Introduction to VLSI Design Flow and Basic MOSFET

Introduction to VLSI

1. VLSI Design Introduction to VLSI

2. Definition of a Circuit A circuit is a closed loop or network of electronic components (e.g., resistors, capacitors, transistors) connected by conductive paths (wires or PCB traces) that enables electric current to flow and perform specific functions, such as amplification, computation, or signal processing. In the evolution of electronics: Initially, circuits were built using discrete components on a Printed Circuit Board (PCB) , where individual elements were physically wired together.

3. Need of Integrated Circuit (IC) Before ICs, electronic systems were built with discrete components (vacuum tubes, then individual transistors, resistors, capacitors). This caused several problems: Size & Weight Issues Circuits with thousands of discrete components were bulky and heavy. Example: Early computers filled entire rooms. High Power Consumption & Heat Vacuum tubes and even discrete transistors consumed a lot of power and generated heat. Reliability Problems More components → more soldering → more failures. Difficult to maintain and repair. Manufacturing & Cost Limitations Building large systems with discrete components was expensive and time-consuming. Need for Miniaturization Military, aerospace, and space missions (like Apollo) required smaller, lighter, more reliable systems.

4. Moore’s Law

5. Evolution of Integrated Circuits Very Large Scale Integration (VLSI)

6. Definition of VLSI VLSI is a process used in electronics to create integrated circuits (ICs) by combining millions (or even billions!) of transistors onto a single chip.

7. VLSI Design Flow 1 2 3 4 5 6 7 8

8. VLSI Design Flow

9. VLSI Design Flow 1. Specification Every chip begins with a clear purpose. The specification stage defines the goals , functionalities , and performance requirements of the chip.  This includes factors like processing speed , power consumption , area constraints , and the specific tasks the chip needs to perform. For example, is it a chip for a smartphone’s processor or an AI accelerator? Specifications serve as a blueprint for all subsequent stages.

10. VLSI Design Flow 2 . Design Architecture Once the specifications are defined, the architecture stage outlines the overall structure of the chip. This is where engineers decide how the chip will function at a high level. They determine the components it will include, such as processors , memory units, communication buses, and interfaces, and how these components will interact. The architecture acts as a roadmap, ensuring all components work together seamlessly to achieve the desired functionality.

11. VLSI Design Flow 3 . Logic Design At this stage, engineers dive into the logical representation of the chip. They use hardware description languages (HDLs) like Verilog or VHDL to describe the chip’s behavior . This is similar to coding software, but instead of programming a computer, engineers are defining how the hardware will behave. For example, they might define how a processor fetches, decodes, and executes instructions or how data flows through a memory subsystem. Simulation tools are used extensively here to verify the logic and identify errors.

12. VLSI Design Flow 4 . Circuit Design The circuit design stage translates the logical design into an actual electronic circuit . Engineers select the types of transistors, logic gates, and other components to implement the desired behavior . They optimize the circuit for performance, power efficiency, and reliability. At this stage, factors like signal timing, voltage levels, and noise immunity are carefully analyzed to ensure the circuit operates correctly in real-world conditions.

13. VLSI Design Flow 5. Physical Design In the physical design stage, the circuit diagram is turned into an actual layout that can be built on a silicon chip. “Logical design is the plan, physical design is the construction on silicon.” Placement: Determining where each component (e.g., logic gates, transistors) will physically reside on the chip. Routing: Connecting these components with metal wires to allow communication. Optimization: Adjusting the layout to minimize delays, power consumption, and chip area while maximizing performance. Engineers use specialized CAD (like Cadence or Synopsys) to perform these tasks. The final output of this stage is a detailed layout file that guides the chip fabrication process.

14. VLSI Design Flow 6. Fabrication With the design finalized, the chip moves to the fabrication stage, which takes place in a semiconductor foundry. Here’s how it works: A silicon wafer is prepared as the base material. Photolithography, etching, and deposition techniques are used to layer the circuits onto the wafer. Transistors, interconnects, and other components are built layer by layer, creating the integrated circuit. This stage requires extreme precision, as the smallest misalignment can render the chip non-functional.

15. VLSI Design Flow 7. Testing and Validation Once the chip is fabricated, it undergoes rigorous testing and validation. This ensures that the chip performs as intended and meets the original specifications. Testing includes: Functional Testing: Verifying the chip’s logical operations. Performance Testing: Checking parameters like speed, power consumption, and thermal stability. Stress Testing: Evaluating how the chip performs under extreme conditions. Any errors or defects identified during testing are analyzed, and the design may be revised to address them before mass production.

16. VLSI Design Flow 8 . Packaging and Deployment After successful validation, the chip is packaged to protect it from physical damage and facilitate its connection to external components. The packaged chips are then integrated into devices like smartphones, computers, cars, or other systems.

17. MOS Layers MOS – Metal Oxide Semiconductor

18. MOS Layers Metal Layer (Gate Terminal) Usually made of aluminium or polysilicon. Serves as the gate electrode where input voltage is applied. Controls the flow of carriers in the semiconductor channel. Oxide Layer (Dielectric Layer) Made of silicon dioxide (SiO₂). Acts as an insulator between the gate (metal) and the semiconductor substrate. Prevents direct current flow while allowing electric field coupling . The thickness of the oxide determines the gate capacitance and switching speed. Semiconductor Layer (Substrate) Typically, silicon (p-type or n-type). Forms the channel region between the source and drain. When a voltage is applied at the gate, carriers (electrons or holes) are induced in this region, forming a conductive channel.

