THEORY EXAMINATION (SEM–VI) 2016-17 ADVANCE SEMICONDUCTOR DEVICES

ADVANCE SEMICONDUCTOR DEVICES (NEC014)

SECTION-WISE SOLVED ANSWERS

SECTION-A – Short Answer Questions

(10 × 2 = 20 marks)

(a) Energy bands and energy gap

Energy bands are ranges of allowed energy levels for electrons in a solid. The energy gap (band gap) is the forbidden energy region between the valence band and conduction band. Its value determines whether a material is a conductor, semiconductor, or insulator.

(b) Optical and thermal properties

Optical properties relate to interaction of light with semiconductors such as absorption and emission. Thermal properties describe how temperature affects carrier concentration, conductivity, and energy band gap.

(c) Depletion region

The depletion region is the area near a p–n junction where mobile charge carriers are absent due to recombination, leaving behind immobile ions that create an electric field.

(d) Single Electron Transistor (SET)

A single electron transistor controls current flow by allowing electrons to tunnel one at a time through a small conducting island, making it useful in nano-electronics.

(e) Non-volatile memory devices

Non-volatile memory devices retain stored data even after power is removed. Examples include ROM, EEPROM, and Flash memory.

(f) Laser operating characteristics

Laser characteristics include threshold current, output power, coherence, monochromaticity, and high directionality.

(g) Laser physics

Laser operation is based on stimulated emission, population inversion, and optical feedback provided by a resonant cavity.

(h) Phototransistor

A phototransistor is a light-sensitive transistor where incident light controls collector current. It provides higher sensitivity than a photodiode.

(i) Different types of diodes

Examples include p–n junction diode, Zener diode, Tunnel diode, Schottky diode, PIN diode, LED, and Laser diode.

(j) Non-uniform doping

Non-uniform doping means dopant concentration varies with position inside the semiconductor, commonly used to improve device performance.

SECTION-B – Long Answer Questions

(Attempt any five – 10 × 5 = 50 marks)

2(a) IMPATT and BRITT diode

IMPATT stands for Impact Ionization Avalanche Transit Time diode. It operates based on avalanche multiplication and carrier transit time to generate microwave frequencies.

The BRITT diode (Barrier Injection Transit Time diode) is a modified IMPATT diode where carriers are injected through a barrier, reducing noise and improving efficiency.

2(b) Tunnel diode and backward diode

A tunnel diode is a heavily doped p–n junction diode that allows electrons to tunnel through the depletion region, resulting in a negative resistance region.

A backward diode operates mainly in the reverse direction and has negligible forward voltage drop, making it useful in microwave detection.

2(c) TRAPATT diode and avalanche velocity

TRAPATT stands for Trapped Plasma Avalanche Triggered Transit diode. It uses plasma formation to achieve high efficiency at microwave frequencies.

Avalanche zone velocity is calculated using:

v=JqNAv = \frac{J}{qN_A}v=qNA​J​

Given values are substituted to find avalanche velocity.

2(d) SCR operation, tunnel diode, and Zener diode

An SCR (Silicon Controlled Rectifier) conducts when gate current is applied and remains ON until current drops below holding current.

Tunnel diode: Uses quantum tunneling and has negative resistance.

Zener diode: Operates in breakdown region to provide voltage regulation.

2(e) N-channel JFET operation and transconductance

In an N-channel JFET, current flows from drain to source through an n-type channel. Pinch-off occurs when depletion regions meet.

Transconductance:

gm=dIDdVGSg_m = \frac{dI_D}{dV_{GS}}gm​=dVGS​dID​​

It represents amplification capability.

2(f) Graded junction and capacitance

A graded junction has gradually varying doping concentration. Junction capacitance is:

Cj=C0(1+VR)mC_j = \frac{C_0}{(1+V_R)^m}Cj​=(1+VR​)mC0​​

where m depends on doping profile.

2(g) Conductivity of doped silicon

Maximum conductivity occurs when majority carriers dominate, while normal conductivity includes both electrons and holes:

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

2(h) n-type and p-type semiconductors & minority carrier lifetime

n-type: Doped with donor impurities (electrons majority).

p-type: Doped with acceptor impurities (holes majority).

Minority carrier lifetime is the average time a minority carrier exists before recombination.

SECTION-C – Very Long Answer Questions

(Attempt any two – 15 × 2 = 30 marks)

3(a) Mobility, mass action law, and Einstein relation

Mobility is the drift velocity per unit electric field:           μ=vdE\mu = \frac{v_d}{E}μ=Evd​​

Mass action law:                                                               np=ni2np = n_i^2np=ni2​

Einstein relation:                                                               Dμ=kTq\frac{D}{\mu} = \frac{kT}{q}μD​=qkT​

It relates diffusion coefficient and mobility.

3(b) MESFET operation

A MESFET (Metal Semiconductor FET) uses a Schottky gate. ON state allows current flow, while OFF state occurs when gate voltage depletes channel. It is widely used in microwave applications.

3(c) Rectifying and ohmic contacts

Rectifying contact allows current in one direction (Schottky contact).
Ohmic contact offers low resistance and allows current in both directions.

4(a) Photodetector and solar cell

A photodetector converts light into electrical current using the photoelectric effect.
A solar cell converts solar energy into electrical energy with I–V characteristics.

4(b) Photoconductivity

Photoconductivity occurs when incident light increases conductivity by generating charge carriers. Used in light sensors and optical switches.

4(c) Diffusion length, carrier lifetime, and recombination

Diffusion length:                                                 L=DτL = \sqrt{D\tau}L=Dτ​

Carrier lifetime is the average time before recombination, which affects device efficiency.

5(a) Charge-Coupled Devices (CCD)

CCD transfers charge packets through potential wells and is used in imaging systems.

5(b) Semiconductor laser

A semiconductor laser uses p–n junctions to produce coherent light through stimulated emission.

5(c) MODFET

MODFET (HEMT) uses heterojunctions for high mobility and is used in high-frequency circuits.