THE ATOM

Structure of an atom: four protons, four neutrons and four electrons.

Atoms are the basic units of matter and the defining structure of elements. They are the smallest
particles of an element. Atoms are made up of three particles: protons, neutrons and electrons.

Protons and neutrons are heavier than electrons and reside in the center of the atom, which is
called the nucleus. Electrons are extremely lightweight and exist in a cloud orbiting the nucleus.
The electron cloud has a radius 10,000 times greater than the nucleus.

Protons and neutrons have approximately the same mass. However, one proton weighs more than
1,800 electrons. Atoms always have an equal number of protons and electrons, and the number
of protons and neutrons is usually the same as well. Adding a proton to an atom makes a new
element, while adding a neutron makes an isotope, or heavier version, of that atom.

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Nucleus

The nucleus was discovered in 1911, but its parts were not identified until 1932. Virtually all the
mass of the atom resides in the nucleus. The nucleus is held together by the "strong force," one
of the four basic forces in nature. This force between the protons and neutrons overcomes the
repulsive electrical force that would, according to the rules of electricity, push the protons apart
otherwise.

Protons

Protons are positively charged particles found within atomic nuclei. They were discovered by
Ernest Rutherford in experiments conducted between 1911 and 1919.

The number of protons in an atom defines what element it is. For example, carbon atoms have
six protons, hydrogen atoms have one and oxygen atoms have weight. The number of protons in
an atom is referred to as the atomic number of that element. The number of protons in an atom
also determines the chemical behavior of the element. The Periodic Table of the Elements
arranges elements in order of increasing atomic number.

Protons are made of other particles called quarks. There are three quarks in each proton — two
"up" quarks and one "down" quark — and they are held together by other particles called gluons.

Electrons

Electrons have a negative charge and are electrically attracted to the positively charged protons.
Electrons surround the atomic nucleus in pathways called orbitals. The inner orbitals surrounding
the atom are spherical but the outer orbitals are much more complicated.

An atom's electron configuration is the orbital description of the locations of the electrons in an
unexcited atom. Using the electron configuration and principles of physics, chemists can predict
an atom's properties, such as stability, boiling point and conductivity.

Typically, only the outermost electron shells matter in chemistry. The inner electron shell
notation is often truncated by replacing the long-hand orbital description with the symbol for a
noble gas in brackets. This method of notation vastly simplifies the description for large
molecules.

For example, the electron configuration for beryllium (Be) is 1s22s2, but it's is written [He]2s2.
[He] is equivalent to all the electron orbitals in a helium atom. The Letters, s, p, d, and f
designate the shape of the orbitals and the superscript gives the number of electrons in that
orbital.

Neutrons

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Neutrons are uncharged particles found within atomic nuclei. A neutron's mass is slightly larger
than that of a proton. Like protons, neutrons are also made of quarks — one "up" quark and two
"down" quarks. Neutrons were discovered by James Chadwick in 1932.

Isotopes

The number of neutrons in a nucleus determines the isotope of that element. For example,
hydrogen has three known isotopes: protium, deuterium and tritium. Protium, symbolized as 1H,
is just ordinary hydrogen; it has one proton and one electron and no neutrons. Deuterium (D or
2
H) has one proton, one electron and one neutron. Tritium (T or 3H) has one proton, one electron
and two neutrons.

What are Semiconductors?

Semiconductors are materials which have a conductivity between conductors (generally metals)
and nonconductors or insulators (such as most ceramics). Semiconductors can be pure elements,
such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. In a
process called doping, small amounts of impurities are added to pure semiconductors causing
large changes in the conductivity of the material.

Due to their role in the fabrication of electronic devices, semiconductors are an important part of
our lives. Imagine life without electronic devices. There would be no radios, no TV's, no
computers, no video games, and poor medical diagnostic equipment. Although many electronic
devices could be made using vacuum tube technology, the developments in semiconductor
technology during the past 50 years have made electronic devices smaller, faster, and more
reliable.

Semiconductors are materials that have properties in between normal conductors (materials
that allow electric current to pass, e.g. aluminium) and insulators (which block electric current,
e.g. sulphur).

Semiconductors fall into two broad categories:

Intrinsic Semiconductors. These are composed of only one kind of material, Silicon and
germanium. They are also called "undoped semiconductors" or "i-type semiconductors".

