p-n junction theory

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ASHVANI SHUKLAManager(c i)Bgr Energy

• P-N JUNCTION

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Introduction

• History of p-n Junction
• In November 16, 1904 first vacuum tube was
  invented by Sir John Ambrose Fleming and it is
  called the Fleming valve, the first thermionic
  valve. There was no existence of p-n junction in
  electronics field. In October 20, 1906 Triode
  Tube had been developed by Dr. Lee de Forest. A
  conceptual figure of vacuum diode is shown below.

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• Here the vacuum tube works mostly like modern
  diode. But its size is larger. It consists of a
  vacuum container with cathode and anode inside.
  This cathode and anode are connected across a
  high voltage source. Generally it works on
  principle of thermo ionic emission. This cathode
  is heated by filament an hence electron get
  emitted from cathode towards anode. So it is also
  known as thermionic tube. Current only flow from
  the anode to cathode i.e. unidirectional flow.
  The V-I characteristics of a vacuum tube is shown
  below.

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How does vacuum tube diode work?
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• Filament creates heat to the cathode to emit
  electrons. Beam of electrons flows from cathode
  to anode through the space between cathode and
  anode. The voltage difference is created across
  the cathode and anode by applying high voltage
  across their terminal. The replacement of the
  electrons in the electrodes is happened by this
  voltage source. Under reverse bias this vacuum
  tube does not work or it does not have any
  breakdown. This vacuum tube was the basic
  component of electronics throughout the first
  half of the twentieth century. It was available
  and common in the circuit of radio, television,
  radar, sound reinforcement, sound recording
  system, telephone , analog and digital computers,
  and industrial process control.

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• Gradually p-n junction semiconductor has come in
  the market and vacuum tubes got replaced by them.
  But till today somewhere vacuum tubes are being
  used widely. These fields for application of the
  vacuum tubes are in Atomic Clocks Audio
  Systems Car Dashboards Cellular Telephone
  Satellites Computer Monitors DVD Players
  Recorders Electromagnetic Testing Electron
  Microscopes Gas Discharge Systems Gas Lasers
  Guitar Amplifiers Ham Radio High-speed
  Circuit Switching Industrial Heating Ion
  Microscopes Ion Propulsion Systems Lasers
  LCD Computer Displays Lighting Microwave
  Systems Microwave Ovens Military Systems
  Mobile Phone, Bluetooth Wi-Fi Microwave
  Components Musical Instrument Amplifiers
  Particle Accelerators Photomultiplier Tubes
  Plasma Panel Displays Plasma Propulsion Systems
  Professional Audio Equipment Radar Systems
  Radio Communications Radio Stations Recording
  Studios Solar Collectors Sonar Systems
  Strobe Lights Satellite Ground Stations
  Semiconductor Vacuum Electronic Systems TV
  Stations Vacuum Electron Devices Vacuum Panel
  Displays

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• Types of Vacuum Diodes
• The vacuum diodes are classified as 1. frequency
  range wise (audio, radio, microwave) 2. power
  rating wise (small signal, audio power) 3.
  cathode/filament type wise (indirectly heated,
  directly heated) 4. application wise (receiving
  tubes, transmitting tubes, amplifying or
  switching) 5. specialized parameters wise (long
  life, very low micro phonic sensitivity and low
  noise audio amplification) 6. specialized
  functions wise (light or radiation detectors,
  video imaging tubes)

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• After the vacuum tubes, p-n junction
  semiconductor came in the market. The circuit
  gets lighter and more compact. The p-n junction
  semiconductor made of either Silicon or Germanium
  material which has four numbers of electrons in
  the valence band. From this valence band the
  electron transit to the conduction band
  penetration the energy gap of one electron volt
  approximately. Generally pure Silicon or
  Germanium has no extra electron available in
  their crystal structure. But applying of thermal
  energy to this crystal some bonds break and some
  electrons get available in the conduction band.
  But current is very small or in the order of
  microampere.

