Unit-1

Question Bank

Q1) Difference between drift and diffusion current?

Sol:

Q2) Explain how the temperature effects the semiconductor parameters?

Sol: The effect of temperature on the semiconductors parameters is listed below

i)                   Intrinsic concentration:The intrinsic carrier concentration depends upon the temperature. For N-type semiconductors the number of holes changes with change in temperature and electrons are not much affected. For P-type semiconductor the number of electrons increases with increase in temperature and hole are not much affected.

Ii)                Forbidden energy gap: The energy required to break the covalent bond is called forbidden energy. This is the difference between energy level of CB and VB. This gap decreases with increase in temperature.

Iii)             Mobility: the mobility decreases in intrinsic semiconductor with temperature. As the temperature increases there are a greater number of charge carriers with high energy, this results in collision of the charge carriers decreasing their mobility. The mobility for Extrinsic semiconductors decreases with increase in temperature.

Iv)              Conductivity: The conductivity increases in intrinsic semiconductors with increase in temperature as there are a greater number of charge carriers at high temperature. For extrinsic semiconductor the conductivity decreases with temperature because the majority carrier concentration is almost constant, but mobility decreases.

Q3) List difference between N-type and P-type semiconductors?

Sol:

Q4) Explain energy band theory of crystals?

Sol: According to Neil Bohr’s theory of atomic structure all atoms have different energy levels. When two or more atoms are placed near to each other their energy levels get transformed to energy band structure. These energy bands are formed due to mutual interaction between the atoms caused by the electromagnetic force between them.

The below figure shows the energy bands of various energy levels. The electrons nearer to nucleus of interacting atom are having energy band 1 and those in outer orbit have E2, E3 so no.

                                             Fig: Energy band in crystal

Energy band in crystals can of different types which are

i)                   Valence Band: The electrons in the outer most orbit of an atom is present in this energy band. This is the highest energy band at room temperature. This band can be completely or partially filled.

Ii)                Conduction Band: This is the lowest energy band containing electrons which are free from the attractive force of atom’s nucleus.

The Valence band has low energy level than conduction band. This is the reason that CB is above the VB separated by energy gap.

Q5) Calculate the drift current density in a semiconductor for a given electric field. For germanium sample at T=300oK where ND = 0 and NA =1x1015cm-3ni= 2.4x1013 cm-3. Assume electron and hole mobility to be 3900 cm2/V-sec and 1900 cm2/V-sec. Applied electric field is E= 40V/cm?

Sol: As NA>>ND

p0 = ½ {(NA-ND) + (NA-ND)2+4ni2)1/2}

p0 = 5x1014cm-3

n= ni2/NA

n= 5.76x1011cm-3

J = q (pp+nn) E

For extrinsic p-type semiconductor

  J = q NAp E =1.6x10-19x1900x1015x40 =12.16 A/cm2

Q6) Determine thermal equilibrium of electron and hole concentration for an n-type silicon semiconductor at T=300oK where ND = 1x1016cm-3 and NA =0. Assume ni = 1.5 x 1010cm-3?

Sol: The majority carrier concentration

n0 = ½ {(ND-NA) + (ND-NA)2+4ni2)1/2}

n0 = 1016 cm-3

The minority carrier concentration

p0 = ni2/ND

p0 = 2.25x x104 cm-3

Q7) Determine thermal equilibrium of electron and hole concentration for doping concentration at T=300oK where ND = 4x1013cm-3 and NA =0. Assume ni = 2.4 x 1013cm-3?

Sol: The majority carrier concentration

n0 = ½ {(ND-NA) + (ND-NA)2+4ni2)1/2}

n0 =3.12x1013 cm-3

The minority carrier concentration

p0 = ni2/ND

p0 = 1.45x x1013 cm-3

Q8) Determine thermal equilibrium of electron and hole concentration in n-type semiconductor at T=300oK where ND = 2x1016cm-3 and NA =12x1015cm-3. Assume ni = 1.5 x 1010cm-3?

Sol: The majority carrier concentration

n0 = ½ {(ND-NA) + (ND-NA)2+4ni2)1/2}

n0 =4x1015 cm-3

The minority carrier concentration

p0 = ni2/ND

p0 = 11.25x x103 cm-3

Q9) Derive continuity equation?

Sol: The equation shows a change in carrier density over time is due to the difference between the incoming and outgoing flux of carriers plus the generation and minus the recombination.

Consider a small area having flux with infinitesimal amount

The above equations are called continuity equations.

Q10) Explain energy band in crystals?

Sol: According to Neil Bohr’s theory of atomic structure all atoms have different energy levels. When two or more atoms are placed near to each other their energy levels get transformed to energy band structure. These energy bands are formed due to mutual interaction between the atoms caused by the electromagnetic force between them.

The below figure shows the energy bands of various energy levels. The electrons nearer to nucleus of interacting atom are having energy band 1 and those in outer orbit have E2, E3 so no.

                                             Fig: Energy band in crystal

Energy band in crystals can of different types which are

i)                   Valence Band: The electrons in the outer most orbit of an atom is present in this energy band. This is the highest energy band at room temperature. This band can be completely or partially filled.

Ii)                Conduction Band: This is the lowest energy band containing electrons which are free from the attractive force of atom’s nucleus.

The Valence band has low energy level than conduction band. This is the reason that CB is above the VB separated by energy gap.