Extrinsic semiconductors are made of intrinsic semiconductors that have had other substances
added to them to alter their properties

--

Intrinsic Semiconductors

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Every atom consists of a nucleus surrounded by a number of electrons. Only the electrons are
involved in electronic processes. The electrons can exist only in certain electron shells around
the atom. There are many shells in each atom.

It requires energy to get an electron from a shell close to the nucleus to one further away, and if
an atom's electrons are in a position which is not the position with least energy (i.e. they are in a
higher (further from nucleus) shell and there is space in a lower shell), energy is given up so the
electrons "fall" into the inner shells. Thereby, the shells closest to the nucleus are filled first, and
then the next closest and so on. It requires more energy for an electron in a shell that is close to
its nucleus to fill an outer shell than it is for an electron on an outer shell to fill an inner shell, so
the inner shell is filled first.

We will consider our material to be arranged in a lattice, which is a regular arrangement, like a
crystal. This helps to describe and explain the principles. In the lattice, each electron can "see"
every atom in the entire lattice, and therefore is not just affected by the presence of electrons in
its own atom, but by all the other atoms in the material. The huge number of atoms (usually
greater than one thousand billion billion in a cube 1mm on a side) means that the number of
electrons in each shell of each atom is not important - the shells "merge" into bands. All that
matters is that if that band is filled, partially filled or empty. The size of bands and the gaps
between them is determined by the nature of the material.

Figure 2: Electronic band structure of an insulator or semiconductor.

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Figure 3: Comparison of the band gaps for a metal, a semiconductor and an insulator.

In a lattice, there will be a set of filled bands, with a full complement of electrons and unfilled
bands which have no electrons (because they are in the lower-energy filled bands). The highest
energy band with electrons in it is called the valence band, from- the chemists' term "valence
electrons" which are the electrons on the outermost shell of the atom which are responsible for
chemical reactions. The conduction band is the band above the valence band. Electrons in the
conduction band are free to move about in the lattice, and can therefore conduct current. The
energy gap between the valence and conduction band is called the band gap.

Every material has associated with it a Fermi energy. Imagine the bands "filling up" from the
bottom up, like water poured into a container. The continuous nature of the filling arises from the
fact that there are such are large number of electrons they are essentially infinite in number. This
behavior does not happen in a single atom, as the small number of electrons means that the
amount of energy is heavily quantized. The Fermi energy is the level of the top of the "sea" that
is formed. This is defined at absolute zero, when there is no thermal energy to allow the
electronics to form "ripples" on the sea.

In insulators, the Fermi level lies between the valence and conduction bands, in one of the
"forbidden zones" where electrons cannot exist. Thus all electrons in the lattice are in the valence
band or a band under that. To get to the conduction band, the electron has to gain enough energy
to jump the band gap. Once this is done, it can conduct. However ,the band gap for insulators is
large (over 3 eV) so very few electrons can jump the gap. Therefore, current does not flow easily
in insulators.

In metals, the conduction band and the valence band overlap or the valence band is only partially
full, both with the Fermi energy somewhere inside. This means that the metal always has
electrons that can move freely and so can always carry current.

In semiconductors, the Fermi energy is between the valence and conduction band, but the band
gap is smaller, allowing electrons to jump the gap fairly easily, given the energy to do it. At
absolute zero, semiconductors are perfect insulators, but at room temperature, there is enough
thermal energy to allow occasional electron jumps, given the semiconductor limited
conductivity, even though, by rights, it should be an insulator.

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If there are no electrons in the conduction band of a semi-conductor it won't conduct. To move
electrons out of the valence band and into the conduction band, one needs to give them energy.
This may be through heat, incident light or high electric field. As most semiconductors operate at
non-zero temperature, there are generally some electrons in the conduction band. This also
means that if the semi-conductor gets too hot (125°C for silicon), excess electrons will exist in
the conduction band, hence the semi-conductor will act more like a conductor.

Because intrinsic semiconductors contain no "extra" electrons from impurities like extrinsic
semiconductors do, every time an electron jumps the band gap, it leaves a hole behind. This hole
represents a positive charge as it is the lack of an electron. Intrinsic semiconductors have exactly
equal numbers of holes and electrons, so, where n is the number of electrons and p is the number
of holes,

Direct and Indirect Semiconductors

Figure 4: Band-gap for silicon

The total energy of an electron is given by its momentum and its potential energy. To move an
electron from the conduction band to the valence band, it may need to undergo a change in
potential energy and a change in momentum. There are two basic material types, in-direct and
direct band gap materials. In an indirect band gap material, such as silicon, shown in figure 4, to
move into the valence band, the electron must undergo a change in momentum and energy[1]. The
chance of this event is small. Typically this process is achieved in several steps. The electron will
first move to a trap site in the forbidden band before moving into the valence band. A change in
potential energy will result in the release of a photon, while a change in momentum will produce
a phonon (a phonon being a mechanical vibration which heats the crystal lattice).

