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Файл:Basics of electronics. Study aid
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It is often said that the Schottky diode is a ‘majority carrier’ semiconductor
device. This means that if the semiconductor body is doped n-type, only the ntype carriers (mobile electrons) play a significant role in normal operation of the
device. The majority carriers are quickly injected into the conduction band of
the metal contact on the other side of the diode to become free moving
electrons. Therefore no slow, random recombination of n- and p- type carriers is
involved, so that this diode can cease the conduction faster than an ordinary
p-n rectifier diode. This property, in turn, allows a smaller device area, which
also makes the transition faster. This is another reason why Schottky diodes are
useful in switch-mode power converters; the high speed of the diode means that
the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing
the use of small inductors and capacitors with greater efficiency than would be
possible with other diode types. Small-area Schottky diodes are used in RF
detectors and mixers, which often operate up to 50 GHz.
The most evident limitations of Schottky diodes are the relatively low
reverse voltage rating for silicon-metal Schottky diodes, 50 V and below, and
a relatively high reverse leakage current. Diode designs have been improving
over time. Nowadays voltage ratings reach 200 V. The reverse leakage
current, as it increases with temperature, leads to a thermal instability issue.
This often limits the useful reverse voltage.
Silicon Carbide Schottky Diode
Since 2001 another important invention was presented by CREE (NC,
USA): a silicon carbide (SiC) Schottky diode. SiC Schottky diodes have
about 40 times lower reverse leakage current compared to silicon Schottky
diodes. In 2011 they were available from several manufacturers in variants up
to 1700 V.
Silicon carbide has a high thermal conductivity and temperature has
little influence on its switching and thermal characteristics. With special
packaging it is possible to have operating junction temperatures of over
500 K, which allows passive radiation cooling in aerospace applications.
Applications
Voltage Clamping
While standard silicon diodes have a forward voltage drop of about
0.6 volts (voltage drop of germanium diodes is about 0.3 volts), Schottky
diodes’ voltage drop at forward biases of around 1 mA is in the range from
0.15 V to 0.46 V, which makes them useful in voltage clamping applications
and in the prevention of the transistor saturation. This is due to the higher
current density in the Schottky diode.
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Reverse Current Protection
Schottky diodes are used in photovoltaic (PV) systems to prevent a
reverse current flowing through the PV modules. For instance, they are used
in stand-alone (‘off-grid’) systems to prevent batteries from discharging
through the solar cells at night. They are used in grid-connected systems with
multiple strings connected in parallel, in order to prevent reverse current
flowing from adjacent strings through shaded strings if the bypass diodes
have failed.
Power Supply
They are also used as rectifiers in switchedmode power supplies; the low forward voltage and
fast recovery time leads to increased efficiency.
Schottky diodes can be used in power supply
‘OR’ing circuits in products that have both an
internal battery and a mains adapter input.
However, the high reverse leakage current presents
a problem in this case, as any high-impedance
voltage sensing circuit detects the voltage from
another power source through the diode leakage.
For example, ST Microelectronics offers Schottky and ultrafast rectifier
solutions for all market requirements. ST’s latest developments include
ULVF™ ultra-low VF diodes, improved avalanche rating, and the integration
of higher currents in low-profile PowerFLAT™ packages.
The range of signal Schottky diodes with the new flip-chip and SOD923 devices helps meet the most precise space-saving requirements,
especially for portable communication equipment.
For power converter applications where silicon diodes reach the limits
of their operating temperature and power density, ST silicon carbide (SiC)
devices take over with optimal reliability.
4.4.3. Photodiode
A photodiode is a type of photodetector capable of converting light into
either the current or the voltage, depending upon the mode of operation.
Photodiodes are similar to regular semiconductor diodes except that they
may be either exposed (to detect vacuum UV or X-rays) or packaged with a
window or optical fibre connection to allow light to reach the sensitive part
of the device. Many diodes designed for use as a photodiode also use a PIN
junction rather than the typical p-n junction.
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Principle of Operation
A photodiode is a p-n junction or PIN structure. When a photon of
sufficient energy strikes the diode, it excites an electron thereby creating a
mobile electron and a positively charged electron hole. If the absorption
occurs in the junction’s depletion region, or one diffusion length away from
it, these carriers are swept from the junction by the built-in field of the
depletion region. Thus holes move toward the anode, and electrons toward
the cathode, and a photocurrent is produced. Fundamentally a photodiode is a
current generator.
