Semiconductor Flashcards
Learn about semiconductor and it's properties
Which band has the highest energy?
Valence band
How are these bands filled?
- Valence band?
- Conduction band?
- Inner orbitals?
- Partially or fully filled
- Empty or partially filled
- Fully filled
What are the properties of conduction band?
It is also called empty band of minimum energy.
This band is partially filled by the electrons. In this band the electrons can gain energy from external electric field.
The electrons in the conduction band
are called the free electrons. They are able to
move anywhere within the volume of the solid.
● Current flows due to such electrons
What are the properties of valence band?
● This is the band of maximum energy.
● Electrons are not capable of gaining energy from external electric field.
● No flow of current due to such electrons.
● The highest energy level which can be occupied by an electron in valence band at 0 K is called fermi level.
- Forbidden Energy Gap (ΔEg)
Energy gap between conduction band and valence band
ΔEg= (C.B.)min - (V.B.)max
a. No free electron present in forbidden
energy gap.
b. Width of forbidden energy gap depends upon the nature of substance.
c. As temperature increases ( ↑ ), forbidden
energy gap decreases (↓) very slightly.
The electrical conductivity of a semiconductor increases when electromagnetic radiation of wavelength shorter than 2480 nm is incident on it. The band gap in (eV) for the semiconductor is
A. 2.5 eV
C. 0.7 eV
B. 1.1 eV
D. 0.5 eV
E = hc/λ = 1240/2480 ev-nm
=0.5ev
Energy gap
Conductors:-Zero or very small Insulators:- Very large; diamond 6 eV Semiconductors:- Very large; diamond 6 eV for Ge Eg = 0.7 eV for Si Eg = 1.1 eV
Condition of
V.B. and C.B. at ordinary
temperature
Conductors:-
V.B. and C.B. are
completely filled or C.B. is somewhat empty
Insulators:-
V.B. – completely
filled
C.B. – completely unfilled
Semiconductors:-
V.B. – somewhat empty
C.B. – somewhat filled
Semiconductor is characterised by -
A. Full filled valence band
B. Partially filled valence band
C. Overlapping of valence band and conduction band
D. None of these
B
Electrical
Conductivity
Conductor > Semiconductor > Insulator
Resistivity
Insulator> Semiconductor> Conductor
Temperature coefficient of resistance (α)
Positive
Zero
Negative
Effect of temperature
on conductivity
Decreases
—
Increases
Effect of temperature on resistance
Increases
—
Decreases
Cu and Ge are cooled to 70K then -
A. The resistance of Cu will increase and that of
Ge will decrease
B. the resistance of Cu will decrease and that of Ge will increase
C. the resistance of both Cu and Ge decrease
D. the resistance of both Cu and Ge increase
B
Choose only false statement from the following -
A. In conductors the valence and conduction band overlap
B. Substance with energy gap of the order of 10 eV are
insulators.
C. The resistivity of a semiconductor increase with increase
in temperature.
D. The conductivity of semiconductor increase with increase in temperature
C
What are holes?
When temperature increases the valence electrons acquires thermal energy to jump to the conduction band (Due to the breaking of covalent bond). If they jumps to C.B. they leaves behind the deficiency of electrons in the valence band. This deficiency of electron is known as hole or cotter
What are charge and magnitude of a hole?
A hole is considered as a seat of positive charge, having magnitude of charge equal to that of an
What are the properties of holes?
- Holes acts as virtual charge, although there
is no physical charge on it. - Effective mass of hole is more than electron.
- Mobility of hole is less than electron.
- The drift velocity of electrons (ve) is greater than that of holes (vh)
The mobility of free electrons is greater than that of free holes because
A. they carry negative charge.
B. mutual collision in them is less.
C. they require low energy to continue their motion
D. none of these
C
What are intrinsic semiconductors?
A Pure Semiconductor is called intrinsic semiconductor. It has thermally generated current carriers.
Properties of intrinsic semiconductors?
- They have four electrons in the outermost orbit of
atom and atoms are held together by covalent bond - Free electrons and holes both are charge carriers and
ne(in C.B.) = nh(in V.B.) - In pure semiconductor, impurity must be less than
1 in 108 parts of semiconductor - Because of less number of charge carriers at room temperature, intrinsic semiconductors have low conductivity so they have no practical use.
- For them fermi energy level lies at the centre
of the C.B. and V.B.
Formula
When some potential difference is applied across a piece of intrinsic semiconductor current flows in it due to both electron and holes
i=eA[ne ve + nh vh]
where ve = drift velocity of electron,
vh = drift velocity of holes, H
If the ratio of the concentration of electrons to that of holes in a semiconductor is 7/5 and the ratio of currents is 7/4, then what is the ratio of their drift velocities?
A. 5/8
B. 4/5
C. 5/4
D. 4/7
ne/nh = 7/5 ie/ih = 7/4
we know, i=neavd
ie/ih=ne ve/nh vh
so, ve/vh = 5/4
What are extrensive semiconductors?
- It is also called impure semiconductor.
2. The process of adding impurity is called Doping.
What are pentavalent impurities?
The elements whose atom has five
valence impurities e.g. As, P, Sb etc. These are also called donor impurities. They donates extra free electron.
Also called N-type Semiconductors
What are trivalent impurities?
The elements whose each atom has three valence electrons are called trivalent impurities
e.g. In, Ga, Al, B, etc.
These impurities are also called acceptor impurities as they accept electron.