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Figure 5: Band-gap for GaAs, a direct semiconductor

In a direct band-gap material such as GaAs, only a change in energy is required, as seen in figure
5. As such GaAs is very efficient at producing light, although in the infrared spectrum.

Extrinsic Semiconductors

One may also dope the semiconductor material. Semi-conductor materials are doped with
impurities chosen to give the material special characteristics. One may want to add extra
electrons or remove electrons.

Figure 6: N-type silicon, doped with phosphorous

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Figure 7: P-type silicon, doped with boron

Doping atoms are chosen from elements in group III or V of the periodic table[1] which are
similar in size to silicon atoms. Thus individual intrinsic semiconductor atoms may be replaced
with dopant atoms to form an extrinsic semi-conductor.

The binding energy of the outer electron added by the impurity is weak. This is represented by
placing the excess electrons just below the conduction band. Thus very little energy is required to
move these electrons into the conduction band. Thus an extrinsic semi-conductor operating at
room temperature will have most of these "extra" electrons existing in the conduction band. Thus
at normal operating temperature,

Where - is the number of conduction electrons and - is the number of dopant

atoms.

. P-Type, N-Type Semiconductors

P-N Junction Diodes are made up of two adjacent pieces of p-type and n-type semiconducting
materials. p-type and n-type materials are simply semiconductors, such as silicon (Si) or
germanium (Ge), with atomic impurities; the type of impurity present determines the type of the
semiconductor. The process of purposefully adding impurities to materials is called doping;
semiconductors with impurities are referred to as "doped semiconductors".

P-TYPE

In a pure (intrinsic) Si or Ge semiconductor, each nucleus uses its four valence electrons to form
four covalent bonds with its neighbors (see figure below). Each ionic core, consisting of the
nucleus and non-valent electrons, has a net charge of +4, and is surrounded by 4 valence

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electrons. Since there are no excess electrons or holes In this case, the number of electrons and
holes present at any given time will always be equal.

An intrinsic semiconductor. Note each +4 ion is surrounded by four electrons.

Now, if one of the atoms in the semiconductor lattice is replaced by an element with three
valence electrons, such as a Group 3 element like Boron (B) or Gallium (Ga), the electron-hole
balance will be changed. This impurity will only be able to contribute three valence electrons to
the lattice, therefore leaving one excess hole (see figure below). Since holes will "accept" free
electrons, a Group 3 impurity is also called an acceptor.

A semiconductor doped with an acceptor. An excess hole is now present.

Because an acceptor donates excess holes, which are considered to be positively charged, a
semiconductor that has been doped with an acceptor is called a p-type semiconductor; "p" stands

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for positive. Notice that the material as a whole remains electrically neutral. In a p-type
semiconductor, current is largely carried by the holes, which outnumber the free electrons. In this
case, the holes are the majority carriers, while the electrons are the minority carriers.

n-type

In addition to replacing one of the lattice atoms with a Group 3 atom, we can also replace it by
an atom with five valence electrons, such as the Group 5 atoms arsenic (As) or phosphorus (P).
In this case, the impurity adds five valence electrons to the lattice where it can only hold four.
This means that there is now one excess electron in the lattice (see figure below). Because it
donates an electron, a Group 5 impurity is called a donor. Note that the material remains
electrically neutral.

A semiconductor doped with a donor. A free electron is now present.

Donor impurities donate negatively charged electrons to the lattice, so a semiconductor that has
been doped with a donor is called an n-type semiconductor; "n" stands for negative. Free
electrons outnumber holes in an n-type material, so the electrons are the majority carriers and
holes are the minority carriers.

Formation of P-type & N-type materials (Extrinsic semiconductor) To produce extrinsic

semiconductor material specific amounts of impurity are added to the pure intrinsic
semiconductor. This process is called doping and the impurity atoms are called donor atoms.
There are two types of extrinsic semiconductor which are manufactured, P type semiconductor
and N type semiconductor. The production of extrinsic semiconductor will be described for the
more common silicon semiconductor material but the process is identical for germanium.