R
S
R
L
I
R
Light intensity
E
E
E
>
>
4
>
3
2
E
R
P
i
1
Ideal
diode
I
D
I
PH
C
S
Dark current
E
=0
1
E
2
E
3
E
4
Load line
Photoconductive
mode
Photovoltaic
mode (solar cell)
Dark current
I
D
Photocurrent
I
v
PH
Diode capacitance
C
S
Parallel resistance
R
P
Noise current
I
R
Series resistance
R
S
Load resistance
R
S
Fig. 4.8. Photodiode operation
Photovoltaic Mode
When used in zero bias or photovoltaic mode, the flow of photocurrent
out of the device is restricted and a voltage builds up. The diode becomes
forward biased and ‘dark current’ begins to flow across the junction in the
direction opposite to the photocurrent. This mode is responsible for the
photovoltaic effect (fig. 4.8), which is the basis for solar cells. In fact, a solar
cell is just an array of large photodiodes.
Photoconductive Mode
In this mode the diode is often (but not always) reverse biased. This
increases the width of the depletion layer, which decreases the junction
capacitance resulting in faster response time. The reverse bias induces only a
small amount of current (known as saturation or back current) along its
direction while the photocurrent remains virtually the same.
63
Although this mode is faster, the photovoltaic mode tends to exhibit less
electronic noise. (The leakage current of a good PIN diode is so low – less
than 1 nA – that the Johnson-Nyquist noise of the load resistance in a typical
circuit often dominates).
4.4.4. Light-Emitting Diode
The first known report of a light-emitting solid-state diode was made in
1907 by the British experimenter H.J. Round. However, no practical use was
made of the discovery for several decades. Independently, Oleg
Vladimirovich Losev published ‘Luminous carborundum [silicon carbide]
detector and detection with crystals’ in the Russian journal Telegrafiya i
Telefoniya bez Provodov (Wireless Telegraphy and Telephony).
A light-emitting diode, usually called an LED, is a semiconductor
diode that emits incoherent narrow-spectrum light when the p-n junction is
electrically biased in the forward direction, as in the common LED circuit.
This effect is a form of electroluminescence.
Blue, green, and red LEDs; these can be combined to produce any color,
including white. Infrared and ultraviolet (UV) LEDs are also available.
A LED is usually a small area light source, often with extra optics added
to the chip that shapes its radiation pattern. LEDs are often used as small
indicator lights on electronic devices and increasingly in higher power
applications such as flashlights and area lighting. The color of the emitted
light depends on the composition and condition of the semiconducting
material used, and can be infrared, visible, or ultraviolet. LEDs can also be
used as a regular household light source. Besides lighting, interesting
applications include water sterilization and disinfection of devices.
LED Panels, Flat Panel LED TV and Others
A combination of red, green and blue LEDs can produce the impression
of white light, though white LEDs today rarely use this principle. Most
‘white’ LEDs in production today are modified blue LEDs: GaN-based,
InGaN-active-layer LEDs emit blue light of wavelengths between 450 nm
and 470 nm.
If the emitting layer material of the LED is an organic compound, it is
known as an Organic Light Emitting Diode (OLED).
Physical Function
Like a normal diode, the LED consists of a chip of semiconducting
material doped with impurities to create a p-n junction. As in other diodes,
the current flows easily from the p-side, or anode, to the n-side, or cathode,
but not in the reverse direction. Charge-carriers – electrons and holes – flow
64
into the junction from electrodes with different voltages. When an electron
meets a hole, it falls into a lower energy level, and releases energy in the
form of a photon.
The wavelength of the light
emitted, and therefore its color, depends
on the band gap energy of the materials
forming the p-n junction. In silicon or
germanium diodes, the electrons and
holes recombine by a non-radiative
transition which produces no optical
emission, because these are indirect
band gap materials. The materials used
for the LED have a direct band gap with
energies corresponding to near-infrared,
visible or near-ultraviolet light.
LED development began with
infrared and red devices made with
gallium arsenide. Advances in materials
Fig. 4.9. A light-emitting diode
construction
science have made possible the
production of devices with ever-shorter wavelengths, producing light in a
variety of colors.
