WEEK ONE

TOPIC: ELECTRONICS

SOLID STATE MATERIALS

CONDUCTORS

INSULATORS

SEMICONDUCTORS

CONDUCTORS

MATERIALS / SUBSTANCES THAT CAN CONDUCT ELECTRICITY, HEAT AND CHARGES CAN ALSO BE TRANSFERRED THROUGH THEM.

EXAMPLES: COPPER IRON SILVER SALT INORGANIC ACID SOLUTIONS

PROPERTIES OF CONDUCTORS

1. ALLOWS THE FLOW OF CURRENT

2. HAS LOW ELECTRICAL RESISTANCE AT ROOM TEMPERATURE

3. HAS A LARGE DENSITY OF MOBILE CHARGE CARRIERS (ELECTRONS)

4. A POSITIVE TEMPERATURE CO-EFFICIENT OF RESISTANCE (COPPER)

INSULATORS

MATERIALS / SUBSTANCES THAT CANNOT ALLOW THE FLOW OF HEAT, ELECTRIC CURRENT OR CHARGES TRANSFER THROUGH THEM.

EXAMPLES: PLASTIC MATERIALS GLASS AIR RUBBER

PROPERTIES OF INSULATORS

1. HIGH ELECTRICAL RESISTANCE

2. NO MOBILE CHARGE CARRIERS

3. NEGATIVE TEMPERATURE COEFFICIENT OF RESISTANCE (RUBBER, PLASTICS ETC)

SEMICONDUCTORS

MATERIALS WHICH HAVE AN ELECTRIC CONDUCTIVITY INTERMEDIATE IN VALUE BETWEEN A GOOD CONDUCTOR (PUREMETAL) AND A GOOD INSULATOR

EXAMPLES

SILICON

GERMANIUM

PROPERTIES OF SEMICONDUCTORS

1. SMALL NUMBER DENSITY OF CHARGE CARRIERS

2. NEGATIVE TEMPERATURE COEFFICIENT OF RESISTANCE

3. INTERMEDIATE RESISTIVITY OF 101AND 103 MM.

FREQUENT SENSITIVITY TO LIGHT

NEARLY OBEYS OHMS’ LAW OVER A WIDE RANGE OF CONDITIONS. EXCEPT AT A JUNTION OF MATERIALS

SILICON AND GERMANIUM HAVE FOUR VALENCE ELECTRONS AT THEIR OUTER SHELLS CRYSTALLINE IN NATURE.

MODES OF CONDUCTION IN SEMICONDUCTORS

IN ELECTROLYTE, TWO TYPES OF IONS ARE CHARGE CARRIERS:CATIONS – POSITIVE CHARGE CARRIERSANIONS – NEGATIVE CHARGE CARRIRS

IN SEMICONDUCTORS, TWO TYPES OF CHARGE CARRIERS:HOLES – POSITIVE CHARGE CARRIERSELECTRONS – NEGATIVE CHARGE

CARRIERS

INTRINSIC AND EXTRINSIC SEMICONDUCTORS

INTRINSIC SEMICONDUCTORSINTRINSIC SEMICONDUCTORS ARE PURE SILICON AND

GERMANIUM (I.E WITHOUT DOPING) HAVE EQUAL NUMBER OF HOLES AND ELECTRONS, BUT DO NOTMAKE EQUAL CONTRIBUTION BECAUSE THEY HAVE DIFFERENT MOBILITIES AND ARE FEW IN NUMBER. CURRENT CAN FLOW IN A PURE (INTRINSIC) SEMICONDUCTOR

THERMAL EFFECT OF SEMICONDUCTORSTHE DENSITY OF CHARGE CARRIERS (HOLES AND ELECTRONS) INA SEMICONDUCTOR INCREASES RAPIDLY WITH TEMPERATURE

EXTRINSIC SEMICONDUCTORS

THESE ARE SEMICONDUCTORS THAT HAVE HAD THEIR SEMICONDUCTING PROPERTIES ARTIFICIALLY MODIFIED BY ADDITION OF IMPURITY ATOMS TO THE CRYSTAL.