N -TYPE SEMICONDUCTOR

The pure silicon is doped with a group 5 element such as phosphorus, antimony or arsenic.
These materials have atoms with five valence electrons (pentavalent atoms). Four of these

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electrons will form covalent bonds with neighbouring silicon atoms. As their are only four
covalent bonds binding the donor atom to the neighbouring silicon atoms the fifth electron is not
part of a covalent bond, and is therefore a free electron. Every impurity atom will produce a free
electron in the conduction band. These electrons will drift to produce an electrical current if a
voltage is applied to the material and the N type semiconductor is a much better conductor than
the intrinsic pure silicon material. The silicon atoms form a square lattice The green atoms
represent the donor atoms Four of the five valence electrons form covalent bonds with
neighbouring silicon atoms The fifth electron has no neighbouring electron to pair with and is a
free electron Each donor atom produces a free electron Note it is important to point out that
the material is called N type semiconductor because the majority of charge carriers which will
contribute to an electrical current through the material are negatively charged free electrons
produced by the doping process. There will be some contribution to the current flow from
positively charge holes due to electron hole pair generation but these holes are the minority
charge carriers in this material. The N type material itself is not negatively charged. The negative
charge of the electrons of the donor atoms is balanced by the positive charge in the nucleus.
Energy band diagram The diagram below shows an energy band diagram for N type
semiconductor. The valence band is completely full as all of the covalent bonds are complete.
The conduction band contains free electrons from the fifth valence electrons in the donor atoms.

Note this diagram does not show the electron hole pairs that would be present due to thermal
energy. The electron hole pairs are minority charge carriers in N type semiconductors, the
majority being the free electrons produced by the doping process.

P TYPE SEMICONDUCTOR

The pure silicon is doped with a group 3 element such as boron, aluminium or indium. These
materials have atoms with three valence electrons (trivalent atoms). The three electrons will form
covalent bonds with neighbouring silicon atoms. However there are not enough electrons to form
the fourth covalent bond. This leaves a hole in the covalent bond structure and therefore a hole in
the valence band of the energy level diagram. Every impurity atom will produce a hole in the
valence band. These holes will drift to produce an electrical current if a voltage is applied to the
material and the P type semiconductor is a much better conductor than the intrinsic pure silicon
material. The silicon atoms form a square lattice The green atoms represent the donor atoms
three of the four covalent bonds are formed with neighbouring silicon atoms The fourth bond

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cannot be formed as there are not enough electrons, this leaves a hole in the valence band Each
donor atom produces a hole in the valence band Note it is important to point out that the
material is called P type semiconductor because the majority of charge carriers which contribute
to an electrical current are positively charged holes produced by the doping process. There will
be some contribution to the current flow from negatively charged electrons due to electron hole
pair generation but these electrons are the minority charge carriers in this material. The P type
material itself is not positively charged because the negative charge of the electrons of the donor
atoms are balance by the positive charge in the nucleus. Energy band diagram The diagram
below shows an energy band diagram for P type semiconductor. The valence band contains holes
due to the incomplete covalent bond around each donor atom. The conduction band is empty as
there are no free electrons. Note this diagram does not show the electron hole pairs that would be
present due to thermal energy. The electron hole pairs are minority charge carriers in P type
semiconductors, the majority being the holes produced by the doping process.

Formation of a Diode

If a P-type and an N-type material are brought close to each other, both of them join to form a
junction, as shown in the figure below.

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A P-type material has holes as the majority carriers and an N-type material has electrons as the
majority carriers. As opposite charges attract, few holes in P-type tend to go to n-side, whereas
few electrons in N-type tend to go to P-side.

As both of them travel towards the junction, holes and electrons recombine with each other to
neutralize and forms ions. Now, in this junction, there exists a region where the positive and
negative ions are formed, called as PN junction or junction barrier as shown in the figure.

The formation of negative ions on P-side and positive ions on N-side results in the formation of a
narrow charged region on either side of the PN junction. This region is now free from movable
charge carriers. The ions present here have been stationary and maintain a region of space
between them without any charge carriers.

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As this region acts as a barrier between P and N type materials, this is also called as Barrier
junction. This has another name called as Depletion region meaning it depletes both the
regions. There occurs a potential difference VD due to the formation of ions, across the junction
called as Potential Barrier as it prevents further movement of holes and electrons through the
junction.

