ESE 372 / Spring 2013 / Lecture 7
pn ‐ Junction.
When these two materials are brought into contact the electrons will go from n
to p and holes from p to n regions by diffusion.
They will be leaving behind immobile charges of ionized donors and acceptors.
acceptors
The corresponding electric field will eventually stop diffusion currents.
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ESE 372 / Spring 2013 / Lecture 7
pn – Junction band diagram.
Donor
concentration
Acceptor
concentration
Surface charge
densities on both
sides of pn-junction
Built-in potential
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ESE 372 / Spring 2013 / Lecture 7
pn – Junction under reverse bias.
External field is trying to move
electrons from p to n and holes
from n to p.
Almost no current can flow since there is small
number of electrons in p and holes in n regions
Junction capacitance decreases
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ESE 372 / Spring 2013 / Lecture 7
pn – Junction under forward bias.
External field is trying to move electrons from n
to p and holes from p to n.
Current should be expected.
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ESE 372 / Spring 2013 / Lecture 7
Current of pn–junction under forward bias.
Reverse saturation
current
Observe accumulation of mobile charges on both
sides on pn-junction under forward bias
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ESE 372 / Spring 2013 / Lecture 7
IV of pn‐junction diode.
Nonideality factor
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ESE 372 / Spring 2013 / Lecture 7
Diode models.
Ideal diode:
Constant voltage drop model.
Battery + resistance model.
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ESE 372 / Spring 2013 / Lecture 7
Reverse breakdown.
Diodes demonstrate unilateral conductance only as long as:
Breakdown voltage
Mechanisms of reverse breakdown
Breakdown current should be limited not to
should be limited not to burn the device
Tunnel
Avalanche
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ESE 372 / Spring 2013 / Lecture 7
Zener diodes
(designed to operate in breakdown regime)
(designed to operate in breakdown regime)
Example of Zener‐based voltage regulator
Unregulated:
Regulated:
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ESE 372 / Spring 2013 / Lecture 7
Effect of Temperature on diode IV
Forward bias:
Reverse bias:
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ESE 372 / Spring 2013 / Lecture 7
Rectifiers
Power outlet
gives sine wave
Electronics uses DC
power supplies
How can we convert 60Hz 120 Vrms
into 5 V DC ?
For sine wave
Rectifier
Filter and regulator
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ESE 372 / Spring 2013 / Lecture 7
Ideal diode in series with AC generator and load.
No DC component
Id l diode
Ideal
di d IV
There is DC component
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ESE 372 / Spring 2013 / Lecture 7
Let’s use more realistic diode model
Ideal
since
We got nonzero DC component at the
output BUT it is not DC voltage yet.
Also, we utilized only half of the input
energy.
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ESE 372 / Spring 2013 / Lecture 7
Diode in series with capacitor
Ideal diode conducts only when
Capacitor directly
connected to AC source
Charge stored on
capacitor plates
Voltage across capacitor is DC voltage for t > T/4
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ESE 372 / Spring 2013 / Lecture 7
Half wave rectifier with filtering capacitor
Capacitor is
charged to
What happens when
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ESE 372 / Spring 2013 / Lecture 7
Half wave rectifier with filtering capacitor
Capacitor is
charged to
Wh
When
initially
Hence diode become reverse biased and
capacitor starts discharging through load.
Rapid charge
through small diode
series resistance
Slow discharge
through
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ESE 372 / Spring 2013 / Lecture 7
Voltage ripple of half wave rectifier
Diode closes
Diode opens
Discharge time
Charging time << discharge time
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ESE 372 / Spring 2013 / Lecture 7
Charging time
Taylor’s series expansion:
Charging time:
Ideally the current flows through diode only during charging time
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Example
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ESE 372 / Spring 2013 / Lecture 7
Example – cont.
Charge lost by capacitor
during discharge time ~ T.
This charge will be replenished during charge time << T.
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ESE 372 / Spring 2013 / Lecture 7
Half wave rectifier circuit
VD
Square wave input
q
p
ID
Vin
ID
For diode with no parasitic capacitance
and no charge accumulation effect we
would obtain:
T
Simplified diode model
RS  0, I S  0
Vin  VB , VTO  0.7V
VD
*Spectrum of square wave signal contains
many frequencies besides f0=1/T.
y –V0 to +V0 square
q
wave:
Fr 50% duty
Vin t  
4 Vin 
sin 6    f 0 

  sin 2    f 0  
 ...
3
 

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ESE 372 / Spring 2013 / Lecture 7
Diode recovery time
VD
ID
Real diodes have depletion and diffusion
capacitances so the transient response
will be complicated.
Vin
ID
Simplified diode model
RS  0, I S  0
Vin  VB , VTO  0.7V
VD
Diode “keeps” forward bias
after input is switched
Diode recovery time.
Determined by how fast the
caps can be discharged.
*Spectrum of square wave signal contains
many frequencies besides f0=1/T.
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ESE 372 / Spring 2013 / Lecture 7
Pn‐junction diode recovery time
Under forward bias the excess minority carriers are injected. Hence, the excess carrier concentrations are present on both
sides
id off pn-junctions.
j
ti
W
We callll it – “charge
“ h
accumulation”
l ti ” and
d characterize
h
t i it b
by so called
ll d diff
diffusion
i capacitance.
it
Recovery time
 REC   S   TR
Discharge of
junction depletion
capacitance
Accumulated charges cannot
disappear immediately after bias is
changed, hence diode would
appear to keep his forward bias VT0
for some time after bias is changed.
Transition time
I TRAN
0.7V

R
Storage time
S 
 TR
QStored
I TRAN
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ESE 372 / Spring 2013 / Lecture 7
Pn‐junction diode recovery time
Consider the case when voltage changes from positive to negative
Large negative current spike but recovery time is decreased as compared to positive-to-zero
transition case since now the same stored charge is being removed by larger current.
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