Transcript p-type semiconductor

The 11th of FEBRUARY
Teacher:
Altynbekov Ayan
• Resistance-Temperature relationship
of a conductor
• Superconductivity
• Electric current in semiconductors
• Doping semiconductors
• Semiconductor diodes
• Transistors
• Electric current in liquids
• Faraday’s law for electrolysis
• Electric currents in gases
Electric current is formed by the
motion of electrons through a
conductor. If we apply a voltage
across a steel wire and then heat
it, we see that the current
passing through it decreases.
This indicates that the current in
a conductor changes with
temperature.
Let the resistance of a conductor at an
initial temperature, t 0 be R 0 and at a final
temperature, t be R. The relationship
between these resistance and temperature
is given by
𝐑 = 𝐑𝟎 𝟏 + 𝛂 𝐭 − 𝐭𝟎
𝐑 = 𝐑𝟎 𝟏 + 𝛂 𝐭 − 𝐭𝟎
Where the proportionality constant, α, is
called the temperature coefficient of the
substance of which the resistor is made.
A light bulb whose filament is constructed of
tungsten, has a resistance of 240Ω, and emits
white light (approximately 1800℃). Find the
approximate resistance of the bulb at room
temperature 20℃ . (Take the temperature
coefficient of resistance for tungsten to be
4,5 ∙ 10−3 ℃ −1 )
Solution
R = R 0 1 + α∆t
R
R0 =
1 + α∆t
240Ω
R0 =
1 + 4,5 ∙ 10−3 ℃−1 1800 − 20 ℃
𝐑 𝟎 = 𝟐𝟔, 𝟔𝟒𝛀
In 1911 a Dutch physicist H.K. Onnes discovered
the phenomenon of superconductivity when he
cooled mercury and observed that its resistance
had decreased to almost zero. When the
temperature was decreased further, at about 4,2K,
the resistance dropped suddenly to zero. This
temperature is called the transition temperature,
Tc .
Semiconductors are materials whose electrical
properties are in between conductors and
insulators. Semiconductors are the basis of
electronics. Silicon (Si) and germanium (Ge)
are the most commonly used semiconductors
in electronic circuits. The resistivity of a
semiconductor decreases quickly as its
temperature increases. Thus they behave just
like an insulator at very low temperatures.
A semiconductor contains four valance
electrons in its outermost shell, their
crystalline structure is shown in figure.
When energy is transferred to a semiconductor
(when it is heated), some valance electrons
begin to move from one atom to a neighbor
atom, like free electrons moving in a metal.
Since the kinetic energy of electrons increase,
they can break their bond with the nucleus.
Thus, when an electric field is applied, an
electric current is formed.
When an electron escapes from its own atom and
moves toward another atom it leaves a hole
(space) behind for an electron which comes from
another neighbor atom, this process continues.
Whenever an electron leaves, it creates a new
hole, this hole (deficiency of electron in the site)
can be thought of as a positive charge, +e, and
acts as a charge carrier.
Thus, as the negative electrons move in one
direction, the positive holes move in the opposite
direction through the semiconductor. In a pure
semiconductor there are equal numbers of
electrons and holes, so it is neutral. The holes
(because they are positive) move in the direction
of the electric field and the electrons move in the
opposite direction to the field.
This electron – hole conduction of current in a
semiconductor can be represented as in figure.
When a semiconductor is heated slightly an
electron is freed, and can break its bond with the
nucleus, leaving a hole behind.
When an electric field is applied, free electrons
begin to move in the opposite direction to the
field. At the same time the hole is filled with
another electron from a nearby atom.
An electron current is produced in the opposite
direction to the electric field. Positive charges
(holes) appear to be flowing in the direction of
the electric field.
The conductivity of semiconductors at room
temperature is not good enough, as it contains a
very small number of free electrons.
When an impurity is added to a semiconductor,
its conductivity increases and it exhibits very
useful properties in electronic circuits.
Adding an impurity to a semiconductor is called
doping the semiconductor.
According to the type of impurity used, two
types of doped semiconductor can be
constructed. If the impurity added to the
semiconductor, is an element with five valance
electrons (such as arsenic), only four electrons
from the arsenic atom will fit into the structure.
The fifth electron cannot fit and thus, moves
freely, just like the free electrons in a metal.
When a semiconductor is doped by an element
having five valance electrons the semiconductor
is called an n-type semiconductor, since the
current is carried by free electrons (negative
charges). The impurity itself is called a donor
atom.
We can add another type of impurity having three
valance electrons, such as gallium or aluminum,
this is called an acceptor atom.
The three electrons of an acceptor atom
(aluminum) cannot fit into the structure. There is
a hole near the impurity, which will be filled by
an electron from a nearby silicon atom, which
again leaves behind a hole. This hole will be
filled by another electron from a neighboring
silicon atom, and so on. In this type of
semiconductor doping, an extra hole is created
and the current is carried by these positive holes.
A semiconductor which has been doped by an
acceptor atom is called a p-type semiconductor.
Note that both n-type and p-type semiconductors
are overall neutral, since each atom in the
semiconductor contains equal numbers of
electrons and protons.
