Diode_page1
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Diode :

        A pure silicon crystal or germanium crystal is known as an intrinsic semiconductor. For most application, there are not
       free  electrons  and  holes  in  an  intrinsic  semi-conductor  to  produce  an  usable  current.  The  electrical  action  of  these
       modified  by  doping  means  adding  impurity  atoms  to  a  crystal  to  increase  either  the  number  of  free  holes  or  no
       electrons. When a crystal has been doped, it is called a extrinsic semi-conductor. They are of two types n-type semico
        having  free electrons as  majority carriers, p-type semiconductor  having  free holes as  majority carriers. By themselves
        doped  materials  are  of  little  use.  However,  if  a  junction  is  made  by  joining  p-type  semiconductor  to  n-type  semicond
        device  is  produced  which  is  extremely  used  known  as  diode.  It  will  allow  current  to  flow  through  it  only  in  one  directio
        unidirectional  properties  of  a  diode  allow  current  flow  when  forward  biased  and  disallow  current  flow  when  reversed
        This is called rectification process and therefore it is called rectifier.

        The  question  now  exists  how  is  it  possible  that  by  properly  joining  two  semi-conductors  each  of  which,  by  itself,  wi
        conduct the current in any direct refuses to allow conduction in one direction.

        Consider first the condition of p-type and n-type germanium just prior to joining fig1 . The majority and minority carriers
        constant motion.
Fig.1
The minority carriers are thermally produced and they exist only for short time after which they recombine and neutraliz
other.  In  the  mean  time,  other minority  carriers  have  been produced  and  this  process  goes  on  and  on.  The  number  o
electron hole pair that exist at any one time depends upon the temperature. The number of majority carriers is howeve
depending on the number of impurity atoms available. While the electrons and holes are in motion but the atoms are
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place and do not move.

As soon as, the junction is formed, the following processes are initiated: Fig2:
•  Holes from the p-side diffuse into n-side where they recombine with free electrons.

•    Free   electrons   from   n-side   diffuse   into   p-side   where   they   recombine   with   free   holes.   The   diffusion   of   el
   and holes is   due   to   the   fact   that   large   no   of   electrons   are        concentrated   in   one   area   and   large   no   o
   are concentrated in     another area.
•
When these electrons and holes begin to diffuse across the junction then they collide each other and negative charg
in the electrons cancels the positive charge of the hole and both will lose their charges.

The  diffusion  of  holes  and  electrons  is  an  electric  current  referred  to  as  a  recombination  current.  The  recomb
process  decay  exponentially  with  both  time  and        distance  from  the  junction.  Thus  most  of  the  recombination
 just  after  the  junction  is  made  and  very  near  to  junction.  A  measure  of  the  rate  of  recombination  is  the
defined as the time required for the density of carriers to decrease to 37% ot the original concentration.
l
The impurity atoms are of course, fixed in their individual places. The atoms itself is a part of the crystal and so can no
When  the  electrons  and  hole  meet,  their  individual  charge  is  cancelled  and  this  leaves  the  originating  impurity  atoms
net  charge,  the  atom  that  produced  the  electron  now  lack  an  electronic  and  so  becomes  charged  positively,  wher
atoms  that  produced  the  hole  now  lacks  a  positive  charge  and  becomes  negative.  The  electrically  charged  atoms  are
ions since they are no longer neutral. These ions produce an electric field as shown. After several collision occurs, the
field is great enough to repel rest of the majority carriers away of the junction. For example, an electron trying to diffuse
to p side is repelled by the negative charge of the p-side. Thus diffusion process does not continue indefinitely but co
as  long  as  the  field  is  developed.  The  net  result  of  this  field  is  that  it  has  produced  a  region,  immediately  surround
junction that has no majority carriers. The majority carriers have been repelled away from the junction and junction is d
from  carriers.  The  junction  is  known  as  the  barrier  region  or  depletion  region  fig3.  The  electric  field  represents  a  p
difference  across  the  junction  also  called  space  charge  potential  or  barrier  potential.  This  potential  is  0.7v  for  S
degree celcious and 0.3v for Ge.
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 The physical width of the depletion region depends on the doping level. If very heavy doping is used, the depletion re
physically  thin  because  diffusion  charge  need  not  travel  far  across  the  junction  before  recombination  takes  place  (s
time). If doping is light, then depletion is more wide (long life time)
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Reverse Bias :

