(07) pn-junction.ppt
TRANSCRIPT
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PN -Junction
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PN-Junction 22 July 2013
PN -junction is formed by growing a singlecrystal of Si or Ge, which is half P -typeand half N -type.
Note that a PN -junction diode cannot bemade by simply pushing a piece of P -typeagainst another of N -type.
PN -junction Diode
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PN-Junction 32 July 2013
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PN-Junction 52 July 2013
Symbol
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PN-Junction 62 July 2013
Formation of Depletion Region
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Because of diffusion of electrons from N -typeinto P- type, and of hole from P- type into N -type, depletion region or space-chargeregion is formed .This region has extremely high resistance.Electric field is called barrier.
Also called barrier potential or contactpotential or diffusion potential .
V 0 = k T ln (N A ND /n i2) VoltV 0 = 0.7 V for Si, 0.3 V for Ge
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PN-Junction 82 July 2013
The barrier discourages the diffusion of majority carriers across the junction.
But the same barrier helps the minoritycarriers to drift across the junction.The minority carriers are constantly
generated due to thermal energy.Does it mean that there should beconstant current due to the minoritycarriers crossing the junction ?If yes, how can there be current withoutany external connection ?
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PN-Junction 92 July 2013
Ans.: The net current across the junctionhas to be zero. The majority carrier havinghigh energy still diffuse across the junction.The two currents counterbalance eachother.
0dif dr I I
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PN-Junction 102 July 2013
PN -Junction with Forward Bias
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PN-Junction 112 July 2013
Applied voltage opposes the contactpotential.
Net barrier potential V B is reduced.Diffusion current increases.Drift current slightly decreases.The forward basing current is
dr dif f I I I
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PN-Junction 122 July 2013
PN -Junction with Reverse Bias
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PN-Junction13
2 July 2013
V B increase.
I dif due to majority carriers reduces toalmost zero.I dr slightly increases.The net current I r remains constanttill breakdown , hence calledsaturation current I s .
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PN-Junction14
2 July 2013
Summary of Biasing Conditions
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PN-Junction 152 July 2013
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PN-Junction 162 July 2013
Reverse Breakdown
At some high reverse voltage, calledbreakdown voltage ( V Z ), the currentabruptly increases. Two phenomena :
1. Zener Breakdown.2. Avalanche Breakdown.
Zener is predominant for V Z less than 4
V. Avalanche is predominant for V Z greaterthan 4 V.
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PN-Junction 172 July 2013
Zener Breakdown
The field across depletion region becomesso high (about 10 7 V/m) that largenumber of covalent bonds break.
Hence large current starts flowing.
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PN-Junction 182 July 2013
Avalanche Breakdown
Increased field causes large increase inthe velocities of minority carriers.These carriers cause impact ionization.Cumulative multiplication of chargecarriers occur.
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PN-Junction 192 July 2013
Junction Capacitances
Two kinds :1. Transition or Depletion Capacitance, C T :
This is predominant in reverse bias. Itdecreases with applied voltage.
2. Storage or Diffusion Capacitance, C D :This is predominant in forward bias. Of
the order of several hundred pF.
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Diode Characteristics 202 July 2013
V-I Characteristics
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Diode Characteristics 212 July 2013
V T is cut-in, knee, offset, turn-on, orthreshold voltage.I s is reverse saturation current.
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Diode Characteristics 222 July 2013
Si Diode versus Ge Diode
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Diode Characteristics 232 July 2013
Temperature Effects
I s almost doubles for every 10 C rise intemperature.V Z increases with temperature.
The width of the depletion layer decreaseand hence barrier potential decreases withrise in temperature.
As a result, V T decreases by 2mV foreach celsius degree rise in temperature.
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Diode Characteristics 242 July 2013
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Diode Characteristics 252 July 2013
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Diode Characteristics 262 July 2013
DIODE RESISTANCE
Two kinds :1. Static or DC Resistance (High Value) : It
is the resistance offered by the diode to a
dc current.2. Dynamic or AC Resistance (Low Value) :
It is the resistance offered by the diodeto ac current.
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Diode Characteristics 272 July 2013
Static or DC Resistance
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Diode Characteristics 282 July 2013
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Diode Characteristics 292 July 2013
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Diode Characteristics 302 July 2013
Dynamic or AC Resistance
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Diode Characteristics 312 July 2013
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Diode Characteristics 322 July 2013
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Diode Characteristics 342 July 2013
Dynamic Resistance of a Diode
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Diode Characteristics 352 July 2013
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Diode Characteristics 362 July 2013
Ideal Diode
Ideally, a diode should haveR F = 0 and R R =
Draw the V-I characteristics of an ideal
diode.
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Diode Characteristics 372 July 2013
Ideal Diode Characteristics
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Diode Characteristics 392 July 2013
Simplified Circuit Model
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Diode Characteristics 402 July 2013
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Diode Characteristics 412 July 2013
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Diode Characteristics 422 July 2013