The document discusses the structure and composition of atomic nuclei. It notes that nuclei are made up of protons and neutrons, which themselves are composed of quarks. The size of nuclei scales approximately with the one-third power of the total number of nucleons. Nuclear forces that bind nuclei include the long-range electromagnetic force and a short-range strong nuclear force that is attractive between nucleons at distances greater than 1-2 femtometers but repulsive at shorter distances. This dual nature of the nuclear force is necessary to maintain nuclear stability.

1. Modern Physics
CONDUCTOR:
The materials having large number of free electrons are called Conductors.
The conductors conduct the current when potential difference is applied. Gold,
Copper and Aluminum are good examples of conductors.
The electrons in the outermost orbit of an atom are called Valence
Electrons. The valence electrons of a conductor which are loosely attached to the
nucleus of an atom are called Free Electrons. The free electrons moving randomly
and continuously in a conductor are called Conduction Electrons.
Each atom of a conductor may contribute one electron and on average such
conductor has 1023 conduction electrons per cm3. The resistivity of conductors is
very small and is of the order of 10-8 ohm-m. The resistivity of Copper is (1.68 x
10-8) ohm-m and resistivity of Aluminum is 2.65 x 10-8 ohm-m.
INSULATORS:
The materials having no free electrons at room temperature are called
Insulators. The insulators don’t conduct current when potential difference is
applied. Glass and Plastic are good examples of Insulators.
Mostly Insulator has one conduction electron per cm3 at room temperature.
The resistivity of insulators is very high and is of the order of 1011 ohm-m. The
resistivity of glass is 9.0 x 1011 ohm-m.
SEMICONDUCTORS:

2. The partial conductors are called Semiconductors. The semiconductors
have few free electrons and holes at room temperature and practically do not
conduct current due to potential difference. Silicon (Si14) and Germanium (Ge32)
are the examples of semiconductors.
The charge carriers are free electrons and holes in semiconductors. The
positively charges are assumed to be fixed. Semiconductor has 1013 conduction
electrons per cm2 at room temperature. The density of charge carriers can be
changed by introducing small impurities (B5, Ga31 and P15, As33, Sb51).
The resistivity of semiconductors has intermediate values between,
conductor and insulators and is of the order of 0.5 ohm-m to 10-4 ohm-m. The
resistivity of Germanium is 0.6 ohm-m and resistivity of Silicon at room
temperature is 3.0 x 10+3 ohm-m
INTRINSIC SEMICONDUCTORS:
The naturally pure, un-doped and crystalline structured semiconductor
materials are called “Intrinsic Semiconductor”. It can conduct a small amount of
current is called “Intrinsic Semiconductor Current”. Si and Ge are the examples
of semiconductor belong to group-IV in periodic table.
EXTRINSIC SEMICONDUCTORS:
A semiconductor in which the charge carrier concentration is dependent
upon impurities or other imperfections is called as “Extrinsic Semiconductor”.
The amount of current pass through the semiconductor may vary by the variation

3. of addition of impurities in the semiconductor material. Amount of current
increases with more addition of impurity atoms and while it decreases with the less
addition of impurity atoms.
The addition of small percentage of impurity into intrinsic semiconductors
such as Ge or Si is called “Doping”. The new materials formed after doping are
called “Extrinsic Semiconductor”. The doping increases the density of charge
carriers (free electrons or holes) and conductivity of Silicon and Germanium
increases. It changes the properties of semiconductors dramatically and has many
applications in electronics.
N-TYPE SEMICONDUCTORS:
Semiconductor materials formed after addition penta-valent impurity such as
(P15, As33 and Sb51) into intrinsic semiconductors is called “N-type
Semiconductor”. The atom of penta-valent impurity has 5 valence electrons in its
outer most shell.
The pentra-valent impurity when added in pure Si or Ge occupies the central
position and four of its valence electrons form covalent bond with four valence
electrons of intrinsic semiconductor. The 5th electron of impurity atom remains
unbound and acts as charge carrier. The impurity atom is called donor atom
because it has donated a free electron. It means addition of donor atoms increases
the density of free electrons in the conduction-band and greatly increases the
conductivity of the intrinsic semiconductor.
The free electrons in conduction-band in N-type material are called Majority
Charge Carriers.

4. The flow of current in N-type semiconductor when connected to a battery is due to
movement of free electrons which are directed towards the positive terminal of
battery is called N-type Conductivity.
P-TYPE SEMICONDUCTORS:
Semiconductor materials formed after addition tri-valent impurity such as
(B5, Ga31) into intrinsic semiconductors is called “P-type Semiconductor”. The
atom of tri-valent impurity has 3 valence electrons in its outer most shell.
The impurity atom when doped with pure Si or Ge occupies the central
position. Its three valence electrons form covalent bond with three valence
electrons of intrinsic semiconductor. A vacancy is formed at the position of 4th
covalent bond. This vacancy is called Hole. It acts as positive charge carrier. The
impurity atom is called Acceptor Atom because it can accept an electron to
complete covalent bond. The holes in valence-band are called Majority Charge
Carriers.
The flow of current in P-type semiconductor when connected to a battery is due to
movement of holes which are directed towards the negative terminal of battery is
called P-type Conductivity.
PN JUNTION:
The junction formed when a piece of N-type material and P-type material is
joined together is called P-N Junction. The P-type material has holes as majority
charge carriers which come from acceptor impurities and N-type material has free
electrons as majority charge carriers which come from donor impurities. The P-N
Junction is formed after diffusion process. The diffusion of further free electrons

