VLSI: As its full name is “ Very large scale integration” , something to do with large no of transistors integration.
Where VLSI is used : it is used to develop any electronic device with a lot of functionalities
Like : Mobile processor , Computer processor /server , Bluetooth device /Wireless Device /Memory
Now if you have to make processor then you need to know how processor works , for that you need to know Digital Logic Design
How NAND, NOR, other logic gates , muxes,decoder, flip flop,latch , counter , moore/mealy machine machine works to get the logic of processor implemented
again to know, how NAND/NOR is designed
you have to know CMOS logic
Using basic switch mechanism you can make all digital component like NOR gate, NAND Gate, OR, AND,.. MUX, Decoder, Flip flop, latch ,../../
CMOS logic is basic component, best switch available into the market ,
CMOS logic : how can you design , how can you set its parameter like W/L ratio ,its Fabrication methodologies , insulating material , doping of source and drain material …/…/ (VLSI Technology)
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Going back to the Digital Logic Design
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How can you implement a certain logic knowing the digital design , you need to know HDL language like System verilog , Verilog, VHDL
Once design is done : Again verification guys should know system verilog/c++/../ to verify the logic design
I took example of design and verification in term of digital design
Same thing is applicable in Analog , instead digital , analog design have to be studied , CMOS is basic component there also
For Analog : Basic Electronics component working should be known to implement any analog component .
All Things to need to studied in details to know how VLSI design works , for specific things one can opt for certain thing .