Download Electronics Engineering (NEC-101)

Solution of CT1
Electronics Engineering (NEC-101)
SECTION – A
(3 marks * 5 Questions = 15 Marks)
Ques1. Attempt any five questions:
Ques a. Draw the characteristics of transition and diffusion capacitance v/s applied bias voltage.
Define transition capacitance.
Ans a. Transition capacitance: A reverse biased PN-junction has a region of high resistivity
(depletion layer) sandwiched in between two regions of relatively low resistivity. The P-N
regions act as the plates of a capacitor and the depletion layer acts as the dielectric. This is
known as the transition capacitance or depletion capacitance.
Ques b Draw the ideal diode equivalent circuit. Explain the circuit. Also, draw its simplified
circuit.
Ans: Since silicon semiconductor diode does not reach the conduction state until VD reaches 0.7
V with a forward bias, a battery VT opposing the conduction direction must appear in the
equivalent circuit. The battery simply specifies that the voltage across the device must be greater
than the threshold battery voltage before conduction through the device in the direction dictated
by the ideal diode can be established. When conductionis established the resistance of the diode
will be the specified value of rav.
Ques c) What is the difference between zener and avalanche breakdown.
Sol: There are two types of breakdown mechanism in diode during reverse bias:
a) Avalanche Breakdown
b) Zener Breakdown
Avalanche Breakdown:
Avalanche breakdown occurs in lightly-doped pn-junctions
where the depletion region is comparatively long. The doping density controls the breakdown
voltage. The temperature coefficient of the avalanche mechanism is positive. Avalanche
breakdown is caused by impact ionization of electron-hole pairs. The electric field in the
depletion region of a diode is very high. Electron/holes that enter the depletion region undergo a
tremendous acceleration. As these accelerated carriers collide with the atoms they can knock
electrons from their bonds, creating additional electron/hole pairs and thus additional current. As
these secondary carriers are swept into the depletion region, they too are accelerated and the
process repeats itself.
Zener Breakdown:
Zener breakdown occurs in heavily doped pn-junctions. The heavy
doping makes the depletion layer extremely thin to allow the electric field across the region to be
very high. The high accelerating field and narrow depletion layer allows electrons to tunnel
through. This is called Zener Breakdown They can withstand large reverse current without
damage.
Ques d Differentiate conductor, semiconductor and insulator.
Sol
Ques e: Differentiate between intrinsic and extrinsic semiconductor with energy band diagram.
Sol: Intrinsic Semiconductor: An intrinsic semiconductor material is chemically very pure and
possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive
carriers (holes). A silicon crystal is different from an insulator because at any temperature above
absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked
loose from its position, leaving behind an electron deficiency called a "hole". If a voltage is applied,
then both the electron and the hole can contribute to a small current flow. The conductivity of a
semiconductor can be modeled in terms of the band theory of solids. The band model of a
semiconductor suggests that at ordinary temperatures there is a finite possibility that electrons can
reach the conduction band and contribute to electrical conduction.
Extrinsic Semiconductor: Where as an extrinsic semiconductor is an improved intrinsic
semiconductor with a small amount of impurities added by a process, known as doping, which
alters the electrical properties of the semiconductor and improves its conductivity. Introducing
impurities into the semiconductor materials (doping process) can control their conductivity.
Doping process produces two groups of semiconductors: the negative charge conductor (n-type)
and the positive charge conductor (p-type). Semiconductors are available as either elements or
compounds. Silicon and Germanium are the most common elemental semiconductors.
Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. Si and Ge both
have a crystalline structure called the diamond lattice. That is, each atom has its four nearest
neighbors at the corners of a regular tetrahedron with the atom itself being at the center. In
addition to the pure element semiconductors, many alloys and compounds are semiconductors.
The advantage of compound semiconductor is that they provide the device engineer with a wide
range of energy gaps and mobilities, so that materials are available with properties that meet
specific requirements. Some of these semiconductors are therefore called wide band gap
semiconductors.
N-Type Semiconductor
The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes
free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous
may be added by diffusion of phosphine gas (PH3).
P-Type Semiconductor
The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic
semiconductor creates deficiencies of valence electrons, called "holes". It is typical to use
B2H6 diborane gas to diffuse boron into the silicon material.
Ques f: What do you understand by dynamic and static resistance of diode?
Sol On the basis of type of applied voltage, diode resistance can be divided into two parts.
1) Static resistance or DC resistance. 2) Dynamic resistance or AC resistance.
Static Resistance- The resistive behavior of diode in presence of DC source is called static resistance
or DC resistance. It can be defined as the ratio of applied DC voltage across diode and its respective
current.
i.e RDC= VDC/IDC
Dynamic Resistance- The resistive behavior of diode in presence of ac source is called dynamic
resistance or ac resistance It is obtained by drawing a tangent to characteristics curve at the operating
point . It is equal to the ratio of change in voltage to change in current. It is given by:
rd= ΛVd/ΛId
∆Vd- is change in voltage across diode in presence of ac source. ∆Id- is change in current through diode in
presence of ac source.
SECTION – B
(5 marks * 1 Questions = 5 Marks)
Ques2. Attempt all question:
a) Explain Semiconductor diode in both forward and reverse bias. Also draw its
characteristics and symbol.
Sol: A p–n junction is a boundary or interface between two types of semiconductor material, ptype and n-type, inside a single crystal of semiconductor. It is created by doping. If two separate
pieces of material were used, this would introduce a grain boundary between the semiconductors
that would severely inhibit its utility by scattering the electrons and holes
Equilibrium (zero bias): In a p–n junction, without an external applied voltage, an equilibrium
condition is reached in which a potential difference is formed across the junction. This potential
difference is called built-in potential
.
After joining p-type and n-type semiconductors, electrons from the n region near the p–n interface
tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (donors) in
the n region. Likewise, holes from the p-type region near the p–n interface begin to diffuse into the ntype region, leaving fixed ions (acceptors) with negative charge. The regions nearby the p–n
interfaces lose their neutrality and become charged, forming the space charge region or depletion
layer (see figure A).
Figure A. A p–n junction in thermal equilibrium with zero-bias voltage applied. Electron and hole concentration are
reported with blue and red lines, respectively. Gray regions are charge-neutral. Light-red zone is positively charged.
Light-blue zone is negatively charged. The electric field is shown on the bottom, the electrostatic force on electrons
and holes and the direction in which the diffusion tends to move electrons and holes.
The electric field created by the space charge region opposes the diffusion process for both
electrons and holes. There are two concurrent phenomena: the diffusion process that tends to
generate more space charge, and the electric field generated by the space charge that tends to
counteract the diffusion. The carrier concentration profile at equilibrium is shown in figure A with blue
and red lines.
Forward bias
In forward bias, the p-type is connected with the positive terminal and the n-type is connected with
the negative terminal.
With a battery connected this way, the holes in the P-type region and the electrons in the N-type
region are pushed toward the junction. This reduces the width of the depletion zone. The positive
potential applied to the P-type material repels the holes, while the negative potential applied to the
N-type material repels the electrons. As electrons and holes are pushed toward the junction, the
distance between them decreases. This lowers the barrier in potential. With increasing forward-bias
voltage, the depletion zone eventually becomes thin enough that the zone's electric field cannot
counteract charge carrier motion across the p–n junction, as a consequence reducing electrical
resistance. The electrons that cross the p–n junction into the P-type material (or holes that cross into
the N-type material) will diffuse in the near-neutral region. Therefore, the amount of minority diffusion
in the near-neutral zones determines the amount of current that may flow through the diode.
Only majority carriers (electrons in N-type material or holes in P-type) can flow through a
semiconductor for a macroscopic length.
Reverse bias
A silicon p–n junction in reverse bias.
Connecting the P-type region to the negative terminal of the battery and the N-type region to
the positive terminal corresponds to reverse bias. Therefore, no current will flow until the diode
breaks down. Because the p-type material is now connected to the negative terminal of the power
supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the
depletion zone to increase. Likewise, because the N-type region is connected to the positive
terminal, the electrons will also be pulled away from the junction. Therefore, the depletion
region widens, and does so increasingly with increasing reverse-bias voltage. This increases the
voltage barrier causing a high resistance to the flow of charge carriers, thus allowing minimal electric
current to cross the p–n junction. The increase in resistance of the p–n junction results in the
junction behaving as an insulator.
Another application of reverse biasing is Varicap diodes, where the width of the depletion
zone (controlled with the reverse bias voltage) changes the capacitance of the diode.
SECTION – C
marks)
Ques3. Attempt all question:
(10 marks * 1 Questions = 10
a) Write a short note on
I.
Varactor Diode

