The document provides information on several electronic devices: 1. It discusses semiconductors and describes intrinsic and extrinsic semiconductors. Extrinsic semiconductors have their electrical conductivity increased by adding impurities. 2. PN junction diodes allow current to flow more easily in one direction. Zener diodes can be used for voltage regulation. Varactor diodes have a capacitance that varies with applied voltage. 3. Tunnel diodes exhibit negative resistance due to quantum tunneling. They can be used in high frequency oscillator circuits and are resistant to nuclear radiation.

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FUNDAMENTALS ON ELECTRONIC DEVICES
M.Prabu, M.E., (Ph.D).,
Assistant Professor,
Department of ECE,
MNM Jain Engineering College,
Chennai-97.
B.Sathya, M.E.,
Assistant Professor,
Department of ECE,
MNM Jain Engineering College,
Chennai-97.

2. Semiconductors
 A semiconductor is a substance which has resistivity in between conductors and insulators.
 It has negative temperature co-efficient of resistance which means the resistance of a semiconductor
decreases with the increase in temperature and vice-versa.
 When a suitable metallic impurity is added to a semiconductor, its current conducting property change
appreciably.
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3. Semiconductors
Intrinsic semiconductor
 An intrinsic semiconductor is a pure semiconductor which does not have any doping agent (an
impurity added to crystal lattice).
 In an intrinsic semiconductors, the number of charge carrier depends upon property of the material
not on impurity present.
 Has little current conduction capability at room temperature.
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4. Semiconductors
Extrinsic Semiconductors
 An extrinsic semiconductor is a semiconductor whose electrical conductivity can be increased by
adding the trace amounts of other elements such as impurities in the material.
 There are two types of impurities added to Ge and Si Crystal.
 Pentavalent material -it is made up of atoms which have five valence electrons.
 Trivalent material: it is materials has three valence electrons.
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5. Semiconductors
Doping: To make the semiconductor conduct electricity, other atoms called impurities must be
added.
“Impurities” are different elements. This process is called doping.
Semiconductors can be Conductors
 An impurity, or element like arsenic, has 5 valence electrons.
 Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring
silicon atoms.
 The one electron left over for each arsenic atom becomes available to conduct current flow.
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6. PN Junction Diode
Source: Wikipedia
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7. PN Junction Diode
 A diode (PN junction) in an electrical circuit allows current to flow more easily in one direction than
another.
 Forward biasing means putting a voltage across a diode that allows current to flow easily, while
reverse biasing means putting a voltage across a diode in the opposite direction.
Forward bias
Reverse bias
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8. PN Junction Diode – Reverse Bias
If a voltage is applied across the diode in such a way that the n-type half of the diode was connected
to the positive terminal of the voltage source and the p-type half was connected to the negative
terminal, electrons from the external circuit would create more negative ions in the p-type region by
"filling the holes" and more positive ions would be created in the n-type region as electrons are
displaced toward the positive terminal of the voltage source.
Source: https://energyeducation.ca
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9. PN Junction Diode – Forward Bias
Source: https://energyeducation.ca
 Forward-biased scenario ensures that the electrons and holes move toward the junction as they are
repelled from the positive and negative terminals of the voltage source respectively.
 Given a great enough applied voltage, both the holes and the electrons would overcome the
depletion region and meet near the junction, where they could combine in a continuous process,
closing the circuit and allowing current flow.
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10. Forward Voltage and Breakdown Voltage
 There is a minimum threshold voltage required to overcome the depletion region, which for most
silicon diodes is a significant 0.7 volts.
 Furthermore, reverse-bias voltage does induce a small amount of current through the diode called
leakage current that is essentially negligible for most purposes.
 Finally, a great enough reverse voltage will result in the complete electronic breakdown of the diode
and allow current to flow through the diode in the reverse direction.
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11. PN Junction Diode
Applications:
 It can be used as a solar cell.
 When the diode is forward-biased, it can be used in LED lighting applications.
 It is used as rectifiers in many electric circuits and as a voltage-controlled oscillator in varactors.
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12. Zener Diode
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Source: Wikipedia

13.  This diode operates similar to the normal diode when in the forward-bias mode. And it has the
turn-on voltage of values between 0.3 V and 0.7 V.
 Whereas when connected it in the reverse mode, which is usual in most of its applications, then a
small leakage current may also flow. Since the reverse voltage increases to the predetermined
breakdown voltage.
Zener Diode
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Source: https://www.electronics-tutorials.ws

15. Zener Diode
Applications:
 Voltage regulator
 Zener diode in overvoltage protection
 Zener Diode Clipping Circuits
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VARACTOR DIODE

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Symbol of Varactor Diode
 The symbol of the varactor diode is similar to that of the PN-junction diode. The diode has two
terminals namely anode and cathode.
 The one end of a symbol consists the diode, and their other end has two parallel lines that
represent the conductive plates of the capacitor. The gap between the plates shows their
dielectric.

