Introduction :

Hello everyone, I hope you are doing well. I'm Wolverine. I've completed my Master's in VLSI, and currently, I'm pursuing my PhD at one of India's leading institutes, IIT Guwahati. My PhD focuses on Radio Frequency Integrated Circuits (RFIC), which is an advanced version of analog VLSI. Throughout my PhD journey, I've completed two tape-outs and worked on multiple RFIC blocks. Today, I'd like to dive into the technical details of my work and guide students interested in this field.

Overview of My PhD Work :

My PhD is basically on analog and RF circuits. The background comes from basic circuits like analog circuits. If you go back to that, you'll find operational amplifiers (op-amps) and other basic building blocks of analog circuits. In my PhD, I work on an advanced version of these analog circuits.

I work on RFIC, which involves circuits like oscillators, mixers, power amplifiers, and distortion circuits. I've done two papers, which are basically on oscillators and some frequency generation circuits.

Key Subjects and Topics in Analog VLSI and RFIC :

If someone wants to explore RFIC or wants to go for a Master's in RFIC, they should have a basic understanding of analog circuits.

1. Basics of Analog Circuits:

   • MOSFET Operation: You should know the basic operation of a MOSFET.

   • Frequency Response of MOSFET: Understand how it behaves, the CGD (Gate-Drain Capacitance), CGS (Gate-Source Capacitance), and how these capacitances affect the frequency response.

   • Basic Amplifier Configurations: Know the basic common source, common drain, and common gate amplifiers. These are the basic building blocks.

   • Cascode and Telescopic Cascode Amplifiers: After understanding the basic amplifiers, you can move on to cascode and telescopic cascode configurations and learn how they work.

   • Differential Amplifiers: From the basic common source amplifier, you progress to differential amplifiers.

   • Operational Amplifiers (Op-Amps): From differential amplifiers, you move on to designing op-amps.

   • Instrumentation Amplifiers and Filters: With op-amps, you can design instrumentation amplifiers, filters, Gm-C filters, and other circuits.

2. Knowledge of Digital Circuits:

   • Digital Bits and Tuning: In RFIC, let's say you want to tune an oscillator. For tuning, you might require some digital bits to turn on or turn off particular parts of the circuit.

   • Switches and Impedance Matching: You should know about switches because to match an impedance, you might need to use switches.

   • Digital Control: Some knowledge of digital circuits is essential for controlling analog and RF circuits.

3. Understanding Filters:

   • Types of Filters: You should know about low-pass filters, high-pass filters, and band-pass filters.

   • Application in RFIC: Filters are used extensively in RFIC for signal conditioning and frequency selection.

Specific Topics in RFIC

RFIC starts from the same basics again—the common source amplifier—but the load in analog is usually a resistor. However, in RF, the load will change to an inductor because RF circuits are tuned circuits. Everything is about frequency tuning.

1. Load Considerations:

   • LC Tank Circuits: In RF, the load will be an LC tank circuit. This consists of an inductor (L) and a capacitor (C) and is used for frequency selection.

2. Frequency Challenges:

   • 2.4 GHz: If you want to work at 2.4 GHz, it has different challenges due to the wavelength and component sizes.

   • 39 GHz: Working at 39 GHz presents challenges like increased parasitic effects and the need for precise component values.

   • 270 GHz: At 270 GHz, you face significant challenges, including device limitations and advanced fabrication requirements.

3. Key RF Blocks:

   • Low Noise Amplifier (LNA): The first block in an RF receiver. It amplifies weak signals received by the antenna with minimal added noise.

   • Impedance Matching:

     • Smith Chart: You should know how to use the Smith Chart for impedance matching to ensure maximum power transfer and minimize reflections.

     • Matching Networks: Designing matching networks using inductors and capacitors.

   • Mixers:

     • Frequency Conversion: Mixers are used to convert signals from one frequency to another.

     • Local Oscillator (LO): For mixers, you need to generate a frequency internally using an oscillator.

   • Oscillators:

     • Frequency Generation: Oscillators generate periodic signals and are critical for timing and frequency references.

     • Types of Oscillators: Understanding different oscillator topologies like LC oscillators, ring oscillators, etc.

   • Power Amplifiers (PA):

     • Signal Transmission: Power amplifiers are used to increase the power level of the signal for transmission.

     • Design Considerations: Efficiency, linearity, and output power are key factors in PA design.

Tools and Software

I work with a tool called Cadence Virtuoso. It's not openly accessible and requires institutional licenses. However, there are tools that students can use:

1. Cadence Virtuoso:

   • Usage: Used for schematic entry, simulation, layout, and verification.

   • Simulations: Allows for advanced simulations like PSS and PNoise.

2. LTSpice:

   • Open Source: An open-source tool that's great for analog circuit simulation.

