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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givennot if 1n Assume ,2-1Factor Ideality

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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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Rectifier Circuits

15

[Karsilayan]

Half-Wave Rectifier

16

[Razavi]

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Voltage Reverse Maximum

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sinFor

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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

onDPR fCR

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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