Biasing of a Diode
When a diode or any two-terminal component is connected in a circuit, it has two biased
conditions with the given supply. They are Forward biased condition and Reverse biased
condition. Let us know them in detail.

Forward Biased Condition

When a diode is connected in a circuit, with its anode to the positive terminal and cathode to
the negative terminal of the supply, then such a connection is said to be forward biased
condition. This kind of connection makes the circuit more and more forward biased and helps in
more conduction. A diode conducts well in forward biased condition.

Reverse Biased Condition

When a diode is connected in a circuit, with its anode to the negative terminal and cathode to
the positive terminal of the supply, then such a connection is said to be Reverse biased
condition. This kind of connection makes the circuit more and more reverse biased and helps in
minimizing and preventing the conduction. A diode cannot conduct in reverse biased condition.

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Let us now try to know what happens if a diode is connected in forward biased and in reverse
biased conditions.

Working under Forward Biased

When an external voltage is applied to a diode such that it cancels the potential barrier and
permits the flow of current is called as forward bias. When anode and cathode are connected to
positive and negative terminals respectively, the holes in P-type and electrons in N-type tend to
move across the junction, breaking the barrier. There exists a free flow of current with this,
almost eliminating the barrier.

With the repulsive force provided by positive terminal to holes and by negative terminal to
electrons, the recombination takes place in the junction. The supply voltage should be such high
that it forces the movement of electrons and holes through the barrier and to cross it to provide
forward current.

Forward Current is the current produced by the diode when operating in forward biased
condition and it is indicated by If.

Working under Reverse Biased

When an external voltage is applied to a diode such that it increases the potential barrier and
restricts the flow of current is called as Reverse bias. When anode and cathode are connected to
negative and positive terminals respectively, the electrons are attracted towards the positive

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terminal and holes are attracted towards the negative terminal. Hence both will be away from the
potential barrier increasing the junction resistance and preventing any electron to cross the
junction.

The following figure explains this. The graph of conduction when no field is applied and when
some external field is applied are also drawn.

With the increasing reverse bias, the junction has few minority carriers to cross the junction. This
current is normally negligible. This reverse current is almost constant when the temperature is
constant. But when this reverse voltage increases further, then a point called reverse breakdown
occurs, where an avalanche of current flows through the junction. This high reverse current
damages the device.

Reverse current is the current produced by the diode when operating in reverse biased condition
and it is indicated by Ir. Hence a diode provides high resistance path in reverse biased condition
and doesn’t conduct, where it provides a low resistance path in forward biased condition and
conducts. Thus we can conclude that a diode is a one-way device which conducts in forward bias
and acts as an insulator in reverse bias. This behavior makes it work as a rectifier, which
converts AC to DC.

Peak Inverse Voltage

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Peak Inverse Voltage is shortly called as PIV. It states the maximum voltage applied in reverse
bias. The Peak Inverse Voltage can be defined as “The maximum reverse voltage that a diode
can withstand without being destroyed”. Hence, this voltage is considered during reverse
biased condition. It denotes how a diode can be safely operated in reverse bias.

Purpose of a Diode

A diode is used to block the electric current flow in one direction, i.e. in forward direction and to
block in reverse direction. This principle of diode makes it work as a Rectifier.

For a circuit to allow the current flow in one direction but to stop in the other direction, the
rectifier diode is the best choice. Thus the output will be DC removing the AC components. The
circuits such as half wave and full wave rectifiers are made using diodes, which can be studied in
Electronic Circuits tutorials.

A diode is also used as a Switch. It helps a faster ON and OFF for the output that should occur in
a quick rate.

V - I Characteristics of a Diode

A Practical circuit arrangement for a PN junction diode is as shown in the following figure. An
ammeter is connected in series and voltmeter in parallel, while the supply is controlled through a
variable resistor.

During the operation, when the diode is in forward biased condition, at some particular voltage,
the potential barrier gets eliminated. Such a voltage is called as Cut-off Voltage or Knee

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Voltage. If the forward voltage exceeds beyond the limit, the forward current rises up
exponentially and if this is done further, the device is damaged due to overheating.

The following graph shows the state of diode conduction in forward and reverse biased
conditions.