   • Capabilities: Can perform AC, DC, and transient analyses.

   • Suitability: More than sufficient for many analog projects.

3. Magic Layout Tool:

   • Learning Layout: An open-source tool to learn the basics of IC layout.

   • Features: Helps understand how to draw and verify layouts.

4. ADS (Advanced Design System):

   • RF Simulation: Specialized for RF and microwave circuit simulation.

   • Usage: Can be used to design and simulate RF circuits like amplifiers, oscillators, and mixers.

Analysis and Simulations in Cadence Virtuoso :

The basic simulation setup includes:

1. Basic Simulations:

   • AC Simulation: To analyze the frequency response of circuits.

   • Transient Simulation: To see how circuits behave over time.

   • DC Simulation: To find the operating point and biasing conditions.

2. Advanced Simulations:

   • Post-Layout Simulations:

     • EMX (Electromagnetic Extraction): Used for extracting parasitic inductance and capacitance in RF components like inductors.

     • PEX (Parasitic Extraction): For extracting parasitic resistances and capacitances from the layout, especially in transistors.

   • Specialized Analyses:

     • PSS (Periodic Steady-State Analysis): Used for circuits like oscillators to find the steady-state behavior.

     • PNoise (Phase Noise Analysis): For analyzing noise performance in oscillators.

     • Transient Noise Analysis: To simulate noise in time-domain simulations.

Details About My PhD Work on Oscillators and Frequency Generation :

1. Oscillator Design:

   • LC Oscillators: An oscillator typically consists of an LC tank circuit and a cross-coupled pair to provide the necessary feedback for oscillations.

   • Innovation: In my PhD, I aim to invent something new in oscillator design as part of contributing original research.

2. Tape-Out Process:

   • EMX and PEX:

     • EMX of Inductors: Electromagnetic extraction to accurately model inductors at high frequencies.

     • PEX of Transistors: Extracting parasitics from transistors to understand their impact on circuit performance.

   • Layout Considerations:

     • Routing: Arranging components and routing interconnections according to chip area constraints.

     • DC Pins: Proper placement and routing of VDD and Ground connections.

     • Verification:

       • LVS (Layout Versus Schematic): Ensuring the layout matches the schematic.

       • DRC (Design Rule Check): Checking that the layout complies with the fabrication process rules.

3. Tape-Out Procedure:

   • Foundry Collaboration: We collaborate with TSMC in Taiwan, which has scheduled tape-out cycles four times a year.

   • Booking Chip Area: We need to book the chip area in advance, for example, 1mm x 1mm.

   • Planning and Timeline:

     • Idea Generation: Should have an idea by January if the tape-out is in July.

     • Design Phase: Start working from February and complete all simulations and modifications by the end of May.

     • Layout Phase: Entire June is dedicated to layout, routing, and final checks.

4. Time Frame:

   • Overall Duration: It takes about 3 to 4 months for a tape-out from design to submission.

   • Intensive Work: The last month before tape-out is very intensive, focusing on layout and ensuring all rules are met.

Challenges Faced During Circuit Design and Physical Design :

1. High-Frequency Design Challenges:

   • Lack of Resources: There is a lack of material and references available online for high-frequency design.

   • Trial and Error: Much of the learning comes from experience and experimenting.

   • Parasitic Effects: Managing parasitic capacitances and inductances becomes critical at high frequencies.

2. Dependency on Foundries:

   • Fabrication: We rely on TSMC in Taiwan for chip fabrication.

   • Lead Times: Any delays or issues can impact research timelines.

3. Post Tape-Out Testing:

   • Receiving the Chip: After tape-out, the chip returns from fabrication after 3 or 4 months.

   • Testing in the Lab: We test the chip using proper probing techniques, which involves connecting wires to the tiny pads on the chip.

   • Probability of Success: The probability of the chip working as intended is about 35%.

   • Patience Required: Debugging can be time-consuming and requires a lot of patience.

4. Debugging Process:

   • Limited Options: After fabrication, you cannot change the internal circuitry.

   • Adjustments:

     • Supply Voltage: Modifying the supply voltage, e.g., increasing from 1.2V to 1.5V.

     • Biasing Conditions: Adjusting external bias currents or voltages.

   • Troubleshooting: Systematically testing and measuring to identify issues.

Recommendations for Students :

1. Projects to Work On:

   • Basic Op-Amp Design: Start with designing basic operational amplifiers to understand analog fundamentals.

   • Basic LNA Design: Work on designing low-noise amplifiers to get introduced to RF concepts.

   • Basic Oscillator Design: Design simple oscillators like LC or ring oscillators to learn about frequency generation.

2. Preparing for PhD Interviews:

   • Problem-Solving Skills: Interviewers may give you circuits or situations to solve, assessing how you approach the problem.