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• This pure semiconductor is called intrinsic
  semiconductor. But some impurities are added to
  the pure semiconductor material like Al, P etc.
  Boron has three electrons in valence band. So one
  Boron atom holds four Silicon atoms with one bond
  with one electron. This deficiency of one
  electron in this bond is called as hole. After
  adding Boron to the intrinsic material this
  semiconductor gets abundance of holes in its
  lattice structure. This semiconductor is called
  extrinsic semiconductor. Due to abundance of
  holes it is known as positive type or p-type
  semiconductor.

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• Phosphorus has five electrons in valence band. So
  one phosphorus atom holds four Silicon atoms. But
  one electron becomes extra. After adding
  phosphorus to the intrinsic material this
  semiconductor gets abundance of electrons in its
  lattice structure. This semiconductor is called
  extrinsic semiconductor. Due to abundance of
  electrons it is known as negative - type or n -
  type semiconductor.

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• A p-n junction is formed by placing p-type and
  n-type semiconductor substrate side by side. It
  has homo junction between p-type and n-type.

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• When p-type and n-type semiconductor comes to
  contact, some interesting cases arise.
• The region of p-type is enriched of holes and the
  region of n-type is enriched of electrons.
• Now Electrons and holes come into action to
  diffuse from zone of high concentration toward
  zone of low concentration, i.e. electrons travel
  from the n-region to the p-region and ionized
  donor atoms are left in this region.
• In the p-region of the p- type substrate the
  electrons recombine with the abundant holes.
  Again, holes come to diffuse from the p-region
  into the n-region. Hence negatively charged
  ionized acceptor atoms are left in the p-region.
• Next, at the contact region in n - type
  semiconductor the holes which come from p - type
  semiconductor recombine with the mobile electrons
  and at the contact region in p - type
  semiconductor electrons come from n - type
  semiconductor recombine with holes. This kind of
  diffusion process will be continuing up to the
  charge balance in two regions.

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• Then a narrow region on both sides of the
  junction is created where no charge carriers
  (electrons or holes) are there. This region is
  called the depletion layer.
• In p - type or n - type region, just after
  creation of depletion region, it contains only
  holes in p-type and electron in the n-type
  semiconductor.
• The depletion layer depends on the impurity level
  or doping level in both type of semiconductor. It
  is inversely proportional to the doping level.
• Now, as a whole joint of two layers looks like a
  depletion region in the middle portion with two
  electric fields at the both end. These electric
  field points to p-type from the n-type region.
• This depletion layer in the middle portion
  creates build-in-potential or contact potential
  with respect to two regions.

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• No net current flows through this depletion
  region. This depletion layer is also known as
  potential barrier.

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• Symbol of p - n Junction Diode
• A p - n junction is nothing but a diode hence an
  p-n junction can also be refereed as p-n junction
  diode.

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• Arrowed portion is called anode or positive
  terminal and bar portion is called cathode or
  negative terminal. Biased p-n Junction
• Forward Bias of p-n Junction
• When the p-type end of a p-n junction is
  connected to the positive end of a battery and
  negative end of this junction is connected to the
  negative of this battery this biasing is called
  as the forward biasing.

