UNIT 1
SEMICONDUCTORS AND RECTIFIERS
Introduction
Because of the very large number of atoms that interact in a solid material,
the energy levels are so closely spaced that they form bands. The highest
energy filled band, which is analogous to the highest occupied molecular
orbital in a molecule (HOMO), is called the valence band. The next higher
band, which is analogous to the lowest unoccupied molecular orbital
(LUMO) in a molecule, is called the conduction band. The energy
separation between these bands is called the energy gap, Eg.
Idealized
representation
of
energy
bands
and
gaps
The filling of these bands and the size of the energy gap determine if a
material is a conductor (a metal), a semiconductor, or an insulator. In
metals there is no energy gap between filled and unfilled energy levels. A
significant number of electrons are thermally excited into empty levels,
creating holes in the filled band. The electrons in a conduction band and
the holes in a valence band can move throughout the material, allowing it
to easily conduct electricity. In semiconductors Eg is small, but large
enough so that a fairly small number of electrons are in the conduction
band due to thermal energy, and these materials conduct poorly. In
insulators Eg is large so that electrons are not promoted to the conduction
band due to thermal energy, and these materials do not conduct electricity.
Intrinsic semiconductor
An intrinsic semiconductor, also called an un doped semiconductor or i-type semiconductor,
is a pure semiconductor without any significant dopant species present. The number
of charge carriers is therefore determined by the properties of the material itself instead of
the amount of impurities. In intrinsic semiconductors the number of excited electrons and
the number of holes are equal: n = p. The electrical conductivity of intrinsic semiconductors
can be due to crystal defects or to thermal excitation. In an intrinsic semiconductor the
number of electrons in the conduction band is equal to the number of holes in the valence
band. An example is Hg0.8Cd0.2Te at room temperature. An indirect band gap intrinsic
semiconductor is one where the maximum energy of the valence band occurs at a different
k (k-space wave vector) than the minimum energy of the conduction band. Examples
include silicon and germanium. A direct band gap intrinsic semiconductor is one where the
maximum energy of the valence band occurs at the same k as the minimum energy of the
conduction band. Examples include gallium arsenide.
A silicon crystal is different from an insulator because at any temperature above absolute
zero temperature, there is a finite probability that an electron in the lattice will be knocked
loose from its position, leaving behind an electron deficiency called a "hole".
If a voltage is applied, then both the electron and the hole can contribute to a small current
flow.
The conductivity of a semiconductor can be modeled in terms of the band theory of solids.
The band model of a semiconductor suggests that at ordinary temperatures there is a finite
possibility that electrons can reach the conduction band and contribute to electrical
conduction.
The term intrinsic here distinguishes between the properties of pure "intrinsic" silicon and
the dramatically different properties of doped n-type or p-type semiconductors.
Extrinsic semiconductor
An extrinsic semiconductor is a semiconductor that has been doped, that is, into which
a doping agent has been introduced, giving it different electrical properties than the intrinsic
(pure) semiconductor.
Doping involves adding dopant atoms to an intrinsic semiconductor, which changes
the electron and hole carrier concentrations of the semiconductor at thermal equilibrium.
Dominant carrier concentrations in an extrinsic semiconductor classify it as either an ntype or p-type semiconductor. The electrical properties of extrinsic semiconductors make
them essential components of many electronic devices
The two types of extrinsic semiconductor
N-type semiconductor
Band
structure of an n-type semiconductor. Dark circles in the
conduction band are electrons and light circles in the valence band are
holes. The image shows that the electrons are the majority charge carrier.
Extrinsic semiconductors with a larger electron concentration than hole
concentration are known as n-type semiconductors. The phrase 'n-type'
comes from the negative charge of the electron. In n-type semiconductors,
electrons are the majority carriers and holes are the minority carriers. Ntype semiconductors are created by doping an intrinsic semiconductor with
donor impurities. In an n-type semiconductor, the Fermi energy level is
greater than that of the intrinsic semiconductor and lies closer to
the conduction band than the valence band.
P-type semiconductor
Band structure of a p-type semiconductor. Dark circles in the conduction
band are electrons and light circles in the valence band are holes. The
image shows that the holes are the majority charge carrier As opposed to
n-type
semiconductors, p-type
semiconductors have
a
larger
hole
concentration than electron concentration. The phrase 'p-type' refers to
the positive charge of the hole. In p-type semiconductors, holes are the
majority
carriers
and
electrons
are
the
minority
carriers.