An important application of p-type and n-type
semiconductors are diodes. A diode is an
electronic device that allows electric current to
pass only in one direction. Let’s have a look at
how it accomplishes this.
When a p-type semiconductor and an n-type
semiconductor are connected, a p-n junction
diode is formed. Remember, that both p-type and
n-type semiconductors are neutral.
Electrons in the n-type, near the junction diffuse
into the holes of the p-type semiconductor. The
n-type semiconductor becomes positively
charged, and the p-type semiconductor becomes
negatively charged.
Thus an electric field is produced in a direction
from the n-type semiconductor to the p-type
semiconductor, preventing further electron
movement.
When a voltage is applied across a diode with its
positive terminal connected to the p-type
material and its negative terminal connected to
the n-type material, the connection is called
forward biased.
The external potential difference is opposite to
the direction of the potential difference formed in
the junction. The positive holes in the p-type
material are repelled by the positive terminal of
the battery. So a current flows through the diode
in a direction from p-type material to n-type
material.
A transistor is formed by three semiconductors;
two of the same type of semiconductor material,
have a semiconductor material of the opposite
type sandwiched between them. There are two
types of transistors: pnp and npn.
The three outer regions of a transistor are called
the collector, base and emitter.
Experiments shows that pure water does not
conduct electric current. This means, there are no
charge carries in pure water. Table salt (sodium
chloride) doesn’t conduct electric current either.
However, when we add some table salt to pure
water, the salt molecules split into Na+ and Cl+
ions. If a battery is applied across this solution,
the ions begin to move under the influence of the
electric field produced by the battery, towards the
electrodes A and C.
The solutions in which electric currents can flow
are called electrolytes. Salty water, sugary water
or solutions of silver nitrate, AgNO3 , are some
common electrolytes. The conducting rods
submerged into an electrolyte (rods A and C) are
called electrodes.
The electrode connected to the positive terminal
of the battery is called the anode and the other
electrode is called cathode. This process of
charge flow through an electrolyte is known as
electrolysis.
Consider two electrodes of different metals, such
as silver and copper submerged in water and
connected to the terminals of a battery.
When some silver nitrate (AgNO3 ) is added into
the water, the Ag + and NO− 3 ions dissolve in the
water. The Ag + ions begin to move to the
cathode (which is made of copper) and accepts
one electron from the cathode. As a result, it
becomes neutral. Contrary to this, the NO− 3 ions
move to the anode and are neutralized there. An
electric current flows and the copper electrode is
plated by the silver atoms.
This is called electroplating in production
technology, and is used to protect metals from
oxidation or for decoration.
Michael Faraday developed an equation showing
the relationship between the amount of substance
which is decomposed at the anode or deposited at
the cathode and the current passing through the
electrolyte.
𝐦 = 𝐤∆𝐪 = 𝐤𝐈𝐭
where k is the electrochemical constant of the
element.
Determine the time required to obtain 6 kg of
copper in an electrolysis experiment, when
passing a current of 100 A through the
electrolyte.
−7 kg
(Take k = 3,3 ∙ 10
A∙s)
Solution
According to Faraday’s law of electrolysis, we
can write
m = kIt
m
6kg
t= =
kg
kI
−7
3,3 ∙ 10
A∙s
t = 50 hours
100A
Gases are insulators. They do not conduct
electric current because they do not contain ions
under normal conditions. A gas can become a
conductor when its atoms become positively or
negatively charged ions in a given process, for
example by heating.
As we heat the gas, the kinetic energy of the
atoms increases, when the kinetic energy of an
atom exceeds the binding energy with its
electrons, they begin to ionize. If the temperature
of the gas is large enough, all atoms ionize and
the gas becomes a mixture of ions and electrons.
This state of a substance is called a plasma. A
substance in a plasma state is a good conductor
of electric current.
Gas atoms can also be ionized by the effects of
electromagnetic radiation as indicated in figure.
Ionization by an external source, such as heating
of electromagnetic radiation is called the
discharge of a gas by external ionizers.
An electric current can flow through a gas
without employing an external ionizer. This
occurs when the electric field inside the gas is
very high, due to the presence of a high voltage.
An electron is accelerated by this electric field
and gains a high enough kinetic energy over a
small distance, that when it strikes an atom, it
knocks out an electron and the atom becomes an
ion.
These two electrons then have enough kinetic
energy to strike other atoms, ionize them also,
and so on. The number of electrons and ions
increase in the gas and an electric current begins
to flow through the gas. This kind of ionization is
called discharge by self-ionizers.
A nickel wire of cross-section area, A = 3mm2
has a resistance of R = 30Ω at a temperature of
t = 900℃. What is the length of the wire?
Copper can be deposited from a copper sulphate
solution ( CuSO4 ) when a voltage of 10V is
applied across it. Find the energy required to
mg
deposit 1 kg of copper. (Take k = 0,329
C)
Find the magnitude of the electric field across a
gas if electrons accelerating over a distance, d =
0,5μm can ionize gas atoms with an ionization
energy of E = 2,4 ∙ 10−18 J.
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Electricity and Magnetism, Zambak publishing,
Ahmet Aki, Salim Gur