Basic Terms used in Semiconductor Physics
Atoms
Atom is the smallest unit of matter that is
composed of a positively charged
centre nucleus surrounded by
negatively charged electrons. Atom does not exist in free state.
all
atoms are composed of these three subatomic particles
Electron
Electron is
fundamental and constituent particle of atom. An electron is represented by “e” and it carries a
negative charge of 1.6 ×10-19 coulombs. Mass of an electron is 9.11 ×´ 10-31 kg.
Hole
The absence of an
electron in a particular place in an atom of a material is called a hole. When a covalent bond is broken, an electron vacates
an energy level in the valence band.The vacancy may be treated as a particle. It is
called a hole. A hole passes from
atom to atom in a semiconductor material.
Charge
Charge is an
intrinsic or basic property of an elementary or subatomic particles due to
which it produces electric and magnetic effects such as attraction, repulsion,
electromagnetic induction current etc. charge is carried by electrons and
protons in equal amount opposite in nature/character.
S.I. unit : unit of electric charge is coulomb in S.I. system.
Charge on
electron = - 1.6 x 10-19 Coulomb
Charge on
proton = +1.6 x 10-19 Coulomb | Charge on
electron | = | charge on proton | = 1.6 x 10-19 Coulomb
Electric
Field
the space
around the charge in which electrostatic force of attraction or repulsion due
to the charge can be experienced by any other charge is called Electric Field.
Figure : electric field lines around a point charges
Electric
Field Intensity
Electric filed intensity at any
point due to source charge is defined as
the force experienced by unit positive test charge placed at that point.
Mathematically, Electric filed intensity is expressed by
Electric dipole
An electric
dipole is system of equal and opposite point charges q and –q, separated by
a distance of 2a. Direction from –q to q is the direction of the dipole. The
mid-point of locations of –q and q is called the center of the dipole.
example: molecule of electrolytic compounds, HCl, H2O.
dipole moment P=q x 2a
Electric Potential
Electric Potential is work done in bringing a unit
positive charge from infinity up to the point in an electric field.
Electron
Volt
electron volt (eV) is the energy acquired by an electron when an electron is accelerated through a potential difference of
1 V. In equation form,
1 eV
= 1.60×10–19 C ✖
1 V
or 1 eV = 1.60×10–19
J
Mobility μ
The drift velocity of electron per unit electric
field applied is called mobility of electron.
Its SI unit is m2s-1V-1.
Electrons
have greater mobility than holes. For this reason, devices with n-type
conduction are mostly preferred to those with p-type conduction.
Drift Velocity vd
Drift velocity is defined as the velocity with which the
free electrons get drifted towards the positive terminal under the effect of the
applied electric field.
Drift
velocity vd is proportional to the electric field strength E.
vd ∝ E
or,
vd = μ E
Drift coefficient is called mobility represented by μ .
Mobility for electron and hole respectively are represented by μe & μh .
drift velocity for electron and hole are represented by Ve and Vh.
Ve = - μe E and
Vh = μh E
Electrical Conductivity 𝝈
The
reciprocal of resistivity is called electrical conductivity. Electrical
conductivity
σ
= 1 / ρ = l / RA = ne2 τ / m
Its
SI units is ohm-1 m-1 or mho m-1 or siemen m-1.
Resistivity 𝜌
The resistivity of a material is the resistance
of a wire of that material of unit length and unit cross-sectional area.
Where ρ (rho) is the proportionality constant. It is
called the electrical resistivity of the material of conductors. The SI unit of
resistivity is the ohm-metre, symbol Ω m.
The resistivity of a material depends on its nature and the temperature of the
conductor, but not on its shape and size.
A good conductor has less resistivity, whereas a bad conductor or insulator has
high resistivity.
Resistivity of materials varies with temperature.
Ohm’s
Law
Ohm’s
Law states that the current flowing through a conductor is directly
proportional to the potential difference applied across its ends, provided the
temperature and other physical conditions remain the same. Mathematically it can be
represented as,
Potential difference ∝ Current
V ∝ I
When the value of V
increases, the value of I increases simultaneously
V = IR
Where,
V
is Voltage in volts (V)
R
is Resistance in ohm (Ω)
I is Current in Ampere (A)
resistance
The hindrance or obstruction offered by any conductor in
the path of flow of current is called its electrical resistance. Resistance is a
property of a material or device. Its SI unit is ohm (Ω)
Electrical resistance, R = V / I
The ratio of voltage to current is also called the resistance of the
material.
1 Ω = 1 V/1A
Resistance in a conductor depends on nature of material, length and area of cross
section of the conductor.
Ohmic Conductors
Those conductors which obey Ohm’s law, are called
ohmic conductors e.g., all metallic conductors are ohmic conductor.
Non-ohmic Conductors
Those conductors which do not obey Ohm’s law, are
called non-ohmic conductors. e.g., diode valve, triode valve, transistor ,
vacuum tubes etc.
Current density J
It
is defined as the current per unit area of cross section of a plane held normal
to the direction of the flow of current in a current carrying conductor.
Its S1 unit is ampere metre-2. It is a
vector quantity.
Relation between
current density (J) and electrical conductivity (σ)
Relation
between current density (J) and electrical conductivity (σ) is given by
J
= σ E
where,
E = electric field intensity.
Electric
Current
The electric current is defined as the charge flowing
through any cross section (perpendicularly) of the conductor in one second.The
electric charge flows from higher potential energy state to lower potential
energy state.
i.e. 1
ampere = 1coulomb/ 1 second
Current is a scalar quantity with its SI unit ampere.
Direction of flow of positive chargeis taken as the
direction of current.
If the current is not varying with time , then the
current is called steady electric or
stationary current.
Ampere
The
current through a conductor is said to be one ampere if one coulomb of charge
is flowing per second through a cross–section of wire.
Types Of Current
The electric current can be classified into the following
categories:
(i) Steady current:- The current
whose magnitude does not change with time is called Steady current. The
variations between current(I) and time(t) for a steady current will be a
straight line.
(ii) Varying current:– The
current whose magnitude changes with time is called varying current. T
(iii) Alternating current:-The
current whose magnitude changes continuously with time and direction changes
periodically is called alternating current. Such a current is represented by a
sine curve or cosine curve .
Charge carries in metal, semiconductor and electrolytes
- In
metal free electrons are the charge carries
- In
semiconductor free electrons and holes
- In
electrolytes and gases +ve and -ve ions
emf
The energy given by a cell in flowing unit positive
charge throughout the circuit completely one time, is equal to the emf of a
cell. Emf is defined as the potential difference between electrodes when there
is no current in the cell(cell is in open circuit). Emf of the cell initiates
the flow of current in the cell.Emf stands for Electro motive Forceof a Cell.
Emf of a cell (E) = W / q.
Its SI unit is volt.
Define mean free path
The
average distance travelled by an electron between two successive collisions
is called
mean free path.
