p-n junction

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2013 Copyright by David Owino
P-n junction
MR. DAVID OWINO
9/10/14
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P-n junction
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Objectives

1. Define the p-n junction
2. Describe the formation and biasing of the p-n
   junction
3. Define semi-conductor diode and sketch its
   circuit symbol
4. Mention at least two types of diodes
5. Sketch current-voltage characteristics for a diode

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2013 Copyright by David Owino
P-n junction

• When an intrinsic s.c simultaneously doped by
  trivalent and pentavalent atoms such that one
  half becomes n-type and the other p-type s.c a
  region is formed
• The boundary or interface separating a p and n
  type material is called p-n junction

N-type
P-type
Junction
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Cont
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Formation of p-n junction

• The free es and holes near the junction diffuses
  across it
• es enter the p-zone as holes move into the n-zone
  as shown
• p-type
  n-type

Depletion layer


-
-


-
-

-
-
-


-



-

-
-


-
-

-


-
-
-


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Cont

• Re-combination of mobile charge carriers takes
  place on either side of the junction
• This depletes mobile charge carriers within the
  region
• The region remain with uncovered fixed ions on
  either side of the junction
• This region occupied by the uncovered fixed ions
  is called depletion layer

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Cont

• The uncovered ions set up a potential
  difference/barrier.
• The barrier in turn sets up a field that stops
  further diffusion of mobile charges.
• The depletion layer has very high resistance to
  the movement of charge carriers.

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CONT
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CONT
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Biasing the p-n junction

• Biasing means connecting /applying p.d across the
  p-n junction
• Biasing can either increase or reduce the
  potential barrier
• There are two ways of biasing a p-n junction to
  e.m.f of the source, namely
• Forward bias
• Reverse bias

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Forward bias

• P-type is connected to ve N-type to the ve
  terminal of an external cell/battery.
• Holes in the p-region are repelled away by the
  ve terminal towards the junction.

VB
holes
P-type
N-type
es
VE
VB-barrier potential
VE-applied potential
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A Forward Biased p-n junction
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Cont

• At the same time, the ve terminal repels the
  free es from n-region towards the junction.
• The potential barrier/depletion layer
  disappears-is reduced considerably.
• Thus, forward resistance is lowered hence
  charge carriers cross the junction with ease.
• The crossing over of charge carriers constitutes
  a very large forward current flowing in the
  circuit.

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CONT
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Cont
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Reverse bias

• P-type is connected to -ve N-type to the ve
  terminal of an external cell/battery.
• Holes in the n-region are attracted by the ve
  terminal of the battery.
• Electrons are also attracted to the ve terminal.

VB
holes
N-type
P-type
es
VE
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A Reverse Biased p-n junction
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Cont

• The thickness of depletion layer increases due to
  high concentration of fixed ve -ve ions.
• Increase in depletion layer increases the
  resistance of the p-n junction.
• Since no majority charge carriers diffuse across
  the junction, no current flows thro the circuit.
• NB
• P-n junction in reverse bias behaves like an open
  switch that blocks the current flow.
• However, small/leakage current flows due to
  minority charge carriers.

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CONT
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Semi-conductor diode

• It is also known as p-n junction diode.
• This is a device that offers high resistance when
  reverse biased low resistance when forward
  biased.
• Or
• Is a one-way conducting device consisting of a
  p-n junction having anode cathode at its
  terminals.
• Anode is ve (p-type)
• cathode is ve ( n-type)

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Circuit symbol for diode
OR
Anode (p-type)
Cathode (n-type)
NB The arrow head indicates the direction of
convectional current when diode is forward biased.
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Biasing a diode
Forward bias
Reverse bias
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Diode characteristics

• Is a graph that shows the relationship btn
  current voltage across a diode.
• There are two types of diode xtics, namely
• Forward xtic
• Reverse xtic

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Cont

• The xtics can be investigated by the circuit
  below.
• With the help of variable resistor, different
  values of V and I are recorded, then
• The values are used to plot a graph of I vs V

mA
-
v

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Forward xtic

• Vo-threshold V/cut in V
• Vo is the V required to overcome barrier
  potential or to start the conduction.
• When the p.b is overcomed, the I increases
  rapidly.
• The curve is non-linear showing that diode is
  non-ohmic.

I
Forward bias
V
Vo
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Reverse xtic
Vb

• Investigated by reversing the terminals of the
  set-up.
• When reverse V is zero, a small leakage (due to
  minority c.c) flows.
• As V is increased, there is no change in I until
  Vb when appreciable I flows.
• At Vb (breakdown V) the diode is damaged.

-V
Reverse bias
-I
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Combined xtic

-Reverse bias I is very small while reverse bias
V is very high.
-Forward bias I is very large while the forward
bias V is very small.
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Cont
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• A diode in reverse bias has very high resistance.
• A damaged diode conducts irrespective of biasing.
• The diode that operate at Vb is called zener
  diode.
• Vb- breakdown V is the voltage at which a diode
  conducts current in its reverse mode.
• It is used for voltage control.
• Its circuit symbol is shown.

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Cont

• Other types of diodes include
• Light Emitting diode (LED)
• Its circuit symbol is
• Laser diodes

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leds
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leds
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leds
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EXAMPLE 1

• The fig. shows two ways of biasing a P.N
  junction.
• In which circuit will current flow? (1mk)
• -y
• Explain your answer in (a) above. (1mk)
• -forward biased depletion is reduced

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EXAMPLE 2

• Study the fig. and use it to answer the questions
  that follow.
• If L1 and L2 are identical bulbs, explain what
  happens when
• Only S1 is closed. (2mks)
• D1 is reverse biased L1 and L2 light with equal
  brightness because they share the voltage.
• Only S2 is closed. (2mks)
• D2 is forward biased L2 is short circuited hence
  does not light L1 lights brightly because all
  the voltage passes across it.
• S1 and S2 are closed. (2mks)
• D2 is forward biased while D1 is reverse biased
  hence L2 is short circuited L1 lights brightly.

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EXAMPLE 3
The fig. shows a circle with two diodes P and Q
and a cell. Explain the observation which
would be made if S is closed.(2mks) P is forward
biased while Q is reverse biased A1
reads/deflects while A2 does not.
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EXAMPLE 4

• Find the current flowing and the voltage across
  each resistor in the circuit.(3mks)

• D2 is reverse biased no I and V (drop) across 8?
  resistor
• D1 is forward biased I flows across 2? and 3?
  resistors.
• Since 2? and 3? resistors are in series

 
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REFERENCES

1. simple-semiconductors.com/8.html
2. Abbort, A.F. (1979). Ordinary level Physics.
   London Heinemann Educational Boooks