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Check out the extensive list of topics we discuss: 

1. Communication Protocols:
   - USB 
   - RS232 
   - Ethernet 
   - AMBA Protocol: APB, AHB and ASB 
   - UART, I2C AND SPI
2. Important concepts in VLSI:
   - Designing a Chip? Here Are the 12 Important Concepts You Need to Know
   - Metastability 
   - Setup time and Hold time
   - Signal Integrity and Crosstalk effect
   - Skews and Slack 
   - Antenna Effect
3. Semiconductor Memories
   
4. Most Frequently Asked Questions in VLSI
5. Transistors:
   - BJT
   - JFET
   - MOSFET
   - CMOS
   - Transmission Gate CMOS
   - Dynamic CMOS
6. Sequential Circuits:
   - Registers
   - Counters
   - Latches
   - Flip Flops
7. FPGA:
   - ASIC vs FPGA
   - FPGA Insights: From Concept to Configuration
   - Full-Custom and Semi-Custom VLSI Designs: Pros, Cons and differences
   - From Theory to Practice: CMOS Logic Circuit Design Rules Made Easy with Examples
8. CMOS Fabrication:
   - CMOS Fabrication
   - Twin-Tub CMOS Technology
9. Combinational Circuits
   - Logic Gates 
   - Boolean Algebra and DeMorgan's Law 
   - Multiplexer (MUX) and Demultiplexer (DEMUX) 
   - Half Adder
   - Full Adder
   - Half Subtractor
   - Full Subtractor
10. Verilog
    - Verilog Datatypes
    - Comments, Numeral Formats and Operators
    - Modules and Ports
    - assign, always and initial keywords
    - Blocking and Non-Blocking Assignments
    - Conditional Statements
    - Looping Statements
    - break and continue Statement
    - Tasks and Functions
    - Parameter and generate
    - Verilog Codes
11. System Verilog: 
    - Disable fork and Wait fork.
    - Fork and Join.
12. Project on Intel Quartus Prime and Modelsim:
    - Vending Machine Controller
13. Xilinx Vivado Projects
    1)VHDL
    - Counters using Testbench code
    - Flip Flops using Testbench code
    - Logic Gates using Testbench code
    - Full Adder using Half Adder and Testbench code
    - Half Adder using Testbench code
    2)Verilog
    - Logic Gates using Testbench code
    - Counters using Testbench code
    - Full Adder using Half Adder and Testbench code
    - Half Adder using Testbench code
14. VLSI Design Flow:
    - Design Flow in VLSI
    - Y chart or Gajski Kuhn Chart
15. Projects on esim:
    - Step-by-Step guide on how to Design and Implement a Full Adder using CMOS and sky130nm PDK
    - Step-by-Step guide on how to Design and Implement a Half Adder using CMOS and sky130nm PDK
    - Step-by-Step guide on how to Design and Implement a 2:1 MUX using CMOS and sky130nm PDK
    - Step-by-Step guide on how to Design and Implement a Mixed-Signal Circuit of 2:1 Multiplexer
16. IoT based project:
    - Arduino
    - Step-by-Step guide on how to Interface Load Cell using Arduino
17. Kmaps:
    - Simplifying Boolean Equations with Karnaugh Maps - Part:2 Implicants, Prime Implicants and Essential Prime Implicants. 
    - Simplifying Boolean Equations with Karnaugh Maps - Part:1 Grouping Rules.
    - Simplifying Boolean Equation with Karnaugh Maps.

Dive deeper into the world of VLSI Design Methodologies

Welcome to our exploration of VLSI (Very Large Scale Integration) design methodologies. In the realm of VLSI, designers have two primary approaches: Full Custom Design and Semi-Custom Design. Each has its strengths and weaknesses, catering to different project requirements and constraints.

1] Full Custom Design: Full Custom Design involves creating and verifying all components of a chip from the transistor level upward. This meticulous process allows for precise optimization of speed, power, and area, making it ideal for mass production. However, it comes with a trade-off — the design and production time of full custom designs are typically longer compared to semi-custom designs.

• Advantages of Full Custom Design:

1. Optimized performance: Full custom designs can achieve higher performance levels compared to semi-custom designs.
2. Area efficiency: By tailoring every component, full custom designs can minimize chip area.
3. Power optimization: Fine-grained control over individual components enables power optimization.

• Disadvantages of Full Custom Design:

1. Longer design and production time: Designing and verifying each component from scratch can be time-consuming.
2. Not cost-efficient for small-scale projects: The high upfront investment in design and fabrication makes full custom design less suitable for low-volume productions.

2] Semi Custom Design: Semi-Custom Design offers a compromise between design flexibility and time-to-market constraints. In this methodology, pre-designed and pre-tested modules are used, with the option to customize and add additional components as needed. While it reduces design time, it may not be as optimized or cost-efficient for mass production compared to full custom design.

• Advantages of Semi-Custom Design:

1. Reduced design time: Leveraging pre-designed modules accelerates the design process.
2. Flexibility: Additional components can be integrated to meet specific requirements without starting from scratch.
3. Suitable for low-volume productions: Semi-custom design offers a balance between customization and cost-effectiveness for smaller-scale projects.

• Disadvantages of Semi-Custom Design:

1. Limited optimization: Pre-designed modules may not offer the same level of performance optimization as full custom designs.
2. Higher production time: Despite faster design time, integrating and customizing modules can extend the overall production time.