        When  the  diode  is  reverse  biased  then  the  depletion  region  width  increases,  majority  carriers  move  away  from  the  j
        and  there  is  no  flow  if  current  due  to  majority  carriers  but  there  are  thermally  produced  electron  hole  pair  also.
        electrons and holes are generated in the vicinity of junction then there is a flow of current. The negative voltage applie
        diode will tend to attract the holes thus generated and repel the electrons. At the same time, the positive voltage will att
        electrons towards the battery and repel the holes. The electron in the p-material and hole in the n-material are being fo
        move forward each other and will probably combine. This will cause current to flow in entire circuit. This current is usua
        small (interms of micro ampher to nano amphere). Since this current is due to minority carriers and these number of m
        carriers are fixed at a given temperature therefore, the current is almost constant known as reverse saturation curren
        actual diode, the current is not almost constant but increases slightly with voltage. This is due to surface leakage curre
        surface  of  diode  follows  ohmic  law  (V=IR).  The  resistance  under  reverse  bias  condition  is  very  high  100k  to  mega
        When the reverse voltage is increased, they and certain value, then breakdown to diode takes place and it conducts
        This is due to avalanche or zener breakdown. Fig4
                                            Fig.4

Forward bias :

        When the diode is forward bias, then majority carriers are pushed towards junction, when they collide and recombinatio
        place. Number of majority carriers are fixed in semi-conductor. Therefore as each electron is eliminated at the junction
        electron must be introduced, this comes from battery. At the same time, one hole must be created in p-layer. This is for
        extracting one electron from p-layer. Therefore, there is a flow of carriers and thus flow of current.

Space Charge Capacitance on Transition Capacitive CT :

       Reverse  bias  causes  majority  carriers  to  move  away  from  the  junction,  thereby  creating  more  ions.  Hence  the  thick
        depletion region increases. This region behaves as the dielectric material used for making capacitors. The p-type and
        conducting on each side of dielectric act as the plate. The incremental capacitance CT is defined by
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Where, dQ is the increase in charge caused by a change dv in voltage.

                                                i = dQ / dt

                                               i = CT dv / dt

 CT  is  not  constant,  it  depends  upon  applied  voltage,  therefore  it  is  defined  as  dQ/dV.  When  p-n  junction  is  forward
then also a capacitance is defined called diffusion capacitance CD (rate of change of injected charge with voltage) to ta
account the time delay in moving the charges across the junction by the diffusion process. Therefore, it cannot be iden
terms  of  a  dielectric  and  plates  as  space  charge  capacitance.  It  must  be  considered  as  a  fictitious  element  that  allo
predict  time  delay.  If  the  amount  of  charge  to  be  moved  across  the  junction  is  increased,  the  time  delay  is  greater,  it
that diffusion capacitance varies directly with the magnitude of forward current.
Real Diode: Small Signal Operation: (Load Line)

       Consider the diode circuit shown in Fig.5
 This equation involves two unknown and can not be solved. The other equation interms of these two variables, is given
static  characteristic.  The  straight  line  represented   by  this  above  equation  is  known  as  the  load  line.  The  load  lin
through two points, I = 0 , VD = V and VD = 0, I = V /  R  L. The slope of this line is equal to 1/ RL. The point of    inter-se
straight line and diode characteristic gives the operating point. Fig.6
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Fig.6
Say V=1V, RL=10ohm. (Fig7).
Fig.7
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Therefore, as the diode voltage varies, diode current also varies, sinusoidally (Fig 8).
Fig.8
 In certain applications only a.c. equivalent circuit is required. Since only a.c. response of the circuit is considered d.c.
is  not  shown  (Fig  9).  The  resistance r f  represents  the  dynamic  resistance  or  a.c.  resistance  of  the  diode  .It  is  obta
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taking the ratio of at operating point.
Fig. 9
Dynamic Resistance
<
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Diode Approximation : (Large signal operations):
1.
Ideal Diode:    When forward biased, resistance offered is zero,
                When reverse biased resistance offered is infinity. It acts as a perfect switch .Fig10
2.
Second Approximation:
when forward voltage is more than 0.7 V, for Si then it conducts and offers zero
   resistance. Fig 11.
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3.
bulk
3rd Approximation:
when forward voltage is more than 0.7 v, then it conduct and offer
        resistance Rf (slope of the current) fig.12
VD = 0.7 + ID Rf.
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When reverse biased resistance offered is very high. (and not infinity). Then the diode
equivalent circuit is as shown in fig13.
Problem:
Calculate the voltage output for following inputs

(a) V1=V2=0.

(b) V1=V, V2=0.

(c) V=V2=V knew voltage =Vr?  .

Forward resistance is Rf.
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Solution:

(a). Vo = 0.