During the reverse bias, current produced through minority carriers exist known as “Reverse
current”. As the reverse voltage increases, this reverse current increases and it suddenly breaks
down at a point, resulting in the permanent destruction of the junction

DIFFERENCE BETWEEN NPN AND PNP TRANSISTOR

The transistors PNP and NPN are BJTs and it is a basic electrical component, used in various
electrical and electronic circuits to build the projects. The operation of the PNP and NPN
transistors mainly utilizes holes and electrons. These transistors can be used as amplifiers,
switches and oscillators. In PNP transistor, the majority charge carriers are holes, where in NPN
the majority charge carriers are electrons. Except, FETs have only one sort of charge carrier. The
major difference between NPN and PNP transistor is, an NPN transistor gets the power when the
flow of current runs through the base terminal of the transistor.

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In NPN transistor, the flow of current runs from the collector terminal to the emitter terminal. A
PNP transistor switches ON, when there is no flow of current at the base terminal of the
transistor. In PNP transistor, the flow of current runs from the emitter terminal to the collector
terminal. As a result, a PNP transistor switch ON by a low signal, where NPN transistor switches
ON by a high signal.

Difference between PNP and NPN

Difference between NPN and PNP Transistor

The main difference between NPN and PNP transistors includes what are PNP and NPN
transistors, construction, working and its applications.

What is a PNP Transistor?

The term ‘PNP’ stands for positive, negative, positive and also known as sourcing. The PNP
transistor is a BJT; in this transistor the letter ‘P’ specifies the polarity of the voltage necessary
for the emitter terminal. The second letter ‘N’ specifies the polarity of the base terminal. In this
kind of transistor, the majority charge carriers are holes. Mainly, this transistor works as the
same as the NPN transistor.

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 PNP Transistor

The required materials used to build the emitter (E), base (B) and collector(C) terminals in this
transistor are diverse from those used in the NPN transistor. The BC terminals of this transistor
are constantly reversed biased, then the –Ve voltage should be used for the collector terminal.
Consequently, the base-terminal of the PNP transistor must be –Ve with respect to the emitter-
terminal, and the collector terminal must be –Ve than the base terminal

PNP Transistor Construction

The PNP transistor construction is shown below. The main characteristics of both the transistors
are similar except that the biasing of the current & voltage directions are inverted for any one of
the achievable 3-configurations namely common base, common emitter and common collector.

PNP Transistor
Construction

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The voltage between the VBE (base and emitter terminal) is –Ve at the base terminal & +Ve at
the emitter terminal. Since for this transistor, the base terminal constantly biased -Ve with
respect to the emitter terminal. Also, the VBE is positive with respect to the collector VCE.

The voltage sources connected to this transistor is shown in the above figure. The emitter
terminal is connected to the ‘Vcc’ with the load resistor ‘ RL’. This resistor stops the current
flow through the device, which is allied to the collector terminal.

The base voltage ‘VB’ is connected to the ‘RB’ base resistor, which is biased negative with
respect to the emitter terminal. To root the base current to flow through a PNP transistor, the
base terminal of the transistor should be more negative than the base terminal by approximately
0.7volts (or) a Si device.

The primary difference between PNP and NPN transistor is the correct biasing of the
transistor joints. The directions of current and the voltage polarities are constantly reverse to
each other.

What is an NPN Transistor?

The term ‘NPN’ stands for negative, positive, negative and also known as sinking. The NPN
transistor is a BJT, in this transistor, the initial letter ‘N’ specifies a negatively charged coating
of the material. Where, ‘P’ specifies a completely charged layer. The two transistors have a
positive layer, which are situated in the middle of two negative layers. Generally, NPN transistor
is used in various electrical circuits for switching and strengthens the signals that exceed through
them.

NPN Transistor

The NPN transistor includes three terminals like base, emitter and collector. These three
terminals can be used to connect the transistor to the circuit board. When the current flows
through this transistor, the base terminal of the transistor gets the electrical signal. The collector

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terminal creates a stronger electric current, and the emitter terminal exceeds this stronger
current on to the circuit. In PNP transistor, the current runs through the collector to the emitter
terminal.

Usually, NPN transistor is used because it is so simple to generate. For an NPN transistor to
function properly, it requires to be created from a semiconductor object, which holds some
current. But not the max amount as extremely conductive materials such as metal. Silicon is one
of the most normally used in semiconductors. These transistors are the simple transistors to build
out of silicon.

