This document provides an overview of device physics, including: - Semiconductors have electrical properties between metals and insulators, with conductivity altered by doping. - Semiconductors form crystal lattices where atoms share valence electrons via covalent bonds. - N-type materials have excess electrons as majority carriers, while P-type have excess holes. - Semiconductors have valence and conduction bands separated by a band gap. - Intrinsic semiconductors are pure, while extrinsic are doped to increase carriers via donors or acceptors. - PN junctions form at the interface of P and N materials, creating a depletion region.

1. DEVICE PHYSICS

2. Introduction
 Semiconductors are materials whose electronic properties are intermediate
between those of Metals and Insulators.
 They have conductivities in the range of 10^-4 to 10^+4 ohm^−1 m^−1.
 Their conductivity and other properties can be altered with the introduction
of impurities, called Doping.
 It has negative temperature co-efficient of resistance which means the
resistance of a semiconductor decreases with the increase in temperature and
vice-versa.
 When a suitable metallic impurity is added to a semiconductor, its current
conducting property change appreciably.

3. Crystal Lattice Structure
 The unique capability of semiconductor atoms is their
ability to link together to form a physical structure
called a crystal lattice.
 The atoms link together with one another sharing their
valence electrons.
 These links are called covalent bonds.

4. Covalent bonds

5. Majority and Minority Carriers
 N-type material has a large number of free electrons and a small
number of holes in which case free electrons are considered majority
carriers and holes are the minority carriers .
 P-type material has a large number of holes and a small number of electrons
in which case holes are considered majority carriers and free electrons are
the minority carriers.

6. Majority and Minority Carriers

7. What Are Valence & Conduction
Bands?
 Semiconductors are also classified by their bands: a fully occupied (with
electrons) valence band, and an unoccupied conduction band.
 A valence band contains an electron orbital where electrons can jump
off and move to the conduction band when they become excited.
 A conduction band is defined as that energy band that consists of free
electrons that are responsible for conduction.

8. BAND GAP

9. FERMI ENERGY & FERMI LEVEL
 Fermi Level is the maximum energy level that an electron could reach at zero
temperature.
 The Fermi level lies between the valence band and conduction band because
at absolute zero temperature, the electrons are all in the lowest energy state.
 Fermi Energy is the maximum kinetic energy an electron can attain at 0K.

10. Types of Semiconductors

11. What is an Intrinsic Semiconductor?
 A semiconductor material in its pure form is known as an intrinsic
semiconductor.
 The intrinsic semiconductors are chemically pure, i.e. they are free from
impurities.
 The number of free electrons is equal to the number of holes in the
intrinsic semiconductor.
 The common examples of the intrinsic semiconductors are germanium
(Ge) and silicon (Si).

12. What is an Extrinsic Semiconductor?
 A semiconductor to which an impurity at a controlled rate is added to
make it conductive is known as an extrinsic semiconductor.
 The process by which an impurity is added to a semiconductor is known
as Doping.
 The purpose of adding impurity in the semiconductor crystal is to
increase the number of free electrons or holes to make it conductive.
 If a Pentavalent impurity, having five valence electrons is added to a
pure semiconductor a large number of free electrons will exist.
 If a trivalent impurity having three valence electrons is added, a large
number of holes will exist in the semiconductor.
 Depending upon the type of impurity added the extrinsic semiconductor
may be classified as n type semiconductor and p type
semiconductor.

13. N-type semiconductor
 An N-type semiconductor is formed when a small amount of
pentavalent impurity is added to a pure germanium or silicon crystal.
 The pentavalent atom has 5 valance electrons, but only 4 form covalent
bonds with the neighbouring atoms. The 5th electron finds no place in
the covalent bonding so becomes free.
 The majority carrier in N-type semiconductor are free electrons. Holes
are also present in the N-type semiconductor as Minority Carriers.
 The pentavalent impurity
atom is called donor
because each donates
a free electron.

14. P-type semiconductor
 In p-type semiconductors, holes are the majority carriers, and electrons
are the minority carriers.
 A p-type semiconductor is formed by adding a III group element(such as B, Al)
as a doping element.
 The impurity atom is surrounded by four silicon atoms. It provides the atoms to
make only three covalent bonds as it has only three valence electrons. The
vacancy that exists in the fourth bond constitutes the hole.
 Trivalent impurities are called the acceptor impurities
 Because each hole it contributes can accept a free
electron during recombination.

