What to learn in lecture3
3.1 PN junction physics, The Ideal Diode
3.2 Terminal Characteristics of Junction Diodes
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p-n Junction
Charge neutrality
• Depletion region
• Large Charge density
• Large electric field
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Band diagram of pn Junction
Potential
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Forward Bias and Reverse Bias
Forward bias
Reverse bias
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I-V characteristics of pn diode
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Pn junction
• I-V
• Variable Capacitances
– Forward bias : very large
– Reverse bias : variable with reverse biases
• Temperature Characteristics
– Constant voltage
– Constant current
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pn Junction applications
• Almost all semiconductor devices
• Bipolar transistor
• Diode, Zener diode
• LD(Laser Diode), LED(light emitting diode),
OLED
• Photodiode, Solar cell
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3.1 Ideal pn Diode  Switch model
off
on
Figure 3.1 The ideal diode: (a) diode circuit symbol; (b) i–v
characteristic; (c) equivalent circuit in the reverse direction; (d)
equivalent circuit in the forward direction.
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Figure 3.2 The two modes of operation of ideal diodes and the use of
an external circuit to limit the forward current (a) and the reverse
voltage (b).
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Applications 1) Rectifier
Figure 3.3 (a) Rectifier circuit. (b) Input waveform. (c) Equivalent
circuit when vI  0. (d) Equivalent circuit when vI 0. (e) Output
waveform.
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Figure E3.1
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Figure E3.2
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EXAMPLE 3.1
- Find the fraction of each cycle during which the diode conducts. And find the
peak diode current and the maximum reverse-bias voltage of the diode.
24 cos 
- The diode conducts when vS exceeds 12 V
24 cos   12    60  Conduction Angle  2  60
- The peak value of the diode current and maximum reverse voltage
I d ,max 
24  12
 0.12 A
100
Vd ,max  24  12  36V
Figure 3.4 Circuit and waveforms for Example 3.1.
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2) Diode logic gate
- Diodes with resistors can be used to implement digital logic functions
Out
A
B
C
=
0
0
0
0
=
0
1
=
1
0
0
1
=
0
1
0
=
1
0
1
0
=
0
0
1
1
=
1
0
1
1
=
0
1
0
0
=
1
1
0
0
=
0
1
0
1
=
1
1
0
1
=
0
1
1
0
=
1
1
1
0
=
0
1
1
1
=
1
1
1
1
=
1
A
B
C
0
0
0
0
0
0
Y  A B C
Out
Y  A B C
Figure 3.5 Diode logic gates: (a) OR gate; (b) AND gate (in a positive-logic
system).
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EXAMPLE 3.2
Figure 3.6 Circuits for Example 3.2.
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I D2 
10  0
 1 mA
10
0  ( 10)
I 1
5
10  0
I D2 
 2 mA
5
0  ( 10)
I2
10
10  ( 10)
I D2 
 1.33 mA
15
VB  10  10  1.33  3.3 V
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Figure E3.4
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3.2 Terminal Characteristics of Junction Diode
Figure 3.7 The i–v characteristic of a silicon junction diode.
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Breakdown voltage
Cut-in voltage
Figure 3.8 The diode i–v relationship with some scales expanded and
others compressed in order to reveal details.
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i  I s (e / nVT  1)
VT 
(3.1)
Is : reverse saturation current
kT
q
(3.2)
Thermal Voltage
For Forward bias region
i
I s e / nVT
(3.3)
i
IS
(3.4)
  nVT ln
1 / nVT
I1  I s e
I2
 e ( 2 1 ) / nVT
I1
 2 / nVT
I2  I se
I2
V2  V1  nVT ln
I1
I2
V2  V1  2.3 nVT log
I1
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n= 1~2
(3.5)
60 meV ~ 120 meV
~0.1V/decade
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Forward bias region
ln I
(log I)
Semi Log graph
ln Is
V
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EXAMPLE 3.3
i  I S e / nVT
I S  ie  / nVT
If n  1: I S  103 e 700 / 25  6.9  1016 A, or about 10-5 A
If n  2: I S  103 e 700 / 50  8.3  1016 A, or about 10-9 A
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Figure 3.9 Illustrating the temperature dependence of the diode
forward characteristic. At a constant current, the voltage drop
decreases by approximately 2 mV for every 1C increase in
temperature.
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Reverse Bias region
i
 IS
Theoretically, Is doubles for every 5 degree C rise in temperature
Reverse current doubles for every 10 degree C rise
Breakdown Voltage
Zener breakdown
Vz < 5V, - Temco
Avalanche breakdown Vz > 5V, + Temco
Vz ~ 5V, 0 Temco
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Temperature Effects
• Vz is function of T
– TC, temco mV/C
– TC depends on the Zener voltage
• Forward conducting diode series with Positive TC Zener
diode (Vz is larger than 5V)
Vz + 0.7 with zero TC
+ 2 mV/C
-2mV/C
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Figure E3.9
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Assignments
1. Explain potential barrier, depletion width, and electric field of
pn junction with respect to applied voltage.
2. Explain temperature effects of I-V characteristics of a pn junction.
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