(b). V1 = V, V2 = V.
(c) V1 = V 2 = V.
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Application_page1
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Applications
Half wave Rectifier (fig.17)
Fig.17
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In  positive  half  cycle,  D  is  forward  biased  (ideal) and  output  voltage  is  same  the  input  voltage.  In  the  negative
half cycle, D is reverse biased, and output voltage is zero.
When the diode is reverse biased, entire transformer voltage appears across the diode. The maximum voltage
across the diode is Vm. The diode must be capable to withstand this voltage. Therefore PIV rating of diode
should be equal to Vm in case of single phase ?  halfwave rectifiers. The avg-current rating must be greater than
Iavg

                                                                                                        Next>>
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Application_page2
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Full Wave Rectifier: (fig.18)
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fig. 18
Center tap transformer. It is more efficient and supplies current in both half cycles.

      z   In the first half cycle D1is forward biased and conducts. D2 is reverse biased .
      z   In the second half cycle D2 is forward biased, and conducts.
     When D1 conducts, then full secondary voltage appears across D2, therefore PIV rating of the diode
should be 2 Vm.
Bridge Rectifier : Fig 19
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Clippers: Fig.20

        Clipping circuits are used to select thatpart of the input wave which lies above or below some reference level.
Fig.20
In this clipper circuit

       Vo = Vi if Vi

       Vo = V R if Vi>VR
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In this clipper circuit

             Vo = Vi

             Vo = VR
if Vi>VR

if Vi
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considering third approximation
Now,
Vo = Vi
if Vi<(V R+V r)
When Vi>(V R+Vr)    diode D conducts and the equivalent circuit becomes
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             y=mx+c

The current i in the circuit is given by
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When Vi < (VR-Vr), D condcuts and the equivalent circuit becomes
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Application_page3
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Therefore
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Clipping at two independent levels

           VR1 < VR2
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<
Next>>>
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Application_page4
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Problem:

       Draw the transfer characteristic of the circuit shown in fig.43
When diode D1 is off,    i1 = 0, D2 must be ON
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Clamper_page1
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Clamper Circuits:

        Claming is a process of introducing d.c. level into a signal . Example. If the input voltage swings from -10 V and
        +10  V  a  positive  d.c.  clamper  will  produce  the  output  that  swings  ideally  from  0  V  to  +  20  V.   The  complete
        waveform is lifted up by +10 V so as to just touch the horizontal axis.

Negative Diode clamper: Fig25
During first positive half cycle as Vi rises from 0 to 10 V, the diode conducts. Assuming an ideal diode, its
voltage, which is also the output must be zero during the time from 0 to t1. Howver, the capacitor charges
during this period to 10 V. With the polarity shown. At the same time (t1). Vi starts to drop which means the
anode of D is negative relative to cathode, (VD = Vi - Vc) thus reverse biasing the diode and preventing the
capacitor from discharging. fig 26 . Since the capacitor is holding its charge it behaves as a D.C. voltage
source while the diode appears as an open circuit the equivalent circuit becomes like as shown in fig .27
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Fig. 26
Fig. 27
Positive Clamper:  Fig.28
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Thus circuit behaves as if a 10 V dc. Source had been placed in series with ac. generate, this shift every voltage by -10 V.

     To clamp the input signal other than 0 V, d.c. source is required. Fig. 30. The d.c. source is reverse biasing the
     diode. In the negative half cycle when the voltage exceed 5V then D conduct. During -5 V to 10 V, the capacitor
     charges to 5 V with the polarity shown. After that D becomes reverse biased. ( VD = V i - 5 V - 5 V ), and open
     circuited. Then complete a.c. signal is shifted upward by 5 V.
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Fig. 30
Next>>
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voltagedoubler_page2
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Voltage Doubler : Fig.32

       Circuit is  shown  in.  At the  peak  of  the  negative  half  cycle  D1  is  forward  based,  and  D2  is  reverse based.  This
       charges C1 to the peak voltage Vp with the polarity shown. At the peak of the positive half cycle D1 is reverse
       biased  and D2  is forward  biased.  Because  the  source  and  C 1  are  in  series, C2  will  change  toward  2  VP.  e.g.
       Capacitor voltage increases continuously and finally becomes 20V.
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voltagedoubler_page2
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fig. 32
During 0 to t1 negative D1contacts
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During t1 to t2, D2 becomes forward biased and conducts and at t2, Vi = 10V total voltage si 20V C1 = C2 = C
i.e both the capacitor charge to +10 V i.e. C1 voltage becomes 0.
t2 to t3 no conduction.