15. Law of Mass Action
 The law of Mass action asserts that at a constant temperature, the product of
the number of electrons in the conduction band and the number of holes in the
valence band remains constant.
 It is expressed Mathematically as
ni
2 =nxp
 Where ni is the intrinsic carrier concentration
 n is number of electrons in the conduction band
 p is number of holes in the valence band.

16. DRIFT CURRENT
 Drift current arises from the movement of carriers in response to an applied
electric field.
 Positive carriers (holes) move in the same direction as the electric field while
negative carriers (electrons) move in the opposite direction.
 The drift velocity increases with increasing electric field and contributes
to the mobility μ of the carriers.

17. DIFFUSION CURRENT
 The movement of charge carriers from higher concentration to lower
concentration generates diffusion current.
 This occurs when a semiconductor is doped non-uniformly then there is a non-
uniform distribution of carriers or a concentration gradient.
 The current moves to the direction where there is initially a higher concentration
of electrons or a lower concentration of holes.

18. Effect of temperature on semiconductor
parameters
Intrinsic concentration (ni) :
 The number of holes or electrons present in an intrinsic semiconductor
at any temperature is called intrinsic carrier concentration (ni).
 In N type semiconductor, the number of free electrons (n) does not
change appreciably with the increase in temperature, but number of
holes (p) increases.
 In P type semiconductor, the number of free electrons (n) increases
with the increase in temperature, but number of holes remains constant.

19. Mobility (µ) :
 The mobility means the movement of charge carriers.
 The mobility of intrinsic semiconductor decreases with increase in temperature.
 Due to the higher collision rate,their effective mobility of the charge carriers
decreases.
Conductivity (σ) :
 The conductivity of an intrinsic semiconductor depends upon the number of
hole electron pairs and mobility.
 The conductivity of an intrinsic semiconductor increases with increase in
temperature.
Forbidden energy gap (EG) :
 The energy required to break a covalent bond in a semiconductor is known as
energy gap.
 The forbidden energy gap decreases with the increase in temperature.

20. Semiconductors
Pn Junction :
 The P-N junction is formed between the p-type and the n-type
semiconductors.
 The p-side or the positive side of the semiconductor has an excess of holes,
and the n-side or the negative side has an excess of electrons.
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21. Semiconductors
Depletion Layer:
 The free electrons of n region begin to diffuse across the junction into the p region
where they combine with the holes near the junction.
 The p region also loses holes as the electrons and holes combine.
 The result is that there are layers of positive charge (pentavalent ions) and
negative charge (trivalent ions).
 These two layers form depletion layer or depletion region
 This layer is free of charge carriers
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22. Semiconductors
 Depletion Layer
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23. BJT(BIPOLAR JUNCTION TRANSISTOR)
 BJT stands for Bipolar Junction Transistor which is a type of transistor
that uses both electrons and holes as charge carriers.
 It’s a 3 terminal device.
 Used in amplification of weak signals and switching operations.
 Plays a vital role in both Analog and digital Electronics.

24. History of its Invention.
 W. Shockley, J. Barden, and W. Brattain invented the first transistor at
Bell Labs in 1948.
 The Inventors received the nobel Prize in physics in 1956.

25. Physical structure of BJT.
 Two types of BJT 1.NPN and 2.PNP.

26. Physical structure of BJT.

27. Working of the Transistor
 The emitter-base junction of a transistor is forward biased whereas collector
base junction is reverse biased.
 At BE junction, the potential barrier decreases with forward bias. So, electron
start flowing from emitter terminal to base terminal.
 As the base is lightly doped terminal, so very little number of electrons from
emitter terminal combine with holes in base terminal.
 Due to combination of electrons and holes, current from base terminal will start
flowing known as Base current (ib).
 While the remaining electrons will flow from the reverse bias collector junction
known as Collector current (ic).
 The total emitter current will be the combination of base current & collector
current given by: ie = ib+ic

32. OPERATING REGIONS

33. Applications of BJT
 Used as an Amplifiers.
 Converters.
 Temperatures sensors.
 Electronic switches.

34. THANKYOU