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1. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Week 8
Diodes
(non-linear devices)

2. Overview
• Overview:
– Terminal characteristics of junction diode
– Modelling the diode forward
5/16/2020Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

3. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Exercise

4. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Exercise
IB
Assuming that D1 and D2 are on:
1) VB=V=0
ID2 = V_10/R_10=10/10=1mA
IB=10/5=2mA
I=IB-ID2 = 2-1=1mA

5. 5/16/2020Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Assuming that D1 and D2 are on:
1) VB=V=0
ID2 = V_10/R_5 = 10/2 =2mA
IB=10/10=1mA
I=IB-ID2 = 1-2=-1mA -> Not both
Diodes are conducting
Assuming that D1 is off and D2 is
ON:
I=0
ID2=IB
ID2=V_tot/R_tot = 20/15=1.33mA
V=V_10 – ID2*R_5=10-1.33*5=
3.36V
IB

6. Terminal Characteristics of Junction Diodes
Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Microelectronic Circuits, Sixth Edition
The i–v characteristic of a
silicon pn junction diode
• The most common implementation of
the diode utilizes a pn junction
• The pn junction can conduct
substantial current in the forward
direction and almost no current in the
reverse direction

7. 1. Forward bias region: v > 0
2. Reverse bias region: v < 0
3. Breakdown region: v < -Vzk
Terminal Characteristics of Junction Diodes
For a “fully conducting” diode, the
voltage drop lies in a narrow range,
approximately 0.6 V to 0.8 V.
Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Microelectronic Circuits, Sixth Edition

8. T
V
v
i  IS (e 1)
q
kT
VT 
Thermal voltage
In the forward region the i–v relationship is closely approximated by
IS is a constant for a given diode at a given temperature. It is
usually called the saturation current and is directly
proportional to the cross-sectional area of the diode (scale
current)
For “small-signal” diodes, which are small-size
diodes intended for low-power applications, IS
is on the order of 10−15 A
At room temperature (20°C) the
value of VT is 25.3 mV (we tend
to use 25mV when we solve the
exercises)
Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Microelectronic Circuits, Sixth Edition
The forward bias region

9. Exercises
v=0.7V, i=1mA
Is?
Is=i/(e^(v/vt))=10^-3/[e^(0.7/25*10^(-3))]=
6.9*10^(-16)
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10. ripple
Figure 4.20 Block diagram of a dc power supply.
Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Rectifier Circuits
One of the most important applications of diodes is in the design of rectifier circuits.
The power supply is fed from the 120-V (rms) 60-Hz ac line, and it delivers a dc voltage Vo
(usually in the range of 4 V to 20 V) to an electronic circuit represented by the load block.
The dc voltage Vo is required to be as constant as possible in spite of variations in the ac
line voltage and in the current drawn by the load
N1/N2 -> 15/1
120V -> 8V

11. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, In c.
Figure 4.23 (a) Half-wave rectifier. (b) Transfer characteristic of the rectifier circuit. (c) Input and
output waveforms.
The half-wave rectifier
Vd

12. The full-wave Bridge rectifier

15. Exercise
A 5.0V stabilised power supply is required to be produced from a 12V DC power supply input
source. The maximum power rating PZ of the zener diode is 2W. Using the zener regulator circuit
circuit above calculate:
a) The maximum current flowing through the zener diode.
b) The minimum value of the series resistor, RS
c) The load current IL if a load resistor of 1kΩ is connected across the zener diode
d) The zener current IZ at full load.

16. a)
b)
c)
d)

17. Schottky Diode
The Schottky-barrier diode (SBD) is formed by bringing
metal into contact with a moderately doped n-type
semiconductor material.
The resulting metal–semiconductor junction behaves like
a diode.
Some characteristics of the Schottky diode:
• They can be switched from on to off, and vice
versa, much faster than is possible with pn-
junction diodes
• Low forward voltage drop: 0.3v - 0.5v (compared to
the 0.6 V to 0.8 V found in silicon pn-junction diodes)
Schottky diodes have high switching speeds and
high-frequency capabilities, which make them
well-equipped for use in radio frequency
applications
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P (W)=VI

18. • If a reverse-biased pn junction is
illuminated—that is, exposed to
incident light—the photons impacting
the junction cause covalent bonds to
break, and thus electron- hole
pairs are generated in the
depletion layer.
• The electric field in the depletion
region then sweeps the liberated
electrons to the n side and the holes
to the p side, giving rise to a reverse
current across the junction. This
current, known as photocurrent,
is proportional to the intensity of
the incident light.
• Such a diode, called a photodiode,
Photodiodes
Examples include fiber-optic
transmission of telephone and television
signals and the use of optical storage in
CD-ROM computer discs

19. LEDs:
- Inverse function of photodiodes: it converts a forward
current into light
• LEDs are very popular devices. They find application in
the design of numerous types of displays
• Laser diodes find application in optical communication
systems and in DVD players, among other things.
Light-Emitting Diodes
(LEDs)
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