Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN325: ElectronicsSpring 2021
Semiconductor pn Junction Diode
Announcements
2
• Lab 4 Due Mar 4 (R)
• Lab 5 Due Mar 9 (T)
• HW4 Due Mar 10
Reading
3
• Razavi Ch2 (optional)• Basic semiconductor device physics, which is
useful to understand how diodes work• Covered in more detail in ECEN 370
• Razavi Ch3• Diode models and circuits
Agenda• Semiconductor pn junction diodes• Diode current-voltage (I-V) characteristics• Constant voltage drop model• Solving circuits with diodes• Diode rectifier circuits
4
Semiconductors
5
• A semiconductor is a material whose conductivity lies somewhere between an insulator and a conductor
• Example: Pure or “intrinsic” Silicon (Si) has 4 valence electrons and is not a very good conductor
• A semiconductor’s conductive properties can be changed by “doping” the material with either n-type dopants (Phosphorous) or p-type dopants(Boron)
• A diode is formed at the boundary or junction of a p and n type semiconductor
Semiconductor pn Junction Diode Physical Schematic
6
Intrinsic Si
p-type Si n-type Sidoping (Boron)
doping (Phosphorous)
[Sedra/Smith]
Diffusion, Drift Current, & Barrier Voltage
7
• “Majority-Carrier” Diffusion Current, ID• Caused by majority carriers diffusing into other region• Near the junction, holes diffusing into the n-region recombine with free electrons,
deplete the carriers close to the junction, and form a positive charged region• Similarly, electrons diffusing into the p-region recombine with free holes, deplete the
carriers close to the junction, and form a negative charged region• This charge separation creates a “Barrier Voltage” which limits the diffusion
current• “Minority-Carrier” Drift Current, IS
• Caused by thermally generated minority carriers sweeping across the junction due to the E-field
[Sedra/Smith]
Operation w/ Different Biases
8
• Open Circuit, ID=IS• Reverse-Biased, IS>ID, Weak Minority Carrier Drift Current• Forward-Biased, ID>>IS, Strong Majority Carrier Diffusion
Current
[Sedra/Smith]
I-V Characteristic
9
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[Sedra/Smith]
Reverse Breakdown
10
• For large negative voltages, the previous exponential equation predicts that the reverse bias current should saturate at –IS
• However, with a large negative voltage “-VZ” the diode “breaks down” and a large negative current exists
• Most diodes should be designed to avoid this reverse-breakdown region
• Special diodes, called Zener diodes, are design to operate in reverse breakdown and used in applications such as voltage regulators
[Sedra/Smith]
Constant-Voltage-Drop Model
11
• Used to simplify analysis• If Vd<Vconstant Id=0
• (Open Circuit)• If Vd>Vconstant Id can go to ,
and Vd clamps at Vconstant• (Battery w/ Vconstant voltage)
• We will assume Vconstant = 0.7V
[Sedra/Smith]
Solving Circuits with Diodes
12
1. A diode will either be “on” or “off”, resulting in 2 possibilities for each diode in the circuit
2. Assume 1 condition and solve the circuit
3. Check solution for consistency with the diode model
4. If it is consistent, the solution is correct and you are done
5. If not consistent, you need to solve the circuit with another possible condition
Diode Circuit Example #1
13
• Solve for Vout and Id• First assume that the
diode is “OFF”, i.e. an open circuit
• Are the diode I-V conditions consistent with the constant-voltage-drop model?
• Vd=10V and Id=0A• This is not consistent with the diode model!• We need to try another diode condition
Diode Circuit Example #1 (cont.)
14
• Now assume that the diode is “ON”, i.e. a 0.7V battery
• Now, Vd=0.7V and Id=4.65mA• This is consistent with the diode model!• This is the correct solution
mAIVV
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V
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OUT
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Rectifier Circuits
15
[Karsilayan]
Half-Wave Rectifier
16
[Razavi]
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Half-Wave Rectifier Transfer Characteristic
17
[Karsilayan]
[Sedra/Smith]
• Only rectifies positive half of the input signal• Lose one diode voltage drop from the peak value
Half-Wave Rectifier w/ a Filter Cap
18
[Razavi]
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11
How Much is the Ripple Voltage?
19
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What is the Peak Diode Current?
20
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[Razavi]
• The peak current is proportional to the product of the RC time constant and the input frequency
Full-Wave Rectifier
21onDP VV ,2Voltage Reverse Maximum
[Sedra/Smith]
• Positive ½ cycle• Top diode on
• Negative ½ cycle• Bottom diode on
[Karsilayan]
Full-Wave Rectifier Transfer Characteristic
22
[Karsilayan]
[Sedra/Smith]
• Rectifies all of the input signal• Lose one diode voltage drop from the peak value
Bridge Rectifier
23
[Karsilayan]
[Sedra/Smith]
• Positive ½ cycle• D1 & D3 on
• Negative ½ cycle• D2 & D4 on
-0.7V
Vs-0.7V-0.7V
Vs-0.7V
onDP VV ,Voltage Reverse Maximum
Vs-2(0.7V)
Bridge Rectifier Transfer Characteristic
24
• Rectifies all of the input signal• Lose two diode voltage drops from the peak value
[Karsilayan]
Full-Wave & Bridge Rectifier w/ a Filter Cap
25
[Sedra/Smith]
• The capacitor only discharges for T/2• Results in ½ Cap size for a given ripple• Roughly ½ diode current due to smaller Cap
inL
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Rectifier Trade-Offs
26
• Half-Wave Rectifier+Simplest design with fewest components- Requires largest capacitor for a given ripple
• Full-Wave Rectifier+Reduces capacitor size by ½ relative to half-wave- Requires center-tapped transformer- Maximum reverse voltage almost double that of half-wave
• Bridge Rectifier+Reduces capacitor size by ½ relative to half-wave+Save maximum reverse voltage as half-wave rectifier- Lose two diode voltage drops in peak value