Ans: The diode consists of two electrodes one is cathode and another is anode. The cathode emits electrons and the anode will attracts the emitted electrons when it is supplied by positive potential.

Ans: The gap between the bottom of conduction band and the top of valence band is called Energy gap. To move the electrons from the valence band to conduction band the supplied external voltage must be equal to energy band gap.

Ans:   The range of energy which is possessed by valence electrons is known as valence band. Here the electrons which are situated at outer most orbits are called valence electrons. The valence band consists of valence electrons which are having highest energy.

Ans:   The range of energies possessed by conducting electrons is known as conduction band. The conduction electrons are responsible for the conduction of current in a conducting material. So, these electrons are called as conduction electrons.

Ans:   Based on the energy gap the solid materials are classified into 3 types they are: conductors, insulators and semi conductors.

Ans:   Conductors: Those substances whose atoms have their outermost orbits incomplete are known as conductors (e.g. Cu, Ag, Au etc.). In conductors, valence and conduction bands are found overlapped into each other i.e. the energy gap is zero.
Insulators: Those substances which have large energy gap between their valence and conduction band, are called insulators (e.g. diamond, wood etc.).
Semi conductors: Those substances which have conductivity and resistivity properties in between conductors and insulators are called semi conductors (e.g. Si, Ge). Energy gap of these semiconductors lies between 0.5 to 1.1eV (Foe Ge it is 0.5 – 0.7eV).

Ans:   Two types of semi conductors are there (i) Intrinsic or pure semi conductors and
(ii) Extrinsic or impure semi conductors.

Ans:   Intrinsic semi conductor: A pure semiconductor is known as intrinsic semi conductor. In these semi conductors, if the temperature increases then the conductivity is also increases. At higher temperatures due to collisions some electrons absorb energy and raises to conduction band then in their places in valence band holes are created. In intrinsic semiconductor number of holes is equal to number of electrons.
Extrinsic semi conductor: A pure semiconductor after doping is called extrinsic or impure semi conductor. Trivalent and penta-valent impurities are added to form P-type and N-type semiconductors respectively.

Ans:   The level upto which all the energy states are filled by electrons is known as Fermi level. The average energy of charge carriers is calculated by Fermi energy level. In pure semi conductors Fermi energy level is at the centre of the valence and conduction bands. In extrinsic/impure P-type (N-type) semiconductor Fermi energy level is near to the valence (conduction) band.

Ans:   The process of adding impurities to a pure semi conductor is called doping
The material added as impurity is called as Dopant.

Ans:   If we add trivalent impurities such as Aluminum to a pure semi conductor then the material is called P-type semi conductor. If a pentavalent impurity such as Arsenic is added to a pure semi conductor then the material is called N-type semi conductor

Ans:   In P-type material 3 electrons of trivalent atom makes covalent bonds with Semiconductors such as Si or Ge and there is a need of one more electron to make the system stable because Si or Ge has 4 electrons in their outermost orbits. For this reason P-type material is also known as Acceptor.
On the other hand, in case of N-type of material 4 electrons of pentavalent atom makes covalent bonds with Semiconductors such as Si or Ge which have 4 electrons in their outermost orbits and hence  there is one free or excess electron remains present in the structure. For this reason N-type material is also known as Donor.
 

Ans:   If P-type and N-type semi conductors are combined to each other then the resultant structure is called P-N junction diode. This means if   trivalent impurity is doped to one end of the pure semi conductor and pentavalent impurity to other end, a P-N junction diode can be formed.

Ans:   When a battery’s positive terminal is connected to P-type material and battery’s negative terminal is connected to N-type material of a P-N junction diode, then this mode of operation is said to be in forward biasing. Here the holes of P are repelled by the positive terminal of the battery and electrons of N are repelled by the negative terminal of the battery and hence both holes and electrons moves towards the junction. As the applied voltage becomes large enough to destroy the depletion barrier diode starts conducting. This Forward Biasing is also called as Low resistance connection. In this mode of biasing the current flow is mainly due to majority charge carriers.

Ans:   When a battery’s positive terminal is connected to N-type material and battery’s negative terminal is connected to P-type material of a P-N junction diode, then this mode of operation is said to be in forward biasing. Here the holes of P are attracted by the negative polarity of the battery and electrons of N are attracted by the positive polarity of the battery and hence both holes and electrons move away from the junction and then this increases the width of depletion layer. This reverse Biasing is also called as High resistance connection. In this bias the current is mainly due to minority charge carriers. In this mode, very small current flows across the junction.