LEDs are usually built on an n-type substrate, with an electrode attached
to the p-type layer deposited on its surface. Less common p-type substrates
occur as well. Many commercial LEDs, especially GaN/InGaN, also use
sapphire substrate. Substrates that are transparent to the emitted wavelength,
and backed by a reflective layer, increase the LED efficiency. The refractive
index of the package material should match the index of the semiconductor,
otherwise the produced light gets partially reflected back into the
semiconductor, where it may be absorbed and turned into additional heat,
thus lowering the efficiency. This type of reflection also occurs at the surface
of the package if the LED is coupled to a medium with a different refractive
index such as a glass fiber or air. The refractive index of most LED
semiconductors is rather high, so almost in all cases the LED is coupled into
a much lower-index medium. The large index difference makes the reflection
quite substantial (per the Fresnel coefficients), and this is usually one of the
dominant causes of LED inefficiency. Often more than half of the emitted
light is reflected back at the LED-package and package-air interfaces. The
reflection is most commonly reduced by using a dome-shaped (half-sphere)
package with the diode in the center so that the outgoing light rays strike the
surface perpendicularly, at which angle the reflection is minimized. An antireflection coating may be added as well. The package may be cheap plastic,
65
which may be colored, but this is only for cosmetic reasons or to improve the
contrast ratio; the color of the packaging does not substantially affect the
color of the light emitted. Other strategies for reducing the impact of the
interface reflections include designing the LED to reabsorb and reemit the
reflected light (called photon recycling) and manipulating the microscopic
structure of the surface to reduce the reflectance, either by introducing
random roughness or by creating programmed special surface patterns.
Conventional LEDs are made from a variety of inorganic semiconductor
materials, producing the following colors:
Aluminium gallium arsenide (AlGaAs) – red and infrared;
Gallium phosphide (GaP) – red, yellow and green;
Indium gallium nitride (InGaN) – 450 nm – 470 nm – near
ultraviolet, bluish-green and blue;
Silicon carbide (SiC) as substrate – blue;
Zinc selenide (ZnSe) – blue;
Diamond (C) – ultraviolet;
and others.
With this wide variety of colors, arrays of multicolor LEDs can be
designed to produce unconventional color patterns.
Types of LEDs
The main types of LEDs are miniature, high power devices and custom
designs such as alphanumeric or multi-color.
Miniature LEDs
These are mostly single die LEDs used as indicators, and come in
various size packages: surface mount; 2 mm; 3 mm; 5 mm. Other sizes are
also available, but less common.
Common package shapes: round, dome top; round, flat top; rectangular,
flat top (often seen in LED bargraph displays); triangular or square, flat top.
The encapsulation may also be clear or semi opaque to improve the contrast
and the viewing angle.
There are 3 main categories of miniature single die LEDs:
Low current – typically rated for 2 mA at around 2 V (approximately
4 mW consumption).
Standard – 20 mA LEDs at around 2 V (approximately 40 mW) for red,
orange, yellow & green, and 20 mA at 4–5 V (approximately 0.1 W) for blue,
violet and white.
Ultra high output – 20 mA at approximately 2 V or 4–5 V, designed for
viewing in direct sunlight.
66
Multicolor LEDs
Bicolor LEDs contain 2 dice of different colors connected back to back,
and can produce any of 3 colors. Current flow in one direction produces one
color, current in the other direction produces the other color, and
bidirectional current produces both colors mixed together.
Tricolor LEDs contain 2 dice of different colors with a 3 wire
connection, available in common anode or common cathode configurations.
The most common form of both the bicolor and tricolor LEDs is red/green,
producing orange when both colors are powered.
RGB LEDs contain red, green and blue emitters, generally using a 4
wire connection with one common (anode or cathode).
Alphanumeric LEDs
LED displays are available in 7 segment and starburst format. 7 segment
displays handle all numbers and a limited set of letters. Starburst displays can
display all letters.
7 segment LED displays were in widespread use in the 1970s and 1980s,
but the increasing use of LCD displays with their lower power consumption
and greater display flexibility has reduced the popularity of numeric and
alphanumeric LED displays.
High-power LEDs (HPLED) can
be driven at the current values from
hundreds of mA to more than an
ampere, compared with tens of mA
for other LEDs. Some can emit over a
thousand lumens. Since the
overheating is destructive, the
HPLEDs must be mounted on a heat
sink to allow for heat dissipation. If
the heat from a HPLED is not
removed, the device will fail in
seconds. One HPLED can often
replace an incandescent bulb in a
torch, or be set in an array to form a
Fig. 4.10. High-power light emitting
diode
powerful LED lamp.
Some well-known HPLEDs in this category are the Osram Opto
Semiconductors Golden Dragon, Cree X-lamp. Some HPLEDs manufactured
by Cree Inc. now exceed 105 lm/W and are being sold in lamps intended to
replace incandescent, halogen, and even fluorescent lights, as LEDs grow
more cost competitive.
67
LEDs developed by Seoul Semiconductor can operate on a.c. power
without a DC converter. For each half-cycle, a part of the LED emits light
and a part is dark, and this is reversed during the next half-cycle. The efficacy
of this type of HPLED is typically 40 lm/W. A large number of LED
elements in series can operate directly from the line voltage.