Also called P-type Semiconductors
Properties of n-type semiconductors
N-type Semiconductors
Majority charge carriers – electrons
Minority charge carriers –
holes
ne»_space; nh ; ie»_space; ih
Conductivity σ ≈ neμee
N-type semiconductor is electrically
neutral (not negatively charged)
Impurity is called Donor impurity because
one impurity atom generate one e–
Donor energy level lies just below the conduction band.
Properties of P-type semiconductors
Majority charge carriers – holes
Minority charge carriers –
electrons
nh»_space; ne ; ih»_space; ie
Conductivity σ ≈ nhμhe
P-type semiconductor is also electrically neutral (not positively charged)
Impurity is called Acceptor impurity.
Acceptor energy level lies just above
the valence band.
A semiconductor is known to have an electron concentration of 8 × 1013 per cm3 and hole concentration of 5 × 1012 per cm3.
The semiconductor is -
A. n-type
B. p-type
C. intrinsic D. none of these
A
If n-type semiconductor is heated then -
A. the number of electrons increases and the
number of holes decrease.
B. the number of holes increases and the number
of electrons decreases.
C. the number of electrons and holes both remain Equal.
D. the number of both electrons and holes increases
D
In N-type semiconductors, the concentration of
minority charge carriers mainly depends upon -
A. the doping technique
B. the doping ratio
C. the temperature of the material
D. None
C
An n-type semiconductor is - A. negatively charged B. positively charged C. neutral D. negatively or positively charged depending upon the amount of impurity
C
What is a Diode?
When a P-type semiconductor is suitably joined
to an N-type semiconductor, then resulting arrangement is called P-N junction or P-N junction diode
In p-n junction what is the direction
of internal electric field -
A. From p to n
B. From n to p
C. Perpendicular to the p-n junction
D. There is no electric field
B
What is potential barrier
The potential difference created across the P-N junction due to
the diffusion of electron and holes is called potential barrier.
For Ge VB = 0.3 V and
for silicon VB= 0.7 V
A potential barrier of 0.50V exists across a P-N junction.
If the depletion region is 5.0 x 10-7 m wide,
the intensity of the electric field in this region is
1.0 x 10^6 V/m
Thus drift current and the diffusion current
are in ___________ directions.
opposite
Forward biasing
Positive terminal of the battery is connected to
the P-crystal and negative terminal of the
battery is connected to N- crystal
Reverse biasing
Positive terminal of the battery is connected to the
N-crystal and negative terminal of the battery is
connected to P-crystal
What is the value of inet in case of steady state?
What is the diode called when no external source is connected?
In steady state idissusion = idrift so inet = 0
When no external source is connected, diode is called unbiased.
RForward and RReverse?
10Ω - 25Ω and 10^5Ω
Potential barrier for forward biasing and reverse biasing?
Forward bias opposes the potential barrier and for V > VB a forward
current is set up across the junction.
Reverse bias supports the potential barrier and no current flows across the junction due to the diffusion of the majority carriers.
(A very small reverse currents may exist)
The ratio of resistance for forward to reverse bias
of P-N junction diode is -
1:10^4
Cut knee Voltage
The voltage at which current starts to increase
For germanium it is 0.3 V and for Si it is 0.7 V
Breakdown voltage
Reverse voltage at which breakdown of semiconductor occurs.
For Ge it is 25V and Si it is 35V
Zener breakdown
When reverse bias is increased
the electric field at the junction also increases. At some stage the electric field becomes so high that it breaks the covalent bonds
creating electron, hole pairs.
Thus a large number of carriers are generated.
This causes a large current to flow.
This mechanism is known as Zener breakdown.
Avalanche breakdown
At high reverse voltage, due to high electric field,the minority charge carriers, while crossing the junction acquires very high velocities.
These by collision breaks down the covalent bonds, generating more carriers. A chain reaction is established,
giving rise to high current.
This mechanism is called avalanche breakdown.
zener diode
It is the highly doped diode in which
reverse current increases very sharply.
It can operate continuously without damaging the junction. The symbol of zener diode is
A zener diode is like an ordinary diode except that it is heavily doped so as to have a sharp breakdown voltage called zener voltage.
Zener diode is always used in reverse bias.
It is used for voltage stabilisation.
LED
When a conduction electron falls into a hole in valence band,
the energy may be emitted as a photon.
For usual diodes the wavelength of photon emitted
lies in infrared region.
If the wavelength of the photons is
in visible range (4000 Å to 7000 Å),
the emitted photon will cause visual effect.
Such a diode is known as light emitting diode and abbreviated as LED. The LEDs are usually made from gallium arsenide (GaAs) or indium
phosphide. LED’s are used in electronic gadgets and indicator lights.
Photodiode
Photodiode is a diode whose function is controlled by the light falling on it.
Light of wavelength λ is sufficient to break the valence bond falls
on the junction, new hole-electron pairs are created.
Circuit current can be controlled by incident light.
For making p-n junction diode in forward biased -
A .same potential is applied
B. greater potential is given to n compared to p
C. greater potential is given to p compared to n
D. unbalanced concentration
C
In the middle of the depletion layer of a
reverse-biased p-n junction, the
A. Electric field is zero
B. Potential is maximum
C. Electric field is maximum
D .Potential is zero
A
When p-n junction diode is forward biased then
A .Both the depletion region and barrier height is reduced
B. The depletion region is widened and barrier height is reduced
C. The depletion region is reduced and barrier height is increased
D. Both the depletion region and barrier height are increased
A
In a full wave rectifier circuit operating from 50 Hz mains frequency, the fundamental frequency in the ripple (output) would be
A. 25 Hz
B. 50 Hz
C. 70.7 Hz
D. 100 Hz
D