   • Fundamentals: Focus on having strong basics rather than just the final answer.

   • Understanding Concepts: Be prepared to explain your reasoning and understanding of core principles.

Pursuing a PhD in VLSI Compared to Other Domains

1. VLSI Specializations:

   • Analog, Digital, and Devices are the three main areas in VLSI.

   • Analog PhD: Involves more challenges due to the necessity of tape-outs and obtaining working silicon.

2. Challenges in Analog PhD:

   • Tape-Out Requirement: For analog and RFIC PhDs, tape-outs are essential to validate designs and publish papers.

   • Difficulty Level: Analog is considered more challenging compared to digital and device domains.

   • Dependence on External Factors: Reliance on fabrication facilities adds complexity.

3. Comparison with Other Fields:

   • Availability of Resources: Fields like chemistry or microbiology often have immediate access to materials and equipment.

   • Research Pace: Other domains may publish papers more frequently due to shorter experimentation cycles.

4. Publication Requirements:

   • Working Chip Needed: For analog PhDs, having a working chip is crucial for publishing papers.

   • Minimum Publications: Most IITs require at least one published paper for an analog PhD.

5. Potential Setbacks:

   • Chip Failure: If the chip doesn't work, it may require investing another year of work.

   • Unexpected Outcomes: Sometimes, designed circuits may not function as intended (e.g., an amplifier may oscillate).

Advice for Students Pursuing PhD in Analog Domain :

1. Patience and Perseverance:

   • Long Development Cycles: Be prepared for longer timelines compared to other fields.

   • Handling Setbacks: Develop resilience to cope with challenges and failures.

2. Passion for the Field:

   • Intrinsic Motivation: Pursuing a PhD in analog and RF requires a genuine interest and passion.

   • Commitment: Understand that the financial incentives during PhD are lower compared to industry jobs, but the experience is rewarding.

3. Focus on Your Journey:

   • Avoid Comparisons: Do not compare your progress with students from other domains.

   • Personal Growth: Focus on building your skills and knowledge.

4. Job Prospects and Motivations:

   • Industry Opportunities: After a Master's in VLSI, there are ample industry opportunities with attractive packages.

   • Academic Aspirations: If you are passionate about research and academia, pursuing a PhD is a fulfilling path.

Future Opportunities and Career Plans

1. Post-PhD Plans:

   • Postdoctoral Research: I aim to pursue a postdoc at a reputed university internationally.

   • Continued Research: Interested in advancing research in analog and RF circuits.

2. Academic and Research Goals:

   • Contribution to the Field: Aspiring to contribute to innovation and development in analog VLSI.

   • Long-Term Vision: Possibly aim for a faculty position or research lead in the future.

3. Notable Universities for Analog VLSI:

   • International Institutions:

     • UCLA (University of California, Los Angeles)

     • University of Minnesota

     • MIT (Massachusetts Institute of Technology)

   • Institutions in India:

     • IIT Madras

     • IIT Bombay

     • IIT Delhi

     • IIT Hyderabad (one or two faculties)

     • IIT Guwahati (one faculty)

Addressing Concerns About the Future of Analog ICs :

I've heard from peers that analog ICs and the analog field might be reaching a dead end due to limited innovation left. I strongly disagree with this notion.

1. Continuous Innovation:

   • Evolving Technologies: Every day, technology is changing, and analog circuits play a crucial role.

   • New Applications: Emerging fields constantly require innovative analog solutions.

2. Examples of Ongoing Innovation:

   • Noise-Canceling Earbuds: Require advanced analog circuits for high-gain microphones and noise cancellation algorithms.

   • Brain-Computer Interfaces: Involve detecting and amplifying extremely small neural signals, which is an analog challenge.

   • Advancements in Communication: Transitioning from 5G to 6G demands new analog front-end designs.

3. No End to Innovation:

   • Limitless Possibilities: There is no saturation point in analog; new challenges arise with every technological advancement.

   • Role of Analog: Analog circuits are essential for interfacing the physical world with digital systems.

4. Misconceptions and Human Mentality:

   • Avoiding Difficult Fields: Some people may say analog is ending because it's a challenging field, and they might prefer easier paths.

   • Persistence of Analog: Despite the challenges, analog continues to be a vital and dynamic field.

Conclusion

Thank you for your time and interest. Analog and RFIC design is a challenging yet incredibly rewarding field with immense potential for innovation. If you're passionate about circuits and willing to embrace the challenges, this field offers endless opportunities.

Remember, success in this domain requires patience, perseverance, and a genuine passion for the subject. Don't be discouraged by setbacks; they are part of the learning process.

Feel free to reach out if you have any questions or need further guidance. Have a great day!