P-type
semiconductors are created by doping an intrinsic semiconductor with
acceptor impurities. P-type semiconductors have Fermi energy levels
below the intrinsic Fermi energy level. The Fermi energy level lies closer to
the valence band than the conduction band in a p-type semiconductor.
Properties of a p-n junction
The p–n junction possesses some interesting properties which have useful applications in
modern electronics. A p-doped semiconductor is relatively conductive. The same is true of
an n-doped semiconductor, but the junction between them can become depleted of charge
carriers, and hence nonconductive, depending on the relative voltages of the two
semiconductor regions. By manipulating this non-conductive layer, p–n junctions are
commonly used as diodes: circuit elements that allow a flow of electricity in one direction
but not in the other (opposite) direction. This property is explained in terms of forward
bias and reverse bias, where the term bias refers to an application of electric voltageto the
p–n junction.
Zener diode
A Zener diode is a type of diode that permits current not only in the forward direction like a
normal diode, but also in the reverse direction if the voltage is larger than the breakdown
voltageknown as "Zener knee voltage" or "Zener voltage". The device was named
after Clarence Zener, who discovered this electrical property. A conventional solidstate diode will not allow significant current if it is reverse-biased below its reverse
breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional
diode is subject to high current due to avalanche breakdown. Unless this current is limited
by circuitry, the diode will be permanently damaged. In case of large forward bias (current
in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in
voltage and internal resistance. The amount of the voltage drop depends on the
semiconductor material and the doping concentrations.
Uses
Zener diodes are widely used as voltage references and as shunt
regulators to regulate the voltage across small circuits. When connected in
parallel with a variable voltage source so that it is reverse biased, a Zener
diode conducts when the voltage reaches the diode's reverse breakdown
voltage. From that point on, the relatively low impedance of the diode
keeps the voltage across the diode at that value. In this circuit, a typical
voltage reference or regulator, an input voltage, UIN, is regulated down to
a stable output voltage UOUT. The intrinsic voltage drop of diode D is
stable over a wide current range and holds UOUT relatively constant even
though the input voltage may fluctuate over a fairly wide range. Because of
the low impedance of the diode when operated like this, Resistor R is used
to limit current through the circuit.
In the case of this simple reference, the current flowing in the diode is determined using
Ohms law and the known voltage drop across the resistor R. IDiode = (UIN - UOUT) / RΩ
The value of R must satisfy two conditions:
1. R must be small enough that the current through D keeps D in
reverse breakdown. The value of this current is given in the data
sheet for D. For example, the common BZX79C5V6[2] device, a 5.6 V
0.5 W Zener diode, has a recommended reverse current of 5 mA. If
insufficient current exists through D, then UOUT will be unregulated,
and less than the nominal breakdown voltage (this differs to voltage
regulator tubes where the output voltage will be higher than
nominal and could rise as high as UIN). When calculating R,
allowance must be made for any current through the external load,
not shown in this diagram, connected across UOUT.
2. R must be large enough that the current through D does not destroy
the
device.
If
the
voltage VBand
its
current
through
maximum
D
is ID,
power
its
breakdown
dissipation PMAX,
then IDVB < PMAX.
A load may be placed across the diode in this reference circuit, and as long as the zener
stays in reverse breakdown, the diode will provide a stable voltage source to the load.
rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically
reverses direction, to direct current (DC), which is in only one direction, a process known
as rectification. Rectifiers have many uses including as components of power supplies and
as
detectors of radio signals.
Rectifiers
may
be
made
of solid
state diodes, vacuum
tube diodes, mercury arc valves, and other components.
Half-wave rectification
In half wave rectification, either the positive or negative half of the AC wave is passed,
while the other half is blocked. Because only one half of the input waveform reaches the
output, it is very inefficient if used for power transfer. Half-wave rectification can be
achieved with a single diode in a one-phase supply, or with three diodes in a three-phase
supply.
The output DC voltage of a half wave rectifier can be calculated with the following two ideal
equations
full-wave rectifier
A full-wave rectifier converts the whole of the input waveform to one of constant polarity
(positive or negative) at its output. Full-wave rectification converts both polarities of the
input waveform to DC (direct current), and is more efficient. However, in a circuit with a
non-center tapped transformer, four diodes are required instead of the one needed for halfwave rectification. (See semiconductors, diode). Four diodes arranged this way are called
a diode bridge or bridge rectifier:
UNIT II