The NPN transistor is used on a computer circuit board to translate the information into binary
code, and this procedure is proficient through a plethora of tiny switches flipping On & OFF on
the boards. A powerful electric signal twists the switch on, while a lack of a signal makes the
switch off.

Construction of NPN Transistor

The construction of this transistor is shown below. The voltage at the transistor’s base is +Ve and
–Ve at the transistors emitter terminal. The base terminal of the transistor is positive at all times
with respect to the emitter, and also collector voltage supply is +Ve with respect to the
transistor’s emitter terminal. In this transistor, the collector terminal is linked to the VCC through
the RL

NPN Transistor
Construction

This resistor restricts the current flow through the highest base current. In NPN transistor, the
electrons flow through the base represents transistor action. The main characteristic of this
transistor action is the connection between the i/p and o/p circuits. Because, the amplifying
properties of transistor come from the resultant control that the base utilizes upon the collector to
emitter current.

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The NPN transistor is a current activated device. When the transistor is turned ON, the huge
current IC supplies between the collector & emitter terminals in the transistor. But, this only
occurs when a tiny biasing current ‘Ib’ flows through the transistor’s base terminal. It is a bipolar
transistor; the current is the relation of two currents (Ic/Ib), named the DC current gain of the
device.

It is specified with “hfe” or these days beta. The beta value can be huge up to 200 for typical
transistors. When the NPN transistor is used in an active region, then base current ‘Ib’ offers the
i/p and collector current ‘IC’ gives the o/p. The current gain of the NPN transistor from the C to
the Eis called alpha (Ic/Ie), and it is a purpose of the transistor itself. As the Ie (emitter current) is
the sum of a tiny base current and huge collector current. The worth of the alpha is very close to
unity, and for a typical low power signal transistor the value ranges from about 0.950- 0.999.

Main Difference Between PNP and NPN

PNP and NPN transistors are three terminal device, which are made up of doped materials,
frequently used in switching and amplifying applications. There are a combined of PN junction
diodes in every bipolar junction transistor. When the couple of diodes connected, then it shapes a
sandwich. That seat a kind of semiconductor in the middle of the similar two types.

Difference between
NPN and PNP Transistor

So, there are only two kinds of bipolar sandwich, that are namely PNP & NPN. In semiconductor
devices, the NPN transistor has typically high electron mobility evaluated to the mobility of a
hole. Thus, it allows a huge amount of current & works very fast. And also, the construction of
this transistor is simple from silicon.

 Both the transistors are collected of special materials and the flow of current in these transistors
is also different.
 In an NPN transistor, the flow current runs from the collector terminal to the Emitter terminal,
whereas in a PNP, the flow of current runs from the emitter terminal to the collector terminal.
 PNP transistor is made up of two P-type material layers with a layer of sandwiched of N-type.
The NPN transistor is made up of two N-type material layers with a layer of sandwiched of P-
type.
 In an NPN-transistor, a +ve voltage is set to the collector terminal to generate a flow of current
from the collector. For PNP transistor, a +ve voltage is set to the emitter terminal to generate
flow of current from the emitter terminal to collector.

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  The main working principle of an NPN transistor is, when the current is increased to the base
terminal, then the transistor switches ON & it performs fully from the collector terminal to
emitter terminal.
 When you reduce the current to the base, the transistor switches ON and the flow of current is
so low. The transistor no longer works across the collector terminal to emitter terminal, and
turns OFF.
 The main working principle of a PNP transistor is, when the current exists at the base of the PNP
transistor, and then the transistor turns OFF. When there is no flow of current at the base of the
transistor, then the transistor switches ON

Difference Between an NPN and a PNP Transistor

Before we talk about the differences between NPN and PNP transistors, we will first discuss
what they are and their similarities.

Both NPN and PNP are bipolar junction transistors (BJTs). BJTs are current-controlled
transistors that allow for current amplification.

A current at the base of the transistor allows for a much larger current across the emitter and
collector leads. NPN and PNPs are exactly the same in their function, they provide amplification
and/or switching capability.

So technically, they achieve and do the same exact thing.

How they differ is how power must be allocated to the terminal pins for them to provide this
amplification or switching. Since they are internally constructed very differently, current and
voltage must be allocated differently in order for them to work. An NPN transistor receives
positive voltage to the collector terminal and positive voltage to the base terminal for proper
operation. A PNP transistor receives positive voltage to the emitter terminal and a negative

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voltage at the base terminal (or rather a more negative or lower voltage than what is supplied at
the emitter terminal).

