(SEM VIII) THEORY EXAMINATION 2021-22 MODELING OF FIELD-EFFECT NANO DEVICES

SECTION A

(Attempt all questions in brief – 10 × 2 = 20 marks)

Q1(a) What is a nano-device?

A nano-device is an electronic device whose critical dimensions are in the nanometer range (1–100 nm). At this scale, quantum effects significantly influence device behavior.

Q1(b) Define modeling.

Modeling is the process of representing a physical device or system using mathematical equations to analyze, predict, and optimize its performance.

Q1(c) What do you mean by transistor?

A transistor is a semiconductor device used to amplify or switch electronic signals by controlling current or voltage.

Q1(d) Define triple gate.

A triple-gate device is a multi-gate transistor where the channel is controlled by three gates to improve electrostatic control and reduce short-channel effects.

Q1(e) What is 0D channel?

A 0D channel refers to a quantum-confined channel where charge carriers are confined in all three dimensions, as in quantum dots.

Q1(f) Define ionizing.

Ionizing refers to the process by which radiation removes electrons from atoms or molecules, creating charged particles (ions).

Q1(g) What is a VT device?

A VT (threshold voltage) device is characterized by the minimum gate voltage required to form a conducting channel between source and drain.

Q1(h) Explain RF circuit.

An RF (Radio Frequency) circuit processes high-frequency signals typically used in wireless communication systems such as transmitters and receivers.

Q1(i) What do you mean by semiconductor?

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator and can be controlled by doping or external fields.

Q1(j) What is a gate stack?

A gate stack consists of the gate electrode, gate dielectric, and interface layers that control the channel conductivity in MOS devices.

SECTION B

(Attempt ANY THREE – 3 × 10 = 30 marks)

Q2(a) MOSFET and MOSFET Scaling

A MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) controls current flow using an electric field created by the gate voltage.
 

MOSFET scaling refers to reducing device dimensions to improve speed, reduce power consumption, and increase packing density. However, scaling leads to short-channel effects, leakage currents, and reliability challenges, which require advanced device structures like multi-gate transistors.

Q2(b) Electrostatics: 1D and 2D MOS Electrostatics

Electrostatics studies electric fields and potentials in devices.
In 1D electrostatics, variations are considered along one direction, suitable for long-channel devices.
In 2D electrostatics, variations along both channel length and depth are considered, which is essential for analyzing short-channel and nano-scale MOS devices.

Q2(c) Carbon Nanotube (CNT)

Carbon nanotubes are cylindrical structures made of rolled graphene sheets. They exhibit high carrier mobility, excellent mechanical strength, and near-ballistic transport, making them suitable for nano-transistors and interconnects.

Q2(d) Ballistic Nano Transistor

A ballistic nano transistor allows charge carriers to travel through the channel without scattering.
In the ballistic model, current is determined by carrier injection from contacts rather than channel resistance, leading to high-speed and low-power operation.

Q2(e) Impact of Device Performance on Digital Circuits

Device performance directly affects speed, power consumption, noise margin, and reliability of digital circuits. Improved nano-device performance enables faster switching, lower power dissipation, and higher integration density in VLSI systems.

SECTION C

Q3(a) Multi-Gate Transistors

Multi-gate transistors use more than one gate to control the channel, improving electrostatic control.

Types:

Double-Gate MOSFET: Two gates control the channel, reducing leakage.

FinFET: A 3D structure where the channel is surrounded by the gate.

Applications:
Used in high-performance processors, low-power ICs, and advanced CMOS technologies.

Q3(b) Quantum Effects in Nano Devices

At nano scale, quantum effects such as tunneling, energy quantization, and wave-particle duality dominate device behavior. These effects influence threshold voltage, leakage current, and transport mechanisms in nano-devices.

Q4(a) Asymmetry Effect

Asymmetry effect arises when source and drain or device geometry are not identical. This leads to unequal carrier injection, threshold voltage variation, and performance degradation, especially in nano-MOSFETs.

Q4(b) (i) Double-Gate MOS System (ii) Scattering

Double-Gate MOS System:
Uses two gates to control the channel, improving scalability and reducing short-channel effects.

Scattering:
Scattering occurs when charge carriers collide with impurities, phonons, or interfaces, reducing mobility and current.

Q5(a) (i) I-V Characteristics (ii) Band Structure of Graphene

I-V Characteristics:
Describe the relationship between current and voltage, helping evaluate device performance.

Graphene Band Structure:
Graphene has a zero bandgap with linear energy dispersion, resulting in very high electron mobility.

Q5(b) Single Electron Charging

Single electron charging occurs when electrons tunnel one at a time through a nano-device, governed by Coulomb blockade. It is used in single-electron transistors for ultra-low-power applications.

Q6(a) Radiation Effects in SOI MOSFETs

Radiation can create trapped charges in oxide layers of SOI MOSFETs, causing threshold voltage shifts, leakage increase, and reliability degradation, especially in space applications.

Q6(b) (i) Scaling Effects (ii) FETs

Scaling Effects:
Include short-channel effects, velocity saturation, and leakage currents.

FETs:
Field Effect Transistors control current using an electric field and are widely used in digital and analog circuits.

Q7(a) SRAM Design

SRAM (Static RAM) uses bistable latch circuits to store data. A typical SRAM cell consists of six transistors and offers fast access and low latency, widely used in cache memory.

Q7(b) (i) Operational Amplifier (ii) Successive Approximation DAC

Operational Amplifier:
An op-amp is a high-gain differential amplifier used for signal amplification, filtering, and analog computation.

Successive Approximation DAC:
This DAC converts digital signals into analog output using a binary search method, offering good speed and accuracy.