Classification and Working of Semiconductors

INTRODUCTION
Department of ECE | VISAT, Elanji
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CLASSIFICATION OF METALS, CONDUCTORS AND
SEMICONDUCTORS
• On the basis of conductivity
• On the basis of energy bands
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*on the basis of conductivity
The solids are broadly classified as:
(i) Metals: They possess very low resistivity (or high
conductivity).
(ii) Semiconductors: They have resistivity or
conductivity intermediate to metals and
insulators.
(iii) Insulators: They have high resistivity (or low
conductivity).
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*on the basis of conductivity
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* On the basis of energy band
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Difference between energy bands of
(a) metals,
(b) insulators and
(c) semiconductors.
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CLASSIFICATION OF
SEMICONDUCTOR
SEMICONDUCTOR
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INTRINSIC
SEMICONDUCTOR
Three-dimensional diamond-
like crystal structure for Carbon,
Silicon or Germanium
Schematic two-dimensional
representation of Si or Ge structure showing
covalent bonds at low temperature
(all bonds intact). +4 symbol
indicates inner cores of Si or Ge.
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FIGURE 14.5
(a) Schematic model of generation of hole at site 1 and conduction electron due to thermal energy at
moderate temperatures.
(b) Simplified representation of possible thermal motion of a hole. The electron from the lower left hand
covalent bond (site 2) goes to the earlier hole site1, leaving a hole at its site indicating an apparent movement
of the hole from site 1 to site 2.
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FIGURE 14.6 (a) An intrinsic semiconductor at T = 0 K behaves like
insulator. (b) At T > 0 K, four thermally generated electron-hole pairs. The
filled circles represent electrons and empty fields represent holes.
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EXTRINSIC SEMICONDUCTORS
• Current flow and electron flow
• ADDING IMPURITY TO INCREASE NUMBER OF
ELECTRONS OR HOLES
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EXTRINSIC SEMICONDUCTORS
• P type
• N type
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PERIODIC TABLE OF SEMICONDUCTOR
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P TYPE SEMICONDUCTOR
These are materials which have Trivalent impurity atoms (Acceptors) added
and conduct by "hole" movement and are called, P-type Semiconductors.
In these types of materials are:
1. The Acceptors are negatively charged.
2. There are a large number of holes.
3. A small number of free electrons in relation to the number of holes.
4. Doping gives:
•negatively charged acceptors.
•positively charged holes
5. Supply of energy gives:
•positively charged holes.
•negatively charged free electrons.
and both P and N-types as a whole, are electrically neutral on their own.
Antimony (Sb) and Boron (B) are two of the most commonly used doping
agents as they are more feely available compared to others and are also
classed as metalloids.
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P type Semiconductors
• P-TYPE
• Ex: Boron, Aluminium
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N type Semiconductor
These are materials which have Pentavalent impurity atoms (Donors) added
and conduct by "electron" movement and are called, N-type Semiconductors.
• In these types of materials are:
1. The Donors are positively charged.
2. There are a large number of free electrons.
3. A small number of holes in relation to the number of free electrons.
4. Doping gives:
• positively charged donors.
• negatively charged free electrons.
5. Supply of energy gives:
• negatively charged free electrons.
• positively charged holes.
EX: Antimony,Phosphorous 18

N type Semiconductor
• N-type
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ENERGY BAND DIAGRAM
.
(b) p-type semiconductor at T > 0K
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PN JUNCTION
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PN Junction Formation Process
PN Junction Formation
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PN Junction at Equilibrium
(a) Diode underequilibrium (V = 0)
(b) Barrierpotential under no bias. 23

Semiconductor Diode
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Types of Semiconductor Diode
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P-N JUNCTION DIODE UNDER
FORWARD BIAS
(a) p-n junction diode under forward bias
(b) Barrier potential
(1) without battery
(2) Low battery voltage
(3) High voltage battery.
Forward bias.
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P-N JUNCTION DIODE UNDER
REVERSE BIAS
Click here
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Animation
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V-I CHARACTERISTICS OF DIODE
Experimental circuit arrangement for studying V-I characteristics of
a p-n junction diode
(a) in forward bias
(b) in reverse bias.
(c) Typical V-I characteristics of a silicon diode
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APPLICATION OF
JUNCTION DIODE AS A
RECTIFIER
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Half Wave Rectifier
Click Here
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FULL WAVE CENTRE TAPPED
(a) A Full-wave rectifier Circuit
(b) Input wave forms given to the diode D1 at A and to the
diode D2 at B
(c) Output waveform across the load RL connected in the
full-wave rectifier circuit.
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(a) A full-wave rectifier with capacitor filter,
(b) Input and output voltage of rectifier in (a)
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SPECIAL PURPOSE PN JUNCTION
DIODES
Zener diode
Vi characteristics of Zener diode 34

Zener diode as a voltage regulator
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Optoelectronic Junction Devices
PHOTODIODE
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LIGHT EMITTING DIODE
Advantages of LED
(i) Low operational voltage and less power.
(ii) Fast action and no warm-up time required.
(iii) The bandwidth of emitted light is 100 Å to
500 Å or in other words it is nearly (but not
exactly) monochromatic.
(iv) Long life and ruggedness.
(v) Fast on-off switching capability.
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Solar cell
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(a) Schematic representations of a n-p-n
transistor and p-n-p transistor
(b) Symbols for n-p-n and p-n-p transistors. WORKING VIDEO
Junction Transistor
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PNP Transistor
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NPN Transistor
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Basic transistor circuit configurations and
transistor characteristics
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INPUT CHARACTERISTICS
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OUTPUT CHARACTERISTICS
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AC Parameters Of Transistors
(i) Input resistance (ri): This is defined as the ratio of
change in base emitter voltage (ΔVBE) to the
resulting change in base current (ΔIB) at constant
collector-emitter voltage (VCE). This is dynamic (ac
resistance) and as can be seen from the input
characteristic, its value varies with the operating
current in the transistor:
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(ii) Output resistance (ro): This is defined as the ratio of change in collector-
emitter voltage (ΔVCE) to the change in collector current (ΔIC) at a constant base
current IB.
(iii) Current amplification factor (β ): This is defined as the ratio of
the change in collector current to the change in base current at a
constant collector-emitter voltage (VCE) when the transistor is in
active state.
This is also known as small signal current gain and its value is very large. If
we simply find the ratio of IC and IB we get what is called dc β of the
transistor. Hence,
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Transistor as a device
• AMPLIFIER
• SWITCH
• OSCILLATOR
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Transistor as a switch
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Transistor as an Amplifier
(CE-Configuration)
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Feedback amplifier and
transistor oscillator
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INTEGRATED CIRCUITS
Depending on nature of input signals, IC’s can be grouped in two categories: (a)
linear or analogue IC’s and (b) digital IC’s.
• The linear IC’s process analogue signals which change smoothly and continuously
over a range of values between a maximum and a minimum. The output is more or
less directly proportional to the input, i.e., it varies linearly with the input. Ex:
operational amplifier.
• The digital IC’s process signals that have only two values. They contain circuits
such as logic gates.
Depending upon the level of integration (i.e., the number of circuit components or
logic gates), the ICs are termed as
• Small Scale Integration, SSI (logic gates < 10);
• Medium Scale Integration, MSI (logic gates < 100);
• Large Scale Integration, LSI (logic gates < 1000);
• Very Large Scale Integration, VLSI (logic gates >1000).
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Classification and Working of Semiconductors

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