Self-Assessment Questions
1. How many terminals does a diode have?
What is the state of a silicon diode if the voltage drop across it is
2.
about 0.7 V and the current of 100 mA flows through it?
What is the resistance value of an ideal diode in the region of
3.
conduction?
4.
Draw the diode characteristic (V/A). Mark the forward voltage
V
drop
.
F
5.
What is the frequency of the output voltage of a full-wave rectifier?
Is the half-wave rectifier easier to filter than a full-wave rectifier for
6.
a given input frequency?
Give the circuit of a single-phase bridge rectifier. Draw the
7.
V
waveform for output voltage if input voltage is sinusoidal (amplitude
frequency
8.
f are known).
Where is the normal operating region for a Zener diode at a V-I
and
m
characteristics?
Draw the symbols of a photodiode. Is it true that a photodiode is
9.
used in a reverse-bias position, and it will increase conduction as the light
intensity increases?
Draw the Zener diode characteristic (V/A). Mark the Zener
10.
voltage
V
.
Z
Why the Schottky diodes are used in very fast-switching circuits?
11.
What do we call the process of emitting photons from a
12.
semiconductive material?
What diode is used in seven-segment displays: Zener, LED or
13.
Schottky?
68
CHAPTER 5. BIPOLAR JUNCTION TRANSISTORS
Transistors are ‘active’ devices of electronics. They are capable of
controlling power from a d.c. power supply, in response to a low-power input
signal, and providing much greater output power. So they can provide the
power gain. This is the basic function of an amplifier. An analogue amplifier
should do this with the minimum distortion of its input waveform. A digital
switch or gate is also required to provide the power gain, but it should
convert any ill-defined input voltage level into a ‘clean’ well-defined voltage
level at its output.
There are two principal types of transistor: the bipolar junction transistor
(BJT) and the field-effect transistor (FET). Both types are described below.
5.1. Construction. Characteristics
Silicon bipolar transistors are three-terminal devices consisting of two pn junctions formed back to back in a single crystal of silicon. They are made
in n-p-n and p-n-p configurations as shown in figures 5.1(a) and (b).
The three regions of the transistor are called
The graphical symbols for the two versions of the device are also shown.
In these symbols the arrow is always on the emitter, and points in the
direction of conventional current flow.
A cross-section of a typical n-p-n transistor is shown in fig. 5.1(c).
Such a device is produced in much the same way as a p-n junction diode
except that there is one additional diffusion. Beginning with an n-type
substrate, a p-type region is diffused into it through a window in the
protective layer, as for a diode. But then a new oxide layer is grown over
the surface, a new and smaller window is etched in it and some donor
material is diffused into the p-type region, converting its surface layer back
into n-type material. This final diffusion produces the emitter region which
is much more heavily doped than the base region, and is therefore labelled
+
n
. This method of making transistors is called the planar process.
The main idea of the transistor action/operation is that a small voltage
applied between the base and emitter terminals makes the current flow in the
collector circuit, as shown in fig. 5.2. A thicker arrow indicates the path of
most of the current. As the base-emitter voltage is varied, producing a small
I
input current
, much larger currents IE and IC vary in response. The reason
B
that the transistor can be used as an effective amplifier or a switch is that a
I
large output power generated from
I
smaller input power from
.
B
or IC can be controlled by a much
E
emitter, base and collector.
69
B
i
C
Collector
C
Base
n
p
n
i
C
Collector
C
B
i
B
Base
p
n
p
i
B
E
i
E
(a)
(c)
Fig. 5.1. Bipolar transistors:
for an n-p-n transistor; (c) a typical structure of an n-p-n transistor
E
Emitter
i
E
Emitter
(b)
BE
p-type n+-type
n-type
C
(a) representation of a p-n-p transistor; (b) the same
aluminium
contacts
silicon
dioxide
Fig. 5.2. Currents in an n-p-n transistor
In the n-p-n transistor, the emitter is n-type and the base is p-type. The
input voltage V
junction. Hence base current I
is of the polarity which forward biases the base-emitter
BE
varies with base-emitter voltage V
B
as shown
BE
in fig. 5.3(a). This variation is of the same exponential form as the
characteristic of a forward biased p-n junction diode.
The corresponding variation of collector current I
for this configuration
C
is very similar to the graph shown in fig. 5.3(a), but the magnitude of the
current is about 20 times larger. The ratio of collector current to base current
is an important transistor parameter, normally referred to as the common-
emitter current gain , where
II
.
CB
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