THEORY EXAMINATION (SEM–VI) 2016-17 ADVANCED SEMICONDUCTOR DEVCES

ADVANCED SEMICONDUCTOR DEVICES (EEC013)

Time: 3 Hours  Max Marks: 100

SECTION – A (Short Answer Questions)

(10 × 2 = 20 Marks)

(a) Simple cubic, BCC and FCC structures

Simple Cubic (SC): Atoms are located at the eight corners of a cube. Coordination number = 6.

Body-Centered Cubic (BCC): Atoms at eight corners and one atom at the center. Coordination number = 8.

Face-Centered Cubic (FCC): Atoms at eight corners and at the center of each face. Coordination number = 12.

(b) Why semiconductor behaves as an insulator at 0 K

At 0 K, all electrons remain in the valence band and no electrons are present in the conduction band. Hence, there are no free charge carriers, so the semiconductor behaves like an insulator.

(c) Fermi level and its significance

The Fermi level is the energy level at which the probability of finding an electron is 50%.
Significance: It indicates carrier concentration, type of semiconductor, and direction of carrier movement.

(d) Impurity scattering and lattice scattering

Impurity scattering: Caused by collision of charge carriers with impurity atoms.

Lattice scattering: Caused by thermal vibrations of lattice atoms.

(e) Difference between drift and diffusion current

Drift current: Caused by applied electric field.

Diffusion current: Caused by carrier concentration gradient.

(f) Mobility of a carrier

Mobility is defined as the drift velocity per unit electric field:

μ=vdE\mu = \frac{v_d}{E}μ=Evd​​ 

(g) Significance of Hall effect

Hall effect helps determine:                                 Type of semiconductor (n or p type)

Carrier concentration                                           Mobility of charge carriers

(h) Significance of negative resistance in tunnel diode

Negative resistance allows high-frequency oscillations, making tunnel diodes suitable for microwave applications.

(i) Delays in IMPATT diode

Delays include:

Avalanche buildup time                                    Carrier transit time
These delays cause phase shift needed for microwave generation.

(j) Solar cell

A solar cell is a p–n junction device that converts solar energy into electrical energy using the photovoltaic effect.

SECTION – B (Long Answer Questions)

(Attempt any FIVE – 5 × 10 = 50 Marks)

2(a) Minimum conductivity condition and Hall voltage

Minimum conductivity occurs when:

n=p=nin = p = n_in=p=ni​

For doped Si with donor concentration ND=1017 cm−3N_D = 10^{17} \, \text{cm}^{-3}ND​=1017cm−3, at 300 K:

σ=q(nμn+pμp)\sigma = q(n\mu_n + p\mu_p)σ=q(nμn​+pμp​)

Hall voltage:

VH=BIqntV_H = \frac{BI}{qnt}VH​=qntBI​                   Substituting given values gives the Hall voltage.

High-field effect: At high electric fields, carrier velocity saturates due to increased scattering.

2(b) Conductivity, mobility, and diffusion

Conductivity:

σ=q(nμn+pμp)\sigma = q(n\mu_n + p\mu_p)σ=q(nμn​+pμp​)

Mobility:

μ=qτm∗\mu = \frac{q\tau}{m^*}μ=m∗qτ​

Diffusion current density:

Jn=qDndndx,Jp=−qDpdpdxJ_n = qD_n\frac{dn}{dx}, \quad J_p = -qD_p\frac{dp}{dx}Jn​=qDn​dxdn​,Jp​=−qDp​dxdp​ 

2(c) Carrier generation, recombination & high-level injection

Carrier generation occurs due to thermal energy or light.
Recombination occurs when electrons recombine with holes.

High-level injection occurs when excess carriers exceed equilibrium majority carriers, altering device behavior.

2(d) Trapping and carrier transport equation

Trapping is capture of carriers by defect states in band gap.

Carrier transport equation:

J=qnμE+qDdndxJ = qn\mu E + qD\frac{dn}{dx}J=qnμE+qDdxdn​ 

2(e) Excess carriers and quasi-Fermi levels

Given optical generation, steady-state excess carriers:

Δn=Gτn,Δp=Gτp\Delta n = G\tau_n, \quad \Delta p = G\tau_pΔn=Gτn​,Δp=Gτp​

Separation between quasi-Fermi levels depends on excess carrier concentration and is shown in energy band diagrams.

2(f) Electric field in linear graded junction

Electric field:

E(x)=qε∫N(x)dxE(x) = \frac{q}{\varepsilon} \int N(x)dxE(x)=εq​∫N(x)dx

Hole current:

Jp=qμppE+qDpdpdxJ_p = q\mu_p pE + qD_p\frac{dp}{dx}Jp​=qμp​pE+qDp​dxdp​

Stored charge:

Q=qA∫ΔpdxQ = qA\int \Delta p dxQ=qA∫Δpdx 

2(g) Forward and reverse current in p–n junction

Forward current:

I=Is(eqV/kT−1)I = I_s(e^{qV/kT} - 1)I=Is​(eqV/kT−1)

Reverse current is approximately equal to saturation current IsI_sIs​.

2(h) I–V characteristics and breakdown mechanism

Real diode I–V curve shows:

Cut-in voltage                                                Finite slope due to resistance

Breakdown mechanisms:                               Zener breakdown (low voltage)

Avalanche breakdown (high voltage)

SECTION – C (Very Long Answer Questions)

(Attempt any TWO – 2 × 15 = 30 Marks)

3. Varactor diode and fill factor

A varactor diode is a voltage-controlled capacitor. Its junction capacitance varies with reverse bias.

Advantages:                                             Electronic tuning

Small size

Applications:                                           FM receivers

Frequency modulators

Fill factor of solar cell:                            FF=VmImVocIscFF = \frac{V_m I_m}{V_{oc} I_{sc}}FF=Voc​Isc​Vm​Im​​ 

4. IMPATT diode and transferred electron mechanism

IMPATT diode operates using avalanche breakdown and transit time delay, producing microwave oscillations.

Transferred electron mechanism occurs in materials like GaAs, where electrons transfer to higher energy valleys with lower mobility.

5. Light Emitting Diode (LED)

LED works on radiative recombination of electrons and holes.

Materials used:                                         GaAs

GaP                                                             GaAsP

Applications:                                             Displays

Optical communication                               Indicators