19. Types of MOSFET MOS – Metal Oxide Semiconductor

20. Enhancement Type nMOS Transistor

21. Depletion Type nMOS Transistor

22. MOS Transistor Symbols (IEEE Standard)

23. MOS Transistor Symbols (IEEE Standard)

24. MOS Transistor Symbols - Simplified For a pMOS transistor , the bubble is drawn at the gate terminal . This indicates that the device turns ON when the gate voltage is LOW . For an nMOS transistor , there is no bubble at the gate. It turns ON when the gate voltage is HIGH .

25. MOS Transistor as Switch

26. Accumulation, Depletion & Inversion Mode Accumulation Mode Depletion Mode Inversion Mode

27. ENHANCEMENT MODE TRANSISTOR ACTION

28. ENHANCEMENT MODE TRANSISTOR ACTION

29. ENHANCEMENT MODE TRANSISTOR ACTION

30. ENHANCEMENT MODE TRANSISTOR ACTION

31. ENHANCEMENT MODE TRANSISTOR ACTION

32. Long Channel I-V Characteristics The long-channel I-V characteristics of a MOSFET describe how the drain current ( ) varies with respect to the applied voltages at the gate , drain , and source terminals. The model assumes that the channel length is long enough that the lateral electric field (the field between source and drain) is relatively low, which is no longer the case in nanometer devices. This model is variously known as the long-channel , ideal , first-order , or Shockley model .  

33. Long Channel I-V Characteristics Cut-Off Region When , the transistor is in OFF state. Therefore, Linear Region We know that charge on each plate of capacitor is . Thus, the charge in the channel is Where, is the amount of voltage attracting charge to the channel beyond the minimum required to invert from p to n.  

34. Long Channel I-V Characteristics The average gate to channel potential is given by If the gate has length L and width W and the oxide thickness is t ox , the Gate Capacitance is is given as Where, The Gate Capacitance per Unit Area is given by  

35. Long Channel I-V Characteristics Therefore equation (3) becomes   Substitute (2) and (3) in (1) Each carrier in the channel is accelerated to an average velocity proportional to the lateral electric field, i.e., the field between source and drain. The constant of proportionality is called mobility .    

36. Long Channel I-V Characteristics The time required for carriers to cross the channel is the channel length divided by the carrier velocity . Therefore, the current between source and drain is the total amount of charge in the channel divided by the time required to cross.   Substitute (4) and (5) in (6)    

37. Long Channel I-V Characteristics     Where,   In this term geometry dependent factor   Equation (II) represents Linear Region of operation .  

38. Long Channel I-V Characteristics SATURATION REGION Saturation occurs when the channel is pinched off at the drain end. This is the saturation condition .   Substituting in equation (II)    

39. Long Channel I-V Characteristics Where,  

40. Career Paths in VLSI Role Responsibilities Key Skills Front-End Design Engineer Logical chip design using HDLs; writing/simulating code; verifying logic design Logic design, HDLs (Verilog, VHDL), simulation tools Back-End Design Engineer Physical chip design; placement and routing; optimizing for power & performance CAD tools (Cadence, Synopsys, Mentor Graphics) Verification Engineer Ensures error-free chips; testbench creation; simulations; debugging UVM/OVM, SystemVerilog, verification methods Analog Design Engineer Designs analog circuits; develops high-performance components; integrates with digital Circuit design, SPICE simulation FPGA Design Engineer FPGA circuit implementation; prototyping and hardware solutions in FPGA FPGA programming, Verilog/VHDL, debugging Embedded Systems Engineer Designs embedded systems; hardware-software integration; system-level solutions Embedded C, RTOS, hardware-software co-design Semiconductor Process Engineer Chip fabrication; process optimization; ensures production quality Semiconductor physics, fabrication processes

41. Career Paths in VLSI Career Path Role Responsibilities Key Skills Front-End Design Engineer Focuses on the logical design of chips using hardware description languages (HDLs) like Verilog and VHDL. Writing and simulating code, ensuring correct functionality, and verifying the logic design. Logic design, HDLs, simulation tools. Back-End Design Engineer Works on the physical design of chips, ensuring that the layout meets performance and area requirements. Placement, routing, and optimizing chip design for power and performance. CAD tools like Cadence, Synopsys, and Mentor Graphics. Verification Engineer Ensures that the chip design is error-free and meets specifications. Creating testbenches, running simulations, and debugging issues. Verification methodologies like UVM/OVM, and programming in SystemVerilog. Analog Design Engineer Designs analog circuits like amplifiers, oscillators, and power management units. Developing high-performance analog components and integrating them with digital systems. Circuit design, SPICE simulation. FPGA Design Engineer Specializes in designing and implementing circuits on Field Programmable Gate Arrays (FPGAs). Developing and testing FPGA-based solutions for applications like prototyping and custom hardware. FPGA programming, VHDL/Verilog, debugging. Embedded Systems Engineer Combines VLSI knowledge with software to design embedded systems for specific applications. Designing system-level solutions and ensuring seamless hardware-software integration. Embedded C, RTOS, hardware-software co-design. Semiconductor Process Engineer Works in chip fabrication, focusing on processes involved in manufacturing integrated circuits. Optimizing fabrication techniques and ensuring high-quality production. Semiconductor physics, fabrication processes.

42. Required Skills for a Career in VLSI

43. Salary of VLSI Designers in India Salaries for VLSI designers in India vary based on factors like experience, location, and the specific employer. Here’s an overview: Entry-Level Positions : Fresh graduates can expect starting salaries ranging from ₹3 lakh to ₹5 lakh per annum. Source: Collegedunia Mid-Level Positions (2–5 years of experience) : Professionals in this bracket typically earn between ₹5 lakh and ₹13 lakh per annum. Source: Collegedunia Senior Positions (5+ years of experience) : Experienced VLSI designers can command salaries upwards of ₹20 lakh per annum, with some earning as high as ₹37 lakh per annum.  Source: AmbitionBox