Chapter 10: Diodes

1
Chapter 10 Diodes
1. Understand diode operation and select diodes
for various applications. 2. Analyze nonlinear
circuits using the graphical load-line
technique. 3. Analyze and design simple
voltage-regulator circuits. 4. Solve circuits
using the ideal-diode model and piecewise-linear
models. 5. Understand various rectifier and
wave-shaping circuits. 6. Understand
small-signal equivalent circuits.
2
Crystal Structure of Silicon
Covalent Bond Two atoms Share two electrons
View along a crystallographic axis of silicon
Silicon atom Four electrons/atom
3
Conductors and Semiconductors
Bar of metal
Bar of silicon
I
V
B

n-Type Silicon

Electron Conduction
Semiconductors
4
Intrinsic Silicon

• Temperature effects
• At absolute zero all electrons are bonded to
  neighbors. There is none available for
  conduction.
• Insulator
• At higher temperatures electrons gain enough
  energy to escape the bonds.
• Some conduction but still basically an insulator
  

Silicon 4 electrons/atom
5
Phosphorus (n-Type) Doped Silicon

• Replace silicon atom by a phosphorus atom which
  had five electrons rather than four (silicon).
  One impurity atom for 106 to 1010 silicon atoms.
• Extra electron is free to move in crystal
  allowing conduction

Phosphorus atom has net () charge which is
fixed in position
Extra electron (-) which is mobile
Phosphorous atom
6
Simplified Representation of n-Type Silicon

• Charges are balanced Number of () charges
  equals number of (-)charges

Charges due to one impurity atom
Shows only impurity atoms

• Net positive charge of each phosphorus atom
• Fixed position in crystal

• Extra electron (- charge) for each phosphorus
  atom
• Mobile Moves around crystal due to E-field and
  diffusion

7
p-Type Silicon

• Example with Boron doping atoms which has three
  electrons/atom.

Charges due to one impurity atom

• Net negative charge of each boron atom
• Fixed position in lattice

• Extra hole ( charge) for each boron atom
• Mobile Moves around crystal due to E-field and
  diffusion

8
Motion of Charges

• Mobile charges move due to two effects
• Forces due to electric field Force charge x
  Electric Field
• The resulting current is called drift current
• Diffusion due to gradient in the charge density
  
• Charges move to be evenly distributed throughout
  space
• Similar to perfume in room or heat in a solid.
• The resulting current is called diffusion
  current

9
Diode Operation Reverse Bias
Junction
Electrons flow to positive terminal
Hole flow to negative terminal

• Regions on both sides of junction become
  depleted of carriers.
• Steady state current approaches zero (after
  initial transient)

10
Diode Operation Forward Bias

• Voltage barrier due to band gap of silicon of
  about 0.6 to 0.7V
• For VD gt 0.6V, current increases exponentially
  with VD

11
Diode Characteristics
Anode (p-type) is positive
Cathode (n-type) is positive. (Anode is negative.)
Section10.1
12
Diode Symbols, etc
Arrow indicates forward current flow
Orthogonal line blocks reverse current flow
Note polarities of vD and iD
Flapper valve analogy
Section10.1
13
Characteristic of Typical Diode
Forward current increases exponentially above
knee on linear plot
Current almost constant
Note scale change
Typically, breakdown is due to avalanche
effects and is relatively soft or gradual
Section10.1
14
The Diode (Shockley) Equation

• IS is a scaling factor proportional to diode
  area
• n is an empirical fudge factor
• VT is the Thermal Voltage (Thermodynamics)
• k 1.38 1023 J/K is Boltzmanns constant
• q 1.60 1019 C is the charge of an electron
• At T 3000K , we can calculate
  .