• Performance analysis of design methodology w.r.t design time:

Performance Graph

From the above graph we can conclude following points:

1. Initial circuit performance of the full custom design is less as compared to semi-custom design as we design it from the basic transistor level.
2. Semi-custom has design less time compared to full custom design to get a stable circuit performance.
3. Circuit performance of the full custom design is higher than the semi-custom design.

• Comparison between Full Custom and Semi-Custom design:

Comparison between Full Custom and Semi-Custom Design

• Classification of Semi-Custom Design:

1] Standard Cell Design:

Standard Cell

• The above diagram shows standard cells semi-custom design.
• Standard cells particularly use standard cell libraries which typically contain a few thousand cells, including inverters, Logic Gates, Adder circuits, Flip flops, AOI/OAI, etc.
• Each gate in this library will have its driving capabilities and parameters based on which we decide which block to be used.
• These standard cells have interconnects between them to perform different functions.
• The masking cost of standard cells is high.
• These cells can be used for both analog and digital designs.
• We don't perform floorplanning in a standard cell as blocks are already placed.
• Examples: Mux, Demux, Flip Flops, Logic Gates, Inverter, Adders, etc.

2] Gate Array Design:

Gate Array

• The above diagram shows the structure of the Gate Array.
• Here, the position of logic blocks is fixed.
• These logic blocks can be connected using interconnection wires to perform different functions.
• Structure also has I/O ports for interfacing.
• Here we program wires and via to implement the desired functions.
• In the gate array, the logic blocks are already placed in pre-defined positions but not connected to each other.
• These gate arrays reduce making costs and also reduce design time.
• As the size, no of logic blocks, transistors are fixed efficiency is less.

3] Programmable Logic Devices:

• These are the IC that can be programmed according to the required specifications.
• Some PLDs are :
  1] SPLD (Simple Programmable Logic Devices)
  2] CPLD (Complex Programmable Logic Devices)
  3] FPGA (Field Programmable Logic Devices)
• Comparison between different Design styles:

Comparison between different Design styles

In conclusion, both full custom and semi-custom design methodologies play crucial roles in VLSI design. While full custom design offers unparalleled optimization and performance, semi-custom design provides flexibility and faster time-to-market for smaller-scale projects. Understanding the strengths and weaknesses of each approach is essential for selecting the most suitable methodology based on project requirements and constraints.

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Fabricating nMOS and pMOS with Twin-Tub CMOS Technology: A Step-by-Step Guide.

 

• In the previous blog CMOS Fabrication Process, we have discussed the CMOS Fabrication Process of nMOS and pMOS on a p-type substrate.
• This fabrication process will have some disadvantages such as the crosstalk effect, latch-up effect, and mutual coupling.
• This will generate issues regarding the speed and performance of CMOS.
• To avoid those issues, we must fabricate nMOS and pMOS using Twin Tub Fabrication Process.
• The below diagram shows the final stage of the Twin Tub Fabrication Method.

• Here, we will have two different wells n-well and p-well.
• nMOS will be fabricated on p-well and pMOS will be fabricated on n-well.
• Previously we have seen that nMOS and pMOS are fabricated on the p-type substrate but here we will have clear isolation between n-well and p-well which will help in avoiding issues regarding latch-up and mutual coupling.
• Steps for Twin Tub Fabrication Process are as follows:

Step 1] First we will have an n-type substrate with high resistance. Due to high resistance substrate current will be less. Higher resistivity can be obtained by having less doped n-type substrate.

Figure 1

Step 2] After that we will grow an Epitaxial Layer of n+ type material on the n-type substrate. This Epitaxial Layer will have a bit higher doping concentration than the n-type substrate. This layer will also have lesser resistance compared to the n-type substrate.

Figure 2

Step 3] Next we will grow a SiO2 layer above Epitaxial Layer and n-type substrate. The oxidation process can be used to grow the SiO2 layer.

Figure 3

Step 4] Now we need to have n-well and p-well for nMOS and pMOS. Hence we will etch the SiO2 layer for n-well and p-well diffusion. SiO2 layer is etched using Masking. Two windows will be provided one for n well and the other for p well.

Figure 4

Step 5] After etching the SiO2 layer, we will be masking the 1st window by having photoresist material. and will have a diffusion of p+ material on the 2nd window to form p well.

Figure 5

Step 6] Now we will mask the 2nd window by having photoresist material and diffuse the 1st window with n+ material to form n well.

Figure 6

Step 7] Now by having thermal oxidation we will be growing a thin SiO2 layer for having a Gate terminal.

Figure 7

Step 8] For photolithography and pattern making we will grow a polysilicon layer above the SiO2 layer.

Figure 8

Step 9] For implant Source and Drain terminals we now etch the polysilicon and SiO2 layer.

Figure 9

Step 10] After having the paths to grow source and drain terminals we will perform the masking steps again to form the two terminals. Here, first we will cover the 2nd window with photoresist material and diffuse the 1st window with p+ material to form Source and Drain.

Figure 10

Step 11] After implanting Source and Drain terminals on n well, we now cover the 1st window and diffuse the 2nd window with n+ material for growing Source and Drain on the p well.

Figure 11

Step 12] After having the complete internal structure we now perform metallization for contact formation.

Figure 12

Step 13] After diffusion we need to cut this metal using metal etching to provide individual contacts for Gate, Source, and Drain.

Figure 13

In this way, to avoid the issues with CMOS Fabrication Process, we use Twin Tub Fabrication Process.

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