TYPES OF EXTRINSIC SEMICONDUCTORS1. N-TYPE SEMICONDUCTORS PRODUCED BY ADDING A TINY AMOUNT OF IMPURITY SUCH AS

ARSENIC OR PHOSPHORUS OR ANTIMONY , PENTAVALENT ELEMENTS TO A PURE SEMICONDUCTOR E.G GERMANIUM

CREATE MORE ELECTRONS THAN HOLES HENCE N-TYPE MAJORITY CARRIERS ARE FREE ELECTRONS (NEGATIVE

CHARGES) MINORITY CARRIERS ARE HOLES (+CHARGES) ARSENIC, PHOSPHORUS AND ANBTIMONY ARE CALLED DONOR ATOMS

2. P-TYPE SEMICONDUCTORS PRODUCED BY ADDING IMPURITIES SUCH AS BORON OR INDIUM IN A

TINY AMOUNT (A THREE-VALENT ELEMENT) TO SILICON, A PURE SEMICONDUCTOR

CREATES MORE HOLES THAN ELECTRONS MAJORITY CARRIERS ARE HOLES MINORITY CARRIERS ARE FREE ELECTRONS BORON AND INDIUM ARE CALLED ACCEPTOR ATOMS

DOPING

DOPING A PROCESS OF ALTERING THE CRYSTALLINE STRUCTURE OF PURE SEMICONDUCTOR (GERMANIUM OR SILICON) BY ADDING IMPURITY CRYSTAL TO PRODUCE MORE FREE ELECTRONS OR MORE HOLES. P-N JUNCTION DIODE A SINGLE SEMICONDUCTOR WHICH CONSISTS A P-TYPE AND N-TYPE SEMICONDUCTOR IN CONTACT. ACTS AS A RECTIFIER (FULL WAVE OR HALF WAVE) FUNCTIONS LIKE THERMIONIC DIODE CHANGING AC TO DC VOLTAGE NON-OHMIC CONDUCTORS SMALLER THAN THERMIONIC DIODE

A DIODE IS FORWARD BIASED WHEN P-TYPE IS CONNECTED TO POSITIVE AND N-TYPE NEGATIVE TERMINAL OF A BATTERY. REVERSE BIASED WHEN N-TYPE IS CONNECTED TO POSITIVE WHILE P-TYPE CONNECTS TO NEGATIVE TERMINAL OF A BATTERY.

A P-N JUNCTION DIODE HAS A LOW RESISTANCE IN FORWARD BIASED AND HIGH IN REVERSE BIASED

SEMICONDUCTOR DEVICES

RECTIFIER

VALVE

DIODE

TRIODE ETC

IN A PURE CONDUCTOR, E.G PURE METAL ITS RESISTANCE INCREASES WITH INCREASE IN TEMPERATURE, BUT DECREASES ITS CONDUCTIVITY.

TRANSISTOR

THIS IS A SEMI CONDUCTOR WHICH CAN AMPLIFY HIGH FREQUENCY SIGNALS AS WELL AS PRODUCE ELECTRICAL OSCILLATIONS.

PERFORMS SIMILAR FUNCTIONS TO THOSE OF A TRIODE

HAS THREE (3) PN JUNCTIONS AND TERMINALS EMITTER COLLECTOR and BASE

TYPES OF TRANSISTOR

1. nPn TRANSISTOR: A THIN LAYER p-TYPE SEMICONDUCTOR SANDWICHEDBETWEEN TWO PIECES OF n-TYPE SEMICONDUCTOR

2. PnP TRANSISTOR:A THIN LAYER n-TYPE SEMICONDUCTOR SANDWICHED BETWEEN TWO PIECES OF p-TYPE SEMICONDUCTOR

ADVANTAGES OF TRANSISTOR:- HAS HIGH VOLTAGE GAIN, BUT LOWER SUPPLY VOLTAGE- NO HEATING EFFECT

DISADVATAGE- LOW POWER DISSIPATION AND LOW INPUT IMPEDENCE- DEPENDENT ON TEMPERATURE

ASSIGNMENT

1. SKETCH A GRAPH TO SHOW HOW CURRENT THROUGH A JUNCTION DIODE VARIES WITH POTENTIAL DIFFERENCE ACROSSIT.

2. STATE THE TWO DEVICES WHICH OHM’S LAW DOES NOT APPLY(SSCE MAY/JUNE 2010)

3. (A) DIFFERENTIATE BETWEEN P-TYPE SEMICONDUCTOR AND N-TYPE SEMICONDUCTOR (B) INTRINSIC AND EXTRINSIC SEMICONDUCTOR.