BETC

Question 1

Define Energy Band, Valence band and conduction band

Answer 1

Energy Band - The range of energy possessed by electrons in an orbit in solids is called Energy Band.

Valence Band - The range of energy possessed by valence electrons is called valence band.

Conduction Band - The range of energy possessed by conduction band electrons is called conduction band.

Question 2

Define Forbidden Energy Gap

Answer 2

Forbidden Energy gap is defined as the separation between the conduction band and the valence band in the energy band diagram.

Question 3

Short notes on Insulators ( 3 points )

Answer 3

• In insulators , the valance band is full and conduction band is empty
• The energy gap is very large
• A large amount of energy is required to push the electrons from valence band to conduction band

Question 4

Short notes on Conductors ( 3 points )

Answer 4

• In conductors, a large number of free electrons are available in conduction band
• The conduction band and valance band overlap
• A slight potential difference across the material can cause free electrons to move and contribute to the electric current

Question 5

Why do conductors show positive coefficient of resistance ?

Answer 5

When conductors get heated, the fixed atoms vibrate and this limits the flow of electrons in conductor which reduces the current flow. This is equivalent to increase in resistance and hence conductor shows positive coefficient of resistance.

Question 6

Short notes on Semiconductors ( 3 points )

Answer 6

• In semiconductors , the energy gap between the valence band and the conduction band is very low
• Very small energy is required to move the electrons from valance to conduction band
• At 0 K, Valance band is completely full and conduction band is completely empty and semiconductor behaves as an insulator at low temperature
• As temperature increases, more number of electrons move from valance band to conduction band and hence the conductivity increases

Question 7

Why do semiconductors show negative coefficient of resistance ?

Answer 7

In Semiconductors, conductivity increases with the rise in temperature and hence it shows negative temperature coefficient of resistance ( As temperature increases, conductivity increases and that means resistance of materials decreases)

Question 8

Donor Doping generates ? in the ? band and acceptor doping generates ? in ? band

Answer 8

Donor Doping generates free electrons in the conduction band and acceptor doping generates holes in valance band

Question 9

Short notes on n-types materials

Answer 9

• Group V elements ( Phosphorous , Arsenic , Antimony etc ) are added to pure semiconductor materials like Ge, and Si to become n type materials
• In n types materials, Majority carriers are holes and minority carriers are electrons
• In n type materials, number of holes > number of electrons

Question 10

Nd + P = Na + n, elaborate for intrinsic materials , n type materials and p type materials

Answer 10

• For intrinsic material, Nd= Na=0 i.e Electron density = Hole density
• For n type material, Na = 0, Nd+ P= n
• For p type material, Nd=0, p= Na + n

; For n type material, Nd= Donor atom concentration
n = Free electron concentration
For p type material, Na = Acceptor Atom concentration
p = hole concentration

Question 11

Define Mass Action Law and give its formula

Answer 11

Mass Action Law : The product of free electron and hole concentration in an intrinsic or extrinsic semiconductor at a given temperature is constant.
np = ni²

; ni = intrinsic constant concentration

For n-type material
Nn.Pn= ni²
Pn= Ni²/Nn = Ni²/ Nd

For p-type material
Pp.Np=Ni²
Np= Ni²/Pp= Ni²/Na

Question 12

Define Drift Current and write its formula

Answer 12

Drift current density is proportional to electric field and concentration of charge carrier 
Current Density J = I/A ( Current through unit area)
I = JA
J= σE
σ= σn + σp = nqrn + pqrp
Hence, Total current I=nqrnEa +pqrnEa
J= Current Density
E= Electric Field
rn= Mobility of electron(m²/vs)
rp=Mobility of Holes
σ= conductivity

Question 13

Define mobility carrier and write its formula

Answer 13

Mobility( μ ) of a carrier is defined as the average drift velocity per unit electric field.
Vn= - μ(n) E
V(p)= μ(p) E
μ(n) > μ(p), electrons have greater mobility than holes due to greater effective mass

Question 14

Write the formula of conductivity

Answer 14

Conductivity σ = 1 / ρ; ρ is resistivity
We know, R = -ρ l / a = l / σ a
where , R = Resistance 
l = length 
and a = area( Cross section )

Question 15

Write the formula of diffusion current density

Answer 15

Diffusion Current Density is given by
J(p) = I(p)/A = -q D(p) dP/dx
J(n)= I(n)/A = q D(n) dn/dx where dp/dx and dn/dx !=0

; J(p) , J(n) are diffusion current density,
dP/dx , dN/dx are hole concentration and electron concentration repectively
Dp, Dn are diffusion constant

Question 16

Einstein’s Relation regarding diffusion current density

Answer 16

D(p)/R(p) = D(n)/ R(n) = V(t) = KT/q = 0.026 V ( at 300 K)
;
V(t) = Thermal Voltage
K = Boltzmann Constant

Question 17

Define Barrier potential

Answer 17

Barrier potential is the potential required to start current through the P-N junction. It is 0.7 V for Si amd 0.3 V for Ge.

Question 18

When is a semiconductor diode said to be forward biased ?

Answer 18

A semiconductor diode is said to be forward biased when p type is connected to positive and n type is connected with negative side of potential supply

Question 19

Short notes on V-I characteristic graph

Answer 19

The plot between I and V is known as V-I characteristic graph of a PN junction diode . For v < v(r), the current is very small in forward bias.
For v > v(r), current rises very sharply . This voltage is called threshold voltage. Generally its 0.7 V for Si and 0.3 V for Ge. At reverse voltage, at ‘P’ the reverse current increases abruptly and almost parallel to y axis. This is called break down region and diode may break down in this region so reverse voltage at PN junction is always less than break down voltage.
The maximum reverse voltage that can be applied to the PN junction without damage to the junction is called peak inverse voltage.

Question 20

What are the 4 advantages that Silicon has over Germanium ?

Answer 20

1. Acts as a better switch
2. Better operable range
3. Better thermal range
4. Easy to find