During t3 to t4 D1 contacts
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During t4 to t5 no conduction

During t5 to t6 D2 contacts
During t6 to t7 no conduction

During t7 to t8 D1 contacts
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During t8 to t9 no conduction

During t9 to t10 D2 contacts
and so on.
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Zener diode
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Zener Diode :  Fig35

       The diodes designed to work in breakdown region are called zener diode. The power handling capacity of these
       diodes  is  better.  The  power  dissipation  of  a  zener  diode  equals  the  product  of  its  voltage  and  current.  PZ  =VZ
       IZ  .  When  zener  is  forward  biased  it  works  as  a  diode  and  drop  across  it  is  0.7V  when  it  works  in  breakdown
        region  the  voltage  across  it  is  constant  Vz  and  the  current  through  it  is  decided  by  external  resistance.  Zener
        diode is used for regulating the the dc voltage. It maintains the output voltage constant even through the current
        through changes.
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                                  fig. 35

To operate the zener in Breakdown region Vs should always be greater then Vz. Rs is used to limit the current.
If the Vs voltage changes operating point also changes simultaneously but voltage across zener is almost
constant. Fig. 37 The first approximation zener diode is a voltage source of Vz magnitude and second
approximation includes the resistance also.
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(Fig.38) The resistance produces more I*R drop as the current increases.

       The voltage at Q1 is

           V1 = R z +Vz

           At Q 2 = I 2 +R 2 +Vz
Change in voltage is
If zener is used to regulate the voltage across a load resistance. The zener is will work in the breakdown region
only if the thevenin voltage across zener is more than V Z
.
If zener is operating in breakdown region, the series current will be given by
Zener Drop Output point :

        For  a  zener  regulator  to  hold  the  output  voltage  constant,  zener  diode  must  remain  in  the  breakdown  region
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Zener diode
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under  all  operating  conditions,  i.e.  there  must  be  zener  current  for  all  source  voltage  load  currents.  The  worst
case occurs for minimum source voltage and maximum load current.
The critical point occurs when maximum load current equals minimum series current
When  the  zener  diode  operates  in  breakdown  region,  the  voltage  Vz  across  it  remains  fairly  constant  even
though the current Iz through it vary considerably. If the load IL should increase, the current Iz should decrease
by  the  same  percentage  in  order  to  maintain  load  current  constant  Is.  This  keeps  the  voltage  drop  across  Rs
constant  and  hence  the  output  voltage.  If  the  input  voltage  should  increase,  the  zener  diode  passes  a  larger
current,  that  extra  voltage  is  dropped  across  the  resistance  Rs.  If  input  voltage  falls,  the  current  Iz  falls  such
that Vz is constant.

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Light Emitting Diode :

       In  a  forward  biased  diode  free  electrons  cross  the  junction  and
      enter    into    p-layer    where    they    recombine    with    holes.    Each
      recombination radiates energy as electron falls from higher energy
      level to  a  lower energy level.  Fig39.  In  ordinary diodes this energy
      is  in  the  form  of  heat.  In  light  emitting  diode,  this  energy  is  in  the
      form  of  light.  Ordinary  diodes  are  made  of  Ge  or  Si  this  material
      block  the  passage  of  light.  LEDs  are  made  of  different  materials
      such  as  gallium,  arsenic  and  phosphorus.  LEDs  can  radiate  red,
      green, yellow, blueorange or infrared (invisible).
Typical LED current is between 10 mA to 50 mA
Seven Segment Display :

        It  contains  seven  rectangular  LEDs.  Each  LED  is  called  a  Segment.  External  resistors  are  used  to  limit  the
        currents to safe Values. It can display any letters A, B, C, D, E, F, I, H, L, P, U. Fig40.a
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The seven segment display are of two types common anode and common cathode. Fig.40.b

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Photo diode :

       When a diode is reversed biased, a reverse current flows due to minority carriers. These carriers exist because
       thermal  energy  keeps  on  producing  free  electrons  and  holes.  The  lifetime  of  the  minority  carriers  is  short,  but
       while they exist they can contribute to the reverse current. When light energy bombards a pn junction, it too can
       produce free electrons. Fig 41
In other words, the amount of light striking the junction can control the reverse current in a diode. A photo diode
is  made  on  the  same  principle.  It  is  sensitive  to  the  light.  In  this  diode,  through  a  window  light  falls  to  the
junction. The stronger the light, the greater the minority carriers and larger the reverse current.

Opto Coupler:

It  combines  a  LED  and  a  photo  diode  in  a  single  package.  LED  radiates  the  light  depending  on  the  current
through  LED.  This  light  fallls  on  photo  diode  and  this  sets  up  a  reverse  current.  The  advantage  of  an  opto
coupler is the electrical isolation between the input and output circuits. The only contact between the input and
output is a beam of light. Because of this, it is possible to have an insulation resistance between the two circuits
of  the  order  of  thousands  of  mega  ohms.  They  can  be  used  to  isolate  two  circuits  of  different  voltage  levels.
Fig42.
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