1. PN JUNCTION DIODE (Characteristics and Applications)
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2. Three important characteristics of a diode are:
PN JUNCTION DIODE
A diode is simply a pn junction, but its applications are extensive in electronic circuits.
Three important characteristics of a diode are:
Forward voltage drop.
Reverse voltage drop.
Reverse breakdown voltage.
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3. A diode is a two-terminal electronic device
PN JUNCTION DIODE
A diode is a two-terminal electronic device
consisting of a single p-n junction. This p-n
junction is usually created on a single block
of silicon by doping the block with donor and
acceptor dopants at opposite ends.
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4. PN JUNCTION DIODE
A diode is a rectifier, allowing current to pass in one direction but not in the opposite direction.
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5. Potential at this junction is called as Barrier Potential
PN JUNCTION
Holes
Free Electrons
P Type
N Type v - +
Potential at this junction is
called as Barrier Potential
Depletion region
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6. The combination of Electrons and holes
PN JUNCTION
The combination of Electrons and holes
depletes the holes in the P region and
electrons in the N region near the junction
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7. The device is called as PN junction Diode
Diode Symbol P N
Anode
Cathode
Cathode
Anode
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8. Forward Biasing of PN – Junction Diode
Connect N type
to negative
of the dc source
Connect P type
to positive
of the dc source
Variable DC Voltage
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9. Forward Biasing of PN – Junction Diode
Forward biasing the p-n junction drives holes to the junction from the p-type material and electrons to the junction from the n-type material.
At the junction the electrons and holes combine so that a continuous current can be maintained.
Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur

10. Reverse Biasing of PN – Junction Diode
Connect P type
to negative
of the dc source
Connect N type
to positive
of the dc source
Variable DC Voltage - +
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11. Reverse Biasing of PN – Junction Diode
The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction.
When the potential formed by the widened depletion layer equals the applied voltage, the current will cease except for the small thermal current.
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12. Diode is unidirectional, i. e
Diode is unidirectional, i.e. current flows in only one direction (anode to cathode internally). When a forward voltage is applied, the diode conducts; and when a reverse voltage is applied, there is no conduction.
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13. VI Characteristics of PN Junction Diode
Vknee mA
(V) μA - +
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14. Reverse saturation current
The Diode Current Equation
Applied Forward
Voltage
Temperature
Equivalent of Volt
Reverse saturation current
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15. Tutorials
A germanium diode has reverse saturation current of 0.19μA. Assuming η =1, find the current in the diode when it is forward biased with 0.3 V at 27oC (Ans: 19.5mA)
The forward current in a Si diode is 15 mA at 27oC. If reverse saturation current is 0.24nA, what is the forward bias voltage? (Ans: 0.93V)
A silicon diode is reverse biased with 5V at room temperature. If reverse sat current is 60 pA, what is the diode current?
A germanium diode carries a current of 10mA when it is forward biased with 0.2V at 27oC. (a) Find reverse sat current. (b) Find the bias voltage required to get a current of 100mA (Ans: 4.42μA, 0.259V)
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16. Avalanche Multiplication
The Diode Breakdown
The reverse break down in diodes can occur due to two mechanisms, each of them require critical electric field at the depletion region of the diode.
They are
Zener breakdown
Avalanche Multiplication
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17. Zener breakdown When the doping is very high (≥ 1025 atoms/m3).
The depletion region is very narrow.
which results in tunneling of electrons from p-type valance band to the n-side conduction band constitutes a reverse current from n to p, this is called Zener effect.
The basic requirement for the tunneling current is a large number of electrons separated from a large number of empty states by a narrow potential barrier.
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18. The electric field resulting due to the depletion region causes field emission where by the force on outer orbit electrons due to field is very high that they are pulled out from the parent nucleus to become free carriers.
This ionization by electro-static attraction is known as “Zener breakdown” and causes an increase in the free carriers density and hence an increase in the reverse current of the junction. Only for the lower level of reverse voltage the zener effect is exhibited.
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19. Avalanche Multiplication
When the diode is reverse biased, carriers acquire sufficient energy from the thermal energy and along with the applied reverse bias results in the high electric field in the depletion region.
An electron entering from the p-side may be accelerated to high kinetic energy to cause ionizing collision. This ionizing collision results in the breakage of covalent bonds, and generate new electron-hole pair. The original electron and generated electron are both swept to the n-side of the junction and generated hole is swept to the p-side.
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20. The generation of electron-hole pair results in the generation of enormous energy by the process called fission.
The liberated fission energy, along with the applied potential and the thermal energy colloid with other non-ionized bonds.
This collision and generation of new electron-hole pairs are continuous and multiplicative, which results in a large amount of charge carriers and thus an increase in the reverse current.
Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur

21. In other words we can say, This phenomenon generally occurs in wider depletion region, where the electric field strength is not strong enough to produce zener breakdown.
Instead, free electrons (minority carriers) accelerated by the electric field collide with electrons in the covalent bonds and breaks it. Thus, an electron-hole pairs are accelerated by the field, resulting in more collision, creating more free electrons. The process quickly avalanches to produce the abrupt rise in current.
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22. Effect of Temperature on the Reverse current
Since the reverse saturation current is temperature dependent parameter, the reverse saturation current approximately doubles for every 10o C rise in temperature. Let I01 is the reverse saturation current at temperature T1 and I02 is the reverse saturation current at temperature T2, where T2 > T1. The rise in reverse saturation current is given by the relation.
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23. The diode current ID=3mA, Reverse saturation current IO= 1 pA,
A Silicon diode has a saturation current of 1pA at 20 Deg C. Find Diode bias voltage when diode current is 3mA. Diode bias current when the temperature is 100 Deg C assuming the diode voltage to be constant.
Solution:
Given
The diode current ID=3mA,
Reverse saturation current IO= 1 pA,
Temperature T=20 Deg C = = 293 K
The diode is silicon η=2
The equation for the diode current ID is given by
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24. The diode current when the temperature is 100OC
The diode bias voltage
The diode current when the temperature is 100OC
The temperature is raised to 100OC (So the reverse saturation current I0 changes) use the relation.
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25. Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur

26. Diode resistances Two types of resistances are defined for a diode :
Static or DC resistance:
It is simply the ratio of diode voltage and diode current
The dc resistance at the knee and below will be greater than the resistance at the linear section of characteristics
The dc resistance in the reverse bias region will naturally be quite high
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27. Diode resistances
(μA)
(mA)
V (volts) I
1μA
0.8V
0.5V
20mA
2mA
–10V C A B
Determine the dc resistances at the three different operating points shown
Ans:
A: 40 Ω
B: 250 Ω
C: 10 MΩ
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28. Diode resistances Dynamic or AC resistance
Often sinusoidal voltages are applied to diode
So the instantaneous operating point moves up and down in the characteristic curve
So DC resistance is not a suitable parameter
Instead, AC resistance is used
It is the change in the diode voltage divided by the corresponding change in the diode current, where the change is as small as possible
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29. Diode resistances
V (volts)
0.8
0.78
I (mA) 30 20
0.6
0.7 4 A B
Determine the AC resistances at operating points A and B
Ans:
A: 2 Ω
B: 25 Ω
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30. Diode resistances
AC resistance is nothing but reciprocal of the slope of the tangent line drawn at that point
Derivative of a function at a point is equal to the slope of the tangent line at that point
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31. Diode resistances
Dynamic resistance can be found using previous equation, no need of characteristic curve
Dynamic resistance in reverse region is very high, since slope of characteristic curve is almost zero
The resistance calculated using equation does not include the resistance due to the metal contact (usually less than 0.1 Ω)
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32. Applied forward voltage= 0.2 V Reverse saturation current IO=1uA,
Find the static and dynamic resistance of a p-n junction germanium diode if the temperature is 27 Deg C and IO=1μA for an applied forward bias of 0.2V.
Solution
Given
Applied forward voltage= 0.2 V
Reverse saturation current IO=1uA,
Temperature T=27 Deg C = = 300 K
The diode is Ge η=1
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33. Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur

34. Diode Equivalent Circuit
Diode is often replaced by its equivalent circuit during circuit analysis and design
Equivalent circuit is obtained by replacing the characteristic curve by straight-line segments Vγ RF A K
Forward bias
Reverse bias Vγ
1/RF
RR = 
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35. Diode Equivalent Circuit
As further approximation, we can neglect the slope of the characteristic i.e., RF = 0 Vγ
RR = 
RF = 0 Vγ A K
Forward bias
Reverse bias
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36. Diode Equivalent Circuit
As third approximation, even the cut-in voltage can be neglected (Ideal diode) A K
RF = 0 A K
Forward bias
RR =  A K
Vγ = 0
Reverse bias
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37. Peak Inverse Voltage (PIV)
This is defined as the voltage that the diode has to withstand under reverse biased condition.
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38. Zener Diode Zener diode is heavily doped P-N junction diode
Optimized to operate in reverse breakdown region
Each zener diode has specific breakdown voltage (VZ). Value of VZ depends on doping level (inversely proportional)
Zener diodes are available with VZ ranging from 1.8V to 200V, power ratings from 250mW to 50W
Symbol of zener diode:
Anode
Cathode P N
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39. Zener Diodes
A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage or "Zener voltage".
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40. A conventional solid-state diode will not let current flow if reverse-biased below its reverse breakdown voltage.
By exceeding the breakdown voltage, a conventional diode is destroyed in the breakdown due to excess current which brings about overheating.
In case of forward-bias (in the direction of the arrow), the diode exhibits a voltage drop of roughly 0.6 volt for a typical silicon diode. The voltage drop depends on the type of the diode.
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41. A Zener diode exhibits almost the same properties, except the device is especially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage.
A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material.
A reverse-biased Zener diode will exhibit a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage
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42. For example, a 3. 2-volt Zener diode will exhibit a voltage drop of 3
For example, a 3.2-volt Zener diode will exhibit a voltage drop of 3.2 volts if reverse biased. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.
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43. Zener Diode characteristics
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44. Zener Diode characteristics
V-I characteristics: VZ
IZK
IZM I V
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45. Zener Diode characteristics
IZK or IZmin – Minimum current necessary to maintain breakdown
IZM or IZMax – Maximum current that can be safely passed through the zener diode
PZM or PZMax – Maximum power dissipation across zener diode
PZM = VZ.IZM
Zener diode is always connected such that it is reverse biased, and it is in zener breakdown region VZ + – IZ
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46. Zener Diode characteristics
V-I characteristics:
When zener diode is forward biased, it acts like ordinary diode – i.e., until certain voltage Vγ is reached, current is zero, then afterwards, current rises exponentially
When zener diode is reverse biased, until the breakdown voltage is reached, current is zero or negligible
When reverse voltage equals zener voltage, current rises exponentially in reverse direction
After the breakdown has occurred, voltage across zener diode remains almost constant at VZ, only the current increases with the increase in applied reverse bias.
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47. Zener Diode characteristics
Equivalent circuits of zener diode
Forward Reverse Breakdown
Note: RZ is usually very small, can be neglected Vγ RF
RR ≈  RZ VZ – + N P
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48. Zener Voltage regulator
Zener diodes are widely used to regulate the voltage across a circuit.
When connected in parallel with a variable voltage source so that it is reverse biased, a zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point it keeps the voltage at that value.
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49. Zener Voltage Regulator
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50. The circuit holds the voltage across the load RL almost equal to the voltage across zener VZ even after the input Vin and load resistor RL undergo changes. If the unregulated dc voltage Vin rises, the current through R increases.
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51. Let I be the current through the resister R , we can write ,
If the load requires more current when RL is decreased, the zener diode can supply the extra current without affecting the load Voltage.
Let I be the current through the resister R , we can write ,
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52. The power dissipated in the diode is Pz=IZ Vz
The selection of Rs is very important here. We have,
After substituting the value of I we get,
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53. For Line regulation RL is constant. is also constant.
And Vin varies between Vin(min) to Vin(max)
For Load Regulation, Vin is constant and RL varies between RLmin and RLmax and load current is given by,
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54. When Vin=Vin(min),and IL is constant then,
Similarly when Vin=Vin(max) we have,
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55. The selected R must be small enough to permit minimum zener current to ensure that the diode is in its breakdown region.
That is R must be small enough to ensure that minimum current IZ(min ) flows under worst condition.
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56. so that power dissipation in the diode will not will not exceed Pz.
This is when Vin falls to its smallest possible value Vinmin and IL is its largest possible value ILmax (Load Regulation).
At the same time R must be selected large enough to ensure that the current through the zener diode should not exceed the maximum zener current Izmax.
so that power dissipation in the diode will not will not exceed Pz.
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57. That is the condition when Vin rises to the value of Vinmax and load current IL to its minimum Ilmin.
So we can write,
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58. Fixed reference voltage
Applications:
As Voltage regulators
As Voltage Limiters
Wave shaping
Protection diode
Fixed reference voltage
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59. LED
The increasing use of digital displays in calculators, watches and all form of instrumentation has contributed to an extensive inherent in structures that emit light when properly biased.
The two types of displays commonly used are light emitting diode (LED) and liquid crystal display(LCD).
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60. LED
Light emitting diode is a diode that gives of visible or invisible (infrared) light when energized.
In any p-n junction there is a recombination of holes and electrons.
During this process energy possessed by the free electron is transferred to another state, some of this energy is transferred into heat and some in the form of photons.
In silicon and germanium greater percentage is converted into heat and the emitted light is insignificant.
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61. Diodes constructed of GaAs emit light in the infrared zone
Diodes constructed of GaAs emit light in the infrared zone. Even though the light is not visible, they have numerous applications like, security systems, industrial processing, optical coupling etc.
By using elements like gallium, arsenic and phosphorous LEDs producing red, green, yellow, blue, orange or infrared (visible). LED’s have replaced incandescent lamps in many applications because of their low voltage, long life, and fast on-off switching.
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62. Problem
In a zener voltage regulator if Vz=10V, Rs=1KΩ, RL=2KΩ. If the input voltage Vin varies from 22 to 40 V, find the maximum and minimum values of zener current.
Solution:
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63. Similarly, Using above relation we can find
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64. Problem
2) Design a Zener voltage regulator for the following specifications: o/p voltage=5v, i/p voltage, Vin = Load current, IL=20 mA Zener power, PZ(max)= 500 mw IZ(min)=2mA.
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65. Solution:
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66. Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur

67. Problem
1. A germanium diode carries a current of 10mA when a forward bias of 0.2V is applied across it at 27 Deg C.
Find the reverse saturation current.
Calculate the bias voltage needed for diode currents of 1mA and 100mA.
Answer:
I0= 4.42 µA
V=0.141,0.259
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68. Problem Answer: a) 4.57 mA b) I0=3.072 µA, I=74.39 mA
2. A silicon diode has a saturation current of 12nA at 20 deg C.
Find its current when it is forward biased by 0.65V.
Find the current in the same diode when the temperature rises to 100 Deg C.
Answer:
a) 4.57 mA
b) I0=3.072 µA, I=74.39 mA
Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur

69. Problem
3) The zener diode in a regulator circuit has a breakdown voltage of 15V and power rating of 0.5 W. if input voltage =40V, what is the minimum value of R that prevents zener diode from being destroyed.
Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur

70. Assignment
In a Zener diode regulator circuit, VL =15V,maximum load current is 100 mA, input voltage range is 18V to 20V, R =10 ohm.
Determine,
Maximum power dissipated by R
Minimum diode current.
The power that must be dissipated by R if the output is accidentally short circuited.
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71. Assignment
A Silicon diode has a saturation current of 0.1pA at 20OC. Find its forward voltage when the current is 0.3mA.
A Germanium diode has IO=10μA. Determine its forward voltage when it is carrying 50mA of current. Compute the dynamic resistance at this operating point.
A Silicon diode at room temperature conducts 5mA at 0.7V. If the voltage increases to 0.8V. Find reverse saturation current.
Calculate the factor by which reverse saturation current IO of Germanium diode is multiplied when the temperature increases from 25 to 100OC.
Department of Electronics and Communication Engineering, School Of Engineering, Manipal University, jaipur