CHSE semiconductor (electronic devices ) best notes
Deleted portions
(p-n junction diodes, LED, photodiode, solar cell and Zener diode and their characteristics, Zener diode as a voltage regulator.)
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1. Solid: Matter having definite shape and definite volume is known as solid. It is two types:
(i)Crystalline solid: it is characterised by
long range order' with natural and regular
geometrical form having sharp melting point
and anistropy.
For Example: NaCI, KCI, Sugar diamond
etc.
ii) Amorphous solid: It is characterised by
"short range order' with irregular
geometrical form having no sharp melting
point and isotropic.
For Example : glass, wax, charcoal, pitch,
rubber etc.
2. Binding in Solids:
(a) lonic Binding: The binding in solid duee
to attraction between ions.
For Example : NaCI, KOH, MgO, CuSO
(b)Covalent Binding: The binding in solid
due to sharing of electrons between
neighbouring atoms.For Example : HCI, CLH, H,O, NH, CH etc.
(C)Metallic Binding: This is due to
electrostatic attraction between the
electrons and ions of different atom.
For Example : Li, Na, K etc.
(d) Vander Waal's Binding: The bond due
to very weak attractive force is known as
Vander Waal's bond.
3. Energy band in Solids: The large number ofenergy levels confined in a small region of energyrange of a given solid, constitute energy bonds.
Thus, the collection of very closely spaced levels is called an energy band. In some solids, there is an energy gap in between the energy bonds. This energy gap is called forbidden gap. The energy
band above forbidden gap is called conduction band and the energy band below the forbidden gap is called valence band. The conductivity of solids depends upon the number of electrons present in the conduction band and number of
holes present in valence band.
4.Classification of metals, insulators and
semiconductors on the basis of energy
band
Metals: The energy band for a metal is
such that the valence band and
conductance band overlap each other and
there is no forbidden gap in between. In
fact it may be considered that a metal has
a single energy band, which is partly filled
and partly empty. The highest filled energy
level in the conduction band at absolute zero
temperature is called fermi level (F) and
the energy corresponding to it is called fermi
energy.
(i) Insulators: Here ordinarily the valence
band is completely filled, the conductance
band is normally empty and forbidden gap
is very large = 6 eV. No electrons from
valence band can cross over to conduction
band at room temperature, even if electric
field is applied.
(ii) Semiconductors: In semiconductors the
valence band is completely filled and the
conduction band is completely empty and
these bands are separated by a small
forbidden gap of about 1.1 eV At 0°K,
there is no conduction in semiconductors
but at room temperature electrons in
valence band acquire energy greater than
forbidden gap and hence move to
conduction band. This makes conduction
through semiconductors possible.
5.Biasing: When a p-n junction is connected to an external voltage source whose magnitude well as direction can be changed, is called biasing of p.n. junction.
i)Forward Biasing: When the positive
terminal ofthe external battery is connected
to p-side and the negative terminal of the
external battery is connected ton-side of
the p.n. junction, the biasing so done is
called forward biasing. In forward biasing
conduction across the P-N junction takes
place due to migration of majority charge
carriers (i.e. electrons from n-side to p-
side and holes from p-side to n-side). The
resistance of the p.n. junction
decreases in forward biasing.
ii) Backward Biasing: When the positive
terminal of the external battery is connected to the n-side and the negative teminal of
the external battery to the p-side of the p.n.
junction, the biasing so done is called
reverse biasing. In reverse biasing
conduction across the p.n. junction takes
place due to minority charge carriers. The
resistance of the p-n junction increases in
reverse biasing. It is also known as
Reverse Biasing.
6.Condition of Doping
(1) The dopant atoms should exactly substitute the semiconductor atoms in the crystal lattice.
(2) The presence of dopant should not distort
the crystal lattice.
(3) The concentration of dopants should not
be large.
7.HoleThe vacancies created in the valence
band due to transference of electrons to the
conduction band are termed as the holes. Thus hole has a positive charge equal to that of an electron.
8.Fermi Level: It is the highest energy level
occupied by an electron at zero kelvin.
Fermi Energy : It is the highest energyy of the electron at zero kelvin.
9.Intrinsic Semiconductors : Intrinsic semi-
conductors are pure semiconducting materials in which no impurity atoms are added. The electrical conductivity of such a pure semiconductor is totally governed by the electrons excited from the valence band into the conduction band and it increases with increase in temperature e.g., Germanium and Silicon.
10.Doping: Doping is the process of deliberate addition of a suitable impurity in a controlled manner to a pure semiconductor so as to modify its electrical properties. The impurity atoms so
added are called dopants.
11. Extrinsic Semiconductor: A doped
semiconductor is called extrinsic semiconductor.
They are of two types:
n-type Semiconductor: When a pure
semiconductor is doped with a controlled
amount of pentavalent atoms (Arsenic,
Antimony, Bismuth), we get n-type
semiconductor or donor semiconductor.
Thus, there is one extra electron per atom
of impurity which remains available as an
almost free electron. In n-type
semiconductor electrons are majority
charge carriers and holes are minority
charge carriers.
(ii) p-type Semiconductor: When a pure
semiconductor is doped with a controlled
amount of trivalent 'atoms (Boron,
Aluminium, Gallium, Indium), we get p type
semiconductor or acceptor semiconductor.
In p-type semiconductor, holes are majority
charge carriers and electrons are minority
charge carriers.
12.Zener Diode: It works on phenomenon of
breakdown at reverse voltage, for which large changes in diode current produced only a small change in diode voltage.
Rectifier: It is, a semiconductor device which
converts alternating current to direct current.
They are of two types:
I) Half-Wave Rectifier: It is the
semiconductor device which converts only
one half of each a.c. cycle to d.c.
II) Full-Wave Rectifier: It is the
semiconductor device which converts full
a.c. cycle to d.c. Junction Transistor: A junction transistor is obtained either by sandwiching anarrow n-type
crystal between two comparatively widerp-type crystals or by.sandwiching a. narrow p-type crystal between two comparatively wider n-type.
13.Semiconductor Diode: When a p-type and
n-type semiconductor are joined, the contae
contact surface dividing the two halves is called pn junction. It is also known as semiconductor diode or crystal diode or junction diode.
14.Effect of temperature on conductivity of
semiconductors: With the increase in
temperature, more number of covalent bonds are broken, resulting large increase in carrier concentration and hence conductivity of a semiconductor increases.
15.P.N. Junction : When a p-type crystal is
brought in close contact withn-type crystal, the resulting conductance is called as P.N junction
16.Potential Barrier: The potential difference
resulting arrangement is called p-n junction.
developed betweenp and n regions about the
junction due to the migration of charge carriers is called the potential barrier. It is also known as turn-on voltage.
17.Depletion Region: On account of the
formation of the potential barrier across tne
junction inap-n crystal, the vicinity of the juncton is devoid of free charge carriers and
immobile ions. This reÄŸion in which no free charge carriers are available is called a depletion region.