Transcript Electronics (2001)

Lecture 2: Semiconductor Diodes
 The pn Junction Diode
 Rectifiers
 Power Supplies
 Diode Clippers
 Diode Clampers
 Diode Thermometer
1
Diodes
• Diodes are semiconductor devices which might be
described as passing current in one direction only. Diodes
however are far more versatile devices than that.
• Diodes may be used as voltage regulators, tuning devices
in RF tuned circuits, frequency multiplying devices in RF
circuits, mixing devices in RF circuits, switching
applications or can be used to make logic decisions in
digital circuits.
• There are also diodes which emit “light”, of course these
are known as light-emitting-diodes or LED’s.
2
A pn Junction
Figure
9.5
3
Forward- and Reverse-biased pn Junctions
Figure
9.7
4
Semiconductor Diode i-v
Characteristic
Semiconductor Diode Circuit
Symbol
Figure 9.8,
9.9
5
Common Diode
• The first diode in the previous Figure is a semiconductor
diode which could be a small signal diode. Notice the
straight bar end has the letter “k”, this denotes the
“cathode” while the “a” denotes anode.
• Current only flows from anode to cathode and not in the
reverse direction, hence the “arrow” appearance. This is
one very important property of diodes.
• The anode terminal must be positive and the cathode
terminal negative for a current to flow.
• In an ideal diode, there is no voltage drop across a diode
when current is flowing.
6
The i-v Characteristic of the Semiconductor Diode
Figure
9.10
7
Large-Signal on/off Diode Model
Figure 9.11
8
Physical Structure of the Junction Diode
9
Forward Biasing the Junction
•
•
Electrons in the p-type material, near
the positive terminal of the supply,
break their electron pair bonds and
enter the supply, thereby producing
new holes. Also electrons from the
negative terminal of the supply enter
the n-type material and migrate towards
the junction.
Free electrons from the n-type then
flow across the junction and move into
the holes which have migrated from the
positive terminal. This current flow will
continue as long as the external supply
is connected and is called forward
current flow.
10
Reverse Biasing the Junction
•
When the polarity of the supply is
reversed, the potential within the
device is effectively reinforced and
the depletion layer becomes wider.
This is because the free electrons in
the n-type are attracted towards the
positive terminal, away from the
junction, while the electrons from
the negative terminal of the supply
enter the p-type and migrate
towards the junction. Current flow
is extremely small and is called
reverse current. Note that this
current is produced by minority
carriers and the device is said to be
reverse biased.
11
Circuit containing ideal diode
Assuming that the ideal diode conducts
Figure
9.12,
9.13,
9.14
Assuming that the ideal diode does not
conduct
12
Zener Diode
• The second of the diodes is a zener diode which are fairly
popular for the voltage regulation of low current power
supplies.
• While it is possible to obtain high current zener diodes,
most regulation today is done electronically with the use of
dedicated integrated circuits and pass transistors.
13
Varactor Diode
• This is actually two varactor diodes mounted back
to back with the DC control voltage applied at the
common junction of the cathodes. These cathodes
have the double bar appearance of capacitors to
indicate a varactor diode.
• When a DC control voltage is applied to the
common junction of the cathodes, the capacitance
exhibited by the diodes (all diodes and transistors
exhibit some degree of capacitance) varies in
accordance with the applied voltage.
14
Vacuum Tube
• The next diode is the simplest form of vacuum
tube or valve. It simply has the old cathode and
anode.
• These terms were passed on to modern solid state
devices. Vacuum tube diodes are mainly only of
interest to restorers and tube enthusiasts.
15
LED
• The last diode depicted is of course a light emitting diode
or LED.
• A LED actually does not emit as much light as it first
appears, a single LED has a plastic lens installed over it
and this concentrates the amount of light.
• Seven LED’s can be arranged in a bar fashion called a
seven segment LED display and when decoded properly
can display the numbers 0 - 9 as well as the letters A to F.
16
Diode Circuits
From Neamen, Electronic Circuit Analysis and Design, McGraw Hill
Input Signal
Rectified Output Signal
17
Ideal Diode Rectifier Input and Output Voltages
Figure
9.20,
9.21
i
T

1T
1 T / 2
  i (t )dt    I P sin t dt   0dt 
T0
T 0
T /2


IP


2irm s

18
Half-Wave Rectifier
(Sedra/Smith Microelectronic Circuits, Oxford)
19
Full-Wave Rectifier
Figure 9.39
20
Full-Wave Rectifier
(Sedra/Smith Microelectronic Circuits, Oxford)
21
Operation of Bridge Rectifier
Figure
9.42
22
The Bridge Rectifier
(Sedra/Smith Microelectronic Circuits, Oxford)
23
DC Power Supply
Figure 9.45
24
(a) A Zener diode voltage regulator; (b) simplified circuit for Zener
regulator
Figure 9.49
25
Two-sided diode clipper
Circuit model for the diode clipper
Figure 9.54,
9.55
26
Two-sided (ideal diode) clipper
input and output voltages
Voltages for the diode clipper
(piecewise linear diode model)
Figure
9.58, 9.60
27
The Filter
(Sedra/Smith Microelectronic Circuits, Oxford)
28