Section10.1
15
Analysis of Simple Diode/Resistor Circuits

• Several Approaches
• Write and solve KVL/KCL using the diode
  (Shockley) equation.
• Load line analysis Actually, the most accurate
  with real circuits.
• Simplified models of diode characteristics
• Sophisticated computer models

Transcendental equation
Very easy for simple circuits
Adequate for many cases
For complex designs where investment is worth
while
16
Graphical Load Line Analysis of Simple
Diode/Resistor Circuit

• Concepts behind for load line analysis

• Both KVL and must be satisfied at all times

Section10.2
17
Current versus Voltage Plots for Diode and
Resistor (connected to VSS).
Diode Current versus vD
Resistor Current versus vD

• We can combine these curves on one plot to do a
  load line analysis (next page).

Section10.2
18
Load Line Analysis
KVL satisfied
RiD
KCL satisfied
Section10.2
19
Example of Load Line Analysis
iD VSS/R 20mA
Operating Point KVL and KCL satisfied
iD 12.5mA vD 1.25V
iD 0
Section10.2
20
Load-Line Analysis of Complex Circuits
Can apply load line analysis
Must be linear to apply Thevenin
Section10.3
21
The Zener Diode

• By proper doping of the silicon, the Zener
  Breakdown can be made to have a very sharp
  breakdown.
• The breakdown voltage is commonly labeled as VZ.

Breaks down at VZ -vD
Symbol
Designed to have a very sharp reverse breakdown
characteristic
Backward Z
Section10.3
22
ZENER DIODE VOLTAGE-REGULATOR No Load
A voltage regulator circuit provides a nearly
constant voltage from a variable source.
Regulates
Doesnt regulate
Section10.3
23
ZENER DIODE VOLTAGE-REGULATOR With Load
Section10.3
24
Example of Zener Voltage Regulator
iD 0 when vD VSS-20V
Correction

• With no load

Operating Point
iD VSS/R -20mA when vD VSS-20V
Section10.3
25
IDEAL-DIODE MODEL
Short circuit for vD 0
Open circuit for vD lt 0

• Similar to ideal check valve for fluids.
• No fluid can flow backward through valve
• No pressure drop when fluid flowing forward
  through valve

Section10.4
26
Analysis of circuits with Ideal Diodes

• Analysis of simple diode/resistor circuit is
  trivial with the ideal diode model.

• If VSS lt 0 Diode is off and iD iR 0

Section10.4
27
Constant Voltage Drop Diode Model

• Very commonly used

Equivalent Circuit (Model)
Section10.5
28
PIECEWISE-LINEAR DIODE MODELS

• The following is used to construct a more
  accurate model.

Section10.5
29
Piecewise Approximation to Diode Characteristics
Linear approximation
Actual characteristic
Intercept 0.6V
Section10.5
30
More on Piecewise Diode Model

• Let us plot the characteristic of the equivalent
  circuit or diode model.
• When vD lt 0.6V, diode is in reverse bias or
  off.
• When vD gt 0.6V, diode is on.
• iD (vD 0.6V)/R
• Plot the curve
• When vD 0, iD 0
• When vD 1.6V
• iD (1.6V-0.6V)/10W 100mA

Section10.5
31
Circuits with Multiple Diodes

• Consider the circuit below assuming ideal diodes
• How do you know whether each diode is on or
  off?

• General Approach
• Assume the state of each of the diodes i.e.,
  on or off.
• Analyze the circuit and check to see if your
  assumptions were correct.
• If not correct try another set of assumptions.

Section10.4
32
Circuits with Multiple Diodes Continued

• Assume D1 is off Replace with open
• Assume D2 is on Replace with short

• vD1 10V 3V 7V
• But this is not possible since the D1 would be
  forward biased or on with vD1 0V.
• We must try another set of assumptions.

Section10.4
33
Circuits with Multiple Diodes Continued

• Assume D1 is on and D2 is off

V1
D1 is forward biased - OK

• iD1 10V/10kW 1mA.
• vD2 3V V1 3V 6V -3V
• We have a valid solution!