Also called as Varicap, VVC (Voltage Variable Capacitance) or
tuning diode.

They are semiconductor, voltage-dependent, variable capacitor.

There mode of operation depends on capacitance that exists at
the p-n junction when the element is reverse biased.

The transition capacitance is CT = ε A
Wd

Where ε is permittivity of the semiconductor material

A is the area of p-n junction and Wd is the depletion width.

As the reverse bias is increased , the width of the depletion region increases
which in turn reduces the transition capacitance.
 In terms of applied reverse bias, the transition capacitance is
approximated by:
CT= K/(VT+VR)n
Where K is constant determined by the semiconductor material
VT is knee potential
VR is magnitude of applied reverse bias
n is ½ for alloy junctions and 1/3 for diffused junction
Applications
• It is used in variable resonant tank LC circuit. Here C part is
varied using varactor diode.
• AFC(Automatic Frequency Control) where in varactor diode is used
to set LO signal.
•Varactor is used as frequency modulator.
• It is used as frequency multiplier in microwave receiver LO.
• It is used as RF phase shifter.
II.
Light Emitting Diode

Light Emitting Diodes or LED´s, are among the most widely used of all the
different types of semiconductor diodes available today. They are the most
visible type of diode, that emit a fairly narrow bandwidth of either visible
light at different coloured wavelengths, invisible infra-red light for remote
controls or laser type light when a forward current is passed through them.
A “Light Emitting Diode” or LED as it is more commonly called, is basically
just a specialized type of PN junction diode, made from a very thin layer of
fairly heavily doped semiconductor material.
When the diode is forward biased, electrons from the semiconductors conduction
band recombine with holes from the valence band releasing sufficient energy to
produce photons which emit a monochromatic (single colour) of light. Because of this
thin layer a reasonable number of these photons can leave the junction and radiate
away producing a coloured light output.
LED Construction
Then we can say that when operated in a forward biased direction Light Emitting
Diodes are semiconductor devices that convert electrical energy into light energy.
The construction of a Light Emitting Diode is very different from that of a normal
signal diode. The PN junction of an LED is surrounded by a transparent, hard plastic
epoxy resin hemispherical shaped shell or body which protects the LED from both
vibration and shock.
Light Emitting Diodes I-V Characteristics.
Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves
showing the different colours available.