5. from N-regrion towards P-region stops when sufficient negative ions are created at
junction boundary. These positive ions and negative ions attract each other and
form a depletion region at junction boundary. The P-N Junction has unidirectional
current flow characteristics. The current flows when P-N Junction is forward
biased and current does not flow when P-N Junction is reverse biased.

6. Size and Structure of the Nucleus:
Nuclei are made up of protons and neutrons. But these particles are not true
elementary particles; these are made up of other particles called quarks. The
masses of the proton and neutron are very similar and they are roughly 2000 times
heavier than the mass of the Electron.
Mass of Proton = Mp = 1.672 x 10-27 kg
Mass of Neutron = Mn = 1.675 x 10-27 kg
Mass of Electron = Me = 9.109 x 10-31 kg
Using of kilograms is not suitable if dealing with such small particles. Instead we
use (E = mc2) to mention the mass of a particle in terms of its rest energy,
(m = E / c2). So mass is measured in units of (MeV / c2)
Mass of Proton = Mp = 938 MeV / c2
Mass of Neutron = Mn = 940 MeV / c2
Mass of Electron = Me = 0.5 MeV / c2
The number of protons in a nucleus is called the atomic number or charge number.
It is represented by Z. The number of neutrons is called the neutron number. It is
represented by N. The total number of nucleon (N + Z) is called the Mass
Number. It is represented by A.
The charge on one proton is (+ e = 1.6 x 10-19 C) and the charge on the electron is
(- e = - 1.6 x 10-19 C), while the neutron is a neutral particle. The nuclei, having
same charge number Z but different mass number is called Isotopes. The nuclei
which are not stable used to emit α, β and γ rays are called radioactive nuclides.

7. The diameter of the nucleus is about 10 million times smaller than the overall
diameter of the atom. Nuclei follow an approximate rule for the radius, 𝑟 ≈ 𝑟0 𝐴
1
3
where 𝑟0 = 1.2 𝑓𝑚 (as 1 fermi = 10-13 cm) and A = Z + N. Now, 𝐴
1
3 increases very
slowly with A. As 16
1
3 = 2.52 while 208
1
3 = 5.93. This means that a very heavy
lead nucleus Pb208 is only about 2.4 times the size of the much lighter O16 nucleus.
To study how protons are distributed inside a nucleus, we send a beam of electrons
at a nucleus and observe how they scatter in different directions. The negatively
charged electrons interact with the positively charged protons, but they obviously
will not see the neutrons. The scattered electrons are captured in a detector which
can be moved around to different angles. In this way one can reconstruct the
charge distribution which caused the electrons to be scattered in that particular
way.
Nuclear Forces:
A nucleus is packed with protons and neutrons. As proton is a positively charged
particle, so, there must be electrostatic repulsion. But the nucleus is very much
rigid and stable. So, there must be some other force inside the nucleus which is
responsible for the stability and rigidity of the nucleus. This force is called Strong
Nuclear Force.
The electrostatic force (Coulomb force) is a long range force, but the Strong
Nuclear Force is a short range force, having the range of 10-15 m. This force binds
every nuclear pair: proton-proton, neutron-neutron and proton-neutron pair with in
the tiny nuclear volume. For the short range of the order of 10-15 m, the Coulomb
force is much smaller than the Strong Nuclear Force. If the separation between the
nucleons is increased beyond 10-15 m, the strong nuclear force drops to zero
rapidly, then the Coulomb’s force of repulsion will be able to break the nucleus.

8. Obviously there must be some attractive force that is stronger than this
repulsion. It is, in fact, called the strong force. From what we have learned so far,
we can guess some of its important features:
a) Since neutron and proton distributions are almost the same, the N-P force
cannot be very different from the N-N or P-P force.
b) Since the density of nucleons in large nuclei is the same as in lighter nuclei,
this means that a given nucleon feels only the force due to its immediate
neighbors and does not interact much with nucleons on the other side of the
nucleus. In other words, the range of the nucleon-nucleon force is very short
and of the order of 1-2 fm only.
The force between two charges is always of one sign - repulsive if the signs are the
same and attractive if they are opposite. In the early years of quantum theory,
people realized that this force comes about because of the exchange of photons
between charges. The nucleon-nucleon force is different. It has to be attractive to
keep the nucleus together, and has to short range. But, to prevent nucleons from
sticking to each other, it must be repulsive at short distances. Now here, "short"
and long means distances on the scale of fermis. Typically the distances between
nucleons are on this scale as well.