Since voltage allocation is different, how current flow works to turn them on is different. An
NPN transistor is powered on when a sufficient current is supplied to the base of the transistor.
Therefore, the base of an NPN transistor must be connected to positive voltage for current to
flow into the base. A PNP transistor is the opposite. In a PNP transistor, current flows out of the
base (negative current to the base) by giving the base terminal a more negative (a lower) voltage
than what is supplied to the emitter terminal. As long as the voltage at the base terminal is lower
than at the emitter terminal in a PNP transistor, the correct biasing and negative current effect
will be achieved.

So knowing this, with an NPN transistor, current needs to be sourced to the base of the transistor
for operation. This means current needs to flow into the base. In a PNP transistor, current is
sourced away or sinked from the base of the transistor to ground for operation. This means
current needs to flow out of the base. So a simple approach of thinking about it is an NPN
transistor requires positive current to the base, while a PNP requires negative current to the base
(current must flow out from the base to ground).

Another concept differentiating NPN and PNP transistors is that since voltage is allocated
differently, they have opposite current flows at the output. In an NPN transistor, output current
flows from the collector to the emitter. In a PNP transistor, output current flows from the emitter
to the collector.

Below we go over the concepts explained above in more depth, with diagrams, to better illustrate
the differences between NPN and PNP transistors.

Voltage Allocation and Current Flow are Switched

Since PNP and NPN transistors are composed of different materials, how voltage is biased to
them to produce current flow is different, and their current flow is opposite as well.

PNP transistors are made up of 2 layers of P material sandwiching a layer of N material, while
NPN transistors are made up of 2 layers of N material sandwiching 1 layer of P material. Really
opposites.

Therefore, to produce current flow in an NPN transistor, positive voltage is given to the collector
terminal and current flows from the collector to the emitter. For a PNP transistor, positive
voltage is given to the emitter terminal and current flows from the emitter to the collector.

This is summarized right below.

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Voltage and Current Biasing

NPN Transistor

An NPN transistor receives positive voltage at the collector terminal. This positive voltage to the
collector allows current to flow across from the collector to emitter, given that there is a
sufficient base current to turn the transistor on.

PNP Transistor

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A PNP transistor receives positive voltage at the emitter terminal. The positive voltage to the
emitter allows current to flow from the emitter to the collector, given that there is negative
current to the base (current flowing out of the base to ground).

How They Operate (Turn On and Off)

NPN Transistor

This is how a NPN transistor works:

As you increase current to the base of a NPN transistor, the transistor is turned on more and more
until it conducts fully from collector to emitter.

And as you decrease current to the base of a NPN transistor, the transistor turns on less and less,
until the current is so low, the transistor no longer conducts across collector to emitter, and shuts
off.

PNP Transistor

A PNP transistor functions the total opposite way.

As current is sinked from the base (flows out from the base to ground), the transistor is on and
conducts across to power on the output load.

So these are the main concepts of NPN vs PNP transistors.

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NPN transistor:

In this type of transistor, p-type semiconductor piece is sandwiched between two pieces of n-
type semiconductor layers.

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As shown in above diagram,the forward bias causes the electrons in the n-type emitter to flow
towards the base which constitutes the emitter current.When these electrons flow towards the
base, it got combined with the majority carriers i.e. holes of the base. As we know, the base is
very thin & lightly doped, therefore, to constitute base current only a few electrons i.e. less than
5% got combine with the holes. The remaining electrons i.e. more than 95% passes to the
collector region to constitute collector current .In this way, the entire emitter current flows in the
collector circuit.

PNP transistor:

In this type of transistor, n-type semiconductor piece is sandwiched between two pieces of p-
type semiconductor layers.

As shown in above diagram, the forward bias causes the holes in the p-type emitter to flow
towards the base which constitutes the emitter current.When these holes flow towards the base, it
got combined with the majority carriers i.e. electrons of the base. As we know, the base is very
thin & lightly doped, therefore to constitute base current only a few holes i.e. less than 5% got
combine with the electrons. The remaining holes i.e. more than 95%passes to the collector
region to constitute collector current. In this way, the entire emitter current flows in the collector
circuit.

Both types of transistors are used in various electronics equipments. In our article, we explain
transistor basics, transistor configurations, transistor as a switch

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