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
- Proton
- Neutron
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.
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.
Figure : Electric 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.
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.
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