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Varactor Diode
Working of Varactor Diode

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 The Varactor diode is made up of n-type and p-type semiconductor material.
 In an n-type semiconductor material, the electrons are the majority charge carrier and in the p-
type material, the holes are the majority carriers.
 When the p-type and n-type semiconductor material are joined together, the p-n junction is
formed, and the depletion region is created at the PN-junction.
 The positive and negative ions make the depletion region. This region blocks the current to enter
from the PN-region.
 The varactor diode operates only in reverse bias. Because of reverse bias, the current does not
flow.
 If the diode is connected in forward biasing the current starts flowing through the diode and their
depletion region become decreases.
 The depletion region does not allow the ions to move from one place to another.
Varactor Diode

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Varactor Diode
 The Varactor diode is used for storing the charge not for flowing the charge.
 In the forward bias, the total charge stored in the diode becomes zero, which is undesirable.
 Thus, the Varactor diode always operates in the reverse bias.
 The capacitance of the varactor diode increases with the increase of n and the p-type region and
decreases with the increases of the depletion region.
 The increase in capacitance means the more charges are stored in the diode.
 For increasing the storage capacity of charge the depletion region (which acts as a dielectric of the
capacitor) of the diode should be kept small.

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Characteristic of Varactor Diode
Varactor Diode
 The characteristic curve of the varactor diode is shown in the figure below.
 The graph shows that when the reverse bias voltage increases the depletion region increases, and
the capacitance of the diode reduces.

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Varactor Diode
Advantages of Varactor Diode:
 The varactor diode produces less noise as less compared to the other diode.
 It is less costly and more reliable.
 The varactor diode is small in size and less in weight.
Varactor Diode Applications:
 Voltage controlled oscillators, VCOs
 RF filters
 Frequency & phase modulators

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TUNNEL DIODE

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Tunnel Diode
 A tunnel diode (also known as a Esaki diode) is a type of semiconductor diode that has effectively
“negative resistance” due to the quantum mechanical effect called tunneling.
 Tunnel diodes have a heavily doped pn junction that is about 10 nm wide.
 The heavy doping results in a broken band gap, where conduction band electron states on the N-
side are more or less aligned with valence band hole states on the P-side.
 The application of transistors in a very high in frequency range are hampered due to the transit
time and other effects.
 Many devices use the negative conductance property of semiconductors for these high frequency
applications.
 A tunnel diode is one of the most commonly used negative conductance devices. It is also known as
Esaki diode after L. Esaki for his work on this effect.

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 The concentration of dopants in both p and n region is very high, at around 1024 – 1025 m-3. The pn
junction is also abrupt. For this reasons, the depletion layer width is very small.
 In the current voltage characteristics of tunnel diode, we can find a negative slope region when a
forward bias is applied.
 The name “tunnel diode” is due to the quantum mechanical tunneling is responsible for the
phenomenon that occurs within the diode.
 The doping is very high so at absolute zero temperature the Fermi levels lies within the bias of the
semiconductors.
Tunnel Diode

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Tunnel Diode Working Phenomenon
Unbiased Tunnel Diode
 In an unbiased tunnel diode, no voltage will be applied to the tunnel diode. Here, due to heavy
doping conduction band of n – type semiconductor overlaps with valence band of p – type material.
Electrons from n side and holes from p side overlap with each other and they will be at same energy
level.
 Some electrons tunnel from the conduction band of n-region to the valence band of p-region when
temperature increases. Similarly, holes will move from valence band of p-region to the conduction
band of n-region. Finally, the net current will be zero since equal numbers of electrons are holes flow
in opposite direction.

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Unbiased Tunnel Diode

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Small Voltage Applied to the Tunnel Diode
 When a small voltage, that has lesser value than the built-in voltage of the depletion layer, is applied
to the tunnel diode, there is no flow of forward current through the junction. Nevertheless, a
minimal number of electrons from the conduction band of n region will start tunneling to valence
band in p region.
 Therefore, this movement creates a small forward biased tunnel current. When a small voltage is
applied, tunnel current starts to flow.

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Small Voltage Applied to the Tunnel Diode

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Increased Voltage Applied to the Tunnel Diode
 When the amount of voltage applied is increased, the number of free electrons generated at n side
and holes at p side is also increased. Due to voltage increase, overlapping between the bands are
also increased.
 Maximum tunnel current flows when the energy level of n-side conduction band and the energy
level of a p-side valence band becomes equal.

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Increased Voltage Applied to the Tunnel Diode

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Further Increased Voltage Applied to the Tunnel Diode
 A further increase in the applied voltage will cause a slight misalignment of the conduction band
and valence band. Still there will be an overlap between conduction band and valence band. The
electrons move from conduction band to valence band of p region. Therefore, this causes small
current to flow. Hence, tunnel current starts decreasing.

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Further Increased Voltage Applied to the Tunnel Diode

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Largely Increased Voltage Applied to the Tunnel Diode
 The tunneling current will be zero when applied voltage is increased more to the maximum. At this
voltage levels, the valence band and the conduction band does not overlap. This makes tunnel
diode to operate same as a PN junction diode.

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Tunnel Diode
Characyteristics

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Tunnel Diode
Tunnel Diode Applications
• Oscillator Circuits
• Microwave Circuits
• Resistant to Nuclear Radiation

38. TRANSISTOR
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