D1 is reverse biased - OK
Section10.4
34
Half Wave Rectifier
Diode Blocks
Diode Conducts
Section10.6
35
Half Wave Rectifier Charging Battery
Assumes Ideal Diode
Diode Blocks
Diode Conducts
Diode conducts only when vs(t)gtVB vD
Section10.6
36
Half-Wave Rectifier with Smoothing Capacitor
Section10.6
37
Half-Wave Rectifier with Smoothing Capacitor
(Continued)
Diode Blocks
Diode Conducts
Section10.6
38
Full Wave Rectifier
Section10.6
39
Diode-Bridge Full Wave Rectifier

• A and B conduct during positive half-cycle
• D and C conduct during negative half-cycle
• Doesnt require center-tapped transformer.

Section10.6
40
Clipper Circuit Waveforms

• Very commonly used to shape and clean up
  waveforms

Diode A Conducts for vingt6V
Diode B Conducts for vinlt-9V
Section10.7
41
Clipper Circuit Transfer Characteristic

• Same circuit as on previous page

vo(t) vin(t)

• Could be used to limit voltage swing of
  amplifier output.

Section10.7
42
Clipper Circuit Using Zener Diodes
D1
D2

• Assume forward voltage drop of 0.6V for both
  diodes
• D1 conducts when vin(t) gt 8.4V 0.6V 9V
• D2 conducts when vin(t) lt -5.4V 0.6V -6V
• Clips at 9V and 6V

Section10.7
43
DC Restore or Clamping Circuit

• During first ¼ cycle the diode conducts and the
  capacitor charges to VP .
• After this, v0 never goes above zero volts, the
  diode does not conduct and the capacitor voltage
  does not change.
• v0 is shifted in the negative direction by 5V.

Section10.7
44
Notation for Currents and Voltages in Electronic
Circuits

• VDQ and IDQ represent the dc voltage and current
  at the quiescent or Q-point.
• vD and iD represent the small signal values.
• vD and iD represent the total values
• vD VD vd and iD ID id

Section10.8
45
Q-Point DC Operating Point
Section10.8
46
Example of Total Current Notation
iD IDQ iD
DC
Total
DC Small signal
Section10.8
47
Small Signal Diode Operation
Section10.8
48
LINEAR SMALL-SIGNAL EQUIVALENT CIRCUITS

• The small-signal equivalent circuit for a diode
  is a resistance.

Corrected Equation
At Q-point
Key equation Must learn
Section10.8
49
Example of Change of rd with Q-Point

• Larger VD gives larger ID
• Larger ID gives smaller rd
• Smaller rd gives more current swing

Section10.8
50
Variable Attenuator
Supplies DC current to change Q-Point of Diode
Small signal resistance of diode changes as
Q-Point changes
Section10.8
51
Equivalent Circuits of Attenuator
DC Circuit
Small Signal Circuit
Section10.8
52
Application of Variable Attenuator

• For maintaining proper signal at head of tape
  recorder.

Feedback loop senses magnitude and adjusts
attenuation to keep it at desired level
Section10.8
53
Device Fabrication
Wafer Fabrication and Polishing
Wafer sliced from ingot
8 or 12 in diameter
Optically Flat
54
Device Fabrication (Continued)
Forming Oxide Layer
Oxidized at about 11000C in Oxygen Environment
Oxide thickness lt 1mm
55
Device Fabrication (Continued)
Lithography

• Resist protects wafer from etching which is next
  step

56
Device Fabrication (Continued)
Etching
Oxide removed by etching

• Etching done by wet acids or by dry etching
  where oxide is immersed in plasma of ionized
  atoms.

57
Device Fabrication (Continued)
Dry Etching
58
Device Fabrication (Continued)
Ion Implanter
Wafer
10,000V to 1,000,000V
59
Device Fabrication (Continued)
Ion Implantation
Atoms stopped by oxide
Atoms buried in silicon substrate
60
Device Fabrication (Continued)

• Photo Exposure Tool
• Early 1990s version
• Modern tools are much more complex
• Most recent immerse lens and wafer in liquid to
  increase optical resolution
• Very expensive

61
Final Diode