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At this biasing condition, the positive potency
always repels the holes of the connected
p-region. Similarly the negative voltage repels
the electrons from the n-type region. Now both
the major carriers i.e. the electrons and the
holes penetrate to the depletion region and
arrive their opposite region. Hence current flows
from the positive region to the negative region.
When battery voltage is applied across the
junction in the forward bias, a current will flow
continuously through this junction.
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IS is Saturation Current (10-9 to 10-18 A) VT is
Volt-equivalent temperature ( 26 mV at room
temperature) n is Emission coefficient (1 n 2
for Si ICs) Actually this expression is
approximated. Reverse Bias of p-n Junction When a
p-n junction is connected across a battery in
such a manner that its n-type region is connected
to the positive potency of the battery and the
p-type region is connected to the negative
potency of the battery. Now the holes are
engulfed by the negative potency of the battery
leaving behind negative static ions in the region
and the electrons are engulfed by the positive
potency of the battery leaving behind positive
static ions in the region . Ultimately the
depletion region at the p-n junction covers total
p and n region of the diode. Hence no current
will flow through this diode.
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iD drops to zero value or very small value. iD
can be written as i0.
IS is Saturation Current (10-9 to 10-18 A) VT is
Volt-equivalent temperature ( 26 mV at room
temperature) n is Emission coefficient (1 n 2
for Si ICs) Actually this expression is
approximated.
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• General Specification of p-n Junction
• A p-n junction is specified in four manners.
  Forward voltage drop (VF) Is the forward
  biasing junction level voltage (0.3V for
  Germanium and 0.7V for Silicon Diode )
• Average forward current (IF) It is the forward
  biased current due to the drift electron flow or
  the majority carriers. If the average forward
  current exceeds its value the diode gets over
  heated and may be damaged.
• Peak reverse voltage (VR) It is the maximum
  reverse voltage across the diode at it reverse
  biased condition. Over this reverse voltage diode
  will go for breakdown due to its minority
  carriers.
• Maximum power dissipation (P) It is the product
  of the forward current and the forward voltage.

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V-I Characteristics of A P-N Junction
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• In the forward bias, the operational region is in
  the first quadrant. The threshold voltage for
  Germanium is 0.3V and for Silicon is 0.7V. Beyond
  this threshold voltage the graph goes upward in a
  non linear manner. This graph is for the dynamic
  Resistance of the junction in the forward bias.
• In the reverse bias the voltage increases in the
  reverse direction across the p-n junction, but no
  current due to the majority carriers, only a very
  small leakage current flows. But at a certain
  reverse voltage p-n junction breaks in
  conduction. It is only due to the minority
  carriers. This amount of voltage is sufficient
  for these minority carriers to break the
  depletion region. At this situation sharp current
  will flow through this junction. This breakdown
  of voltage is of two types. (a) Avalanche
  breakdown it is not properly sharp, rather
  inclined linear graph i.e. after break down small
  increase in reverse voltage causes more sharp
  current gradually. (b) Zener Breakdown this
  breakdown is sharp and no need to increase
  reverse bias voltage to get more current, because
  current flows sharply.

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• Resistances of p-n Junction
• Dynamic Resistance of p - n Junction
• From V-I characteristics of a p-n junction, it is
  clear that graph is not linear. The forward
  biased p-n junction resistance is rd ohm it is
  called AC resistance or dynamic resistance. It is
  equivalent to slope of voltage current of the
  PN junction.

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• Average AC Resistance of p - n Junction
• Average AC resistance is determined by the
  straight line that is drawn linking the
  intersection of the minimum and maximum values of
  external input voltage.

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• Some important terms related to p-n Junction
  Transition Capacitance of p-n Junction
• When depletion region exist in the common
  junction around, the diode acts as a capacitor.
  Here the depletion region is the dielectric and
  two regions (p-type and n-type) at both ends act
  as the charged plates of a capacitor. As the
  depletion layer decreases the capacitance value
  goes down. Diffusion Capacitance of p-n Junction
• It the capacitance of the diode in forward biased
  condition and it is defined as the ratio of
  transiting charge created to the differential
  change in voltage. When the current through the
  junction increases the diffusion capacitance also
  increases. Along with this increase in current,
  the forward biased resistance also decreases.
  This diffusion capacitance is somewhat greater
  than the Transition capacitance. Storage Time of
  p-n Junction
• It is the time taken by the electrons to move
  from n-type region to p-type region and p-type
  region to n-type region by applying simultaneous
  forward and reverse bias voltage during switching.

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• Transition Time of p-n Junction
• It is the time taken by the current to decrease
  to reverse leakage current. This transition time
  can be determined by geometry of P-N junction and
  concentration of the doping level.

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• Reverse Recovery Time of p-n Junction
• It is sum of the storage time and transition
  time. It is the time for diode to raise applied
  current to get 10 of the constant state value
  from the reverse leakage