUS IN THIS WAY AN EXTRA ELECTRON WILL REMAIN IN THE VOID AND FREE SPACE THIS FREE ELECTRON GOES TO CONDUCTION BAND AND BY NUMBER OF SUCH PHOSPHOROUS ATOM THE CONDUCTION OF THE FREE ELECTRON IN COVALENT BOND WILL RISE WITH RESPECT TO HOLES IN THE VALANCE BAND

DOPING OF:-Si-TETRAVALENT ATOM

p-PENTAVALENT ATOM

P-TYPE SEMICONDUCTOR

THE EXTRINSIC P-TYPE SEMICONDUCTOR IS FORMED WHEN A TRIVALENT IMPURITY IS ADDED TO A PURE SEMICONDUCTOR IN A SMALL AMOUNT, AND AS A RESULT, A LARGE NUMBER OF HOLES ARE CREATED IN IT. A LARGE NUMBER OF HOLES ARE PROVIDED IN SEMICONDUCTOR MATERIAL BY THE ADDITION OF TRIVALENT IMPURITIES LIKE GALLIUM AND INDIUM.

EFFECT OF TEMPERATURE ON N-TYPE AND P-TYPE(FERMI-LEVEL)

ON INCREASING TEMPERATURE OF AN INTRINSIC TYPE FEW ELECTRONS GET EXCITED TOWARD THE CONDUCTION BAND BUT DUE TO THE NUMBER OF ELECTRONS OF DOPED PHOSPHOROUS ATOM THE CONDUCTION OF THE NEGATIVE CHARGE INCREASES IN THE COVALENT BOND. AS A RESULT, IF WE INCREASE THE TEMPERATURE FURTHER THEN THE EXCITED ELECTRON WILL STAY AT A TEMPORARY LEVEL JUST BELOW THE COVALENT BOND (0.01ev) THIS LEVEL IS CALLED AS FERMI LEVEL OR DONER LEVEL. THEREFORE WITH THE RISE OF TEMPERATURE, THIS TEMPORARY LEVEL WILL BE REPLACED BY THE FREE ELECTRON IN THE CONDUCTION BAND AND IT MOTION RESULT IS A DECREASE IN ENERGY FORBEDEN BAND. HENCE CONDUCTIVITY OF THE N-TYPE WILL INCREASE. A SIMILAR EFFORT IS ALSO HAPPENS IN P-TYPE THE FERMI LEVEL IS ALSO KNOWN AS ACCEPTOR LEVEL AND ITS UPWARD MOTION ALSO DECREASES THE ENERGY GAP.