Upon successful completion of this laboratory session, students will be able to:
● Navigate and effectively operate the core components of the Electronic Design Automation (EDA) tool suite designated for VLSI design (e.g., schematic editor, simulator, waveform viewer).
● Establish and manage a new design project, including proper directory structuring and library association.
● Perform schematic capture of fundamental circuit elements, including placing and wiring MOS transistors and voltage sources.
● Configure and execute basic SPICE (Simulation Program with Integrated Circuit Emphasis) simulations for DC and Transient analysis.
● Simulate and meticulously analyze the fundamental Current-Voltage (I-V) characteristics of both NMOS and PMOS transistors.
● Simulate and analyze the essential Capacitance-Voltage (C-V) characteristics of MOS transistors.
● Extract key transistor parameters such as threshold voltage (Vt) from simulated I-V curves.
● Understand and explain the profound impact of the Width-to-Length (W/L) ratio on MOS transistor electrical characteristics.

Integrated circuits, the foundation of modern electronics, are built from billions of interconnected transistors. The most prevalent type of transistor used in VLSI is the Metal Oxide Semiconductor (MOS) Field-Effect Transistor (MOSFET). Understanding its fundamental electrical characteristics is paramount for any VLSI designer.

The MOS transistor operates as a voltage-controlled switch or current source. Its behavior is primarily governed by the voltages applied to its four terminals: Gate (G), Drain (D), Source (S), and Bulk (B).

● NMOS (N-channel MOSFET): Conducts current when a sufficiently positive voltage is applied to its gate, creating an N-type channel between the source and drain. The bulk (substrate) is typically connected to the lowest potential (ground, GND).
● PMOS (P-channel MOSFET): Conducts current when a sufficiently negative voltage is applied to its gate (or sufficiently low relative to its source), creating a P-type channel. The bulk (N-well) is typically connected to the highest potential (VDD).

An understanding of the three main operating regions is critical for designing digital circuits:
● Cutoff Region: The transistor is OFF, acting as an open switch. For NMOS, this occurs when VGS < Vt (threshold voltage). For PMOS, when |VGS| < |Vt|.
● Triode Region (Linear Region): The transistor is ON and acts like a voltage-controlled resistor. For NMOS, VGS > Vt and VDS < (VGS - Vt). For PMOS, |VGS| > |Vt| and |VDS| < (|VGS| - |Vt|).
● Saturation Region: The transistor is ON and acts like a voltage-controlled current source, with drain current relatively independent of VDS. For NMOS, VGS > Vt and VDS >= (VGS - Vt). For PMOS, |VGS| > |Vt| and |VDS| >= (|VGS| - |Vt|).

These plots illustrate the relationship between the currents flowing through the transistor (specifically Drain Current, ID) and the voltages applied across its terminals.
● ID-VDS Curves (Output Characteristics): Shows ID vs. VDS for different fixed VGS values. These curves clearly delineate the triode and saturation regions.
● ID-VGS Curves (Transfer Characteristics): Shows ID vs. VGS for a fixed (typically high) VDS. This curve is used to extract the threshold voltage (Vt), the minimum gate voltage required to turn the transistor on.

MOS transistors exhibit various internal capacitances (e.g., gate-to-source Cgs, gate-to-drain Cgd, gate-to-bulk Cgb, and total gate capacitance Cgg). These capacitances are not constant; they vary with the applied terminal voltages and are crucial for determining circuit delay and dynamic power consumption.
● The gate capacitance, in particular, varies significantly as the transistor transitions through cutoff, depletion, and inversion, influencing how quickly the transistor can switch.

The physical dimensions of the transistor's channel (Width W and Length L) are critical design parameters. The W/L ratio profoundly influences the transistor's current driving capability (strength) and its associated parasitic capacitances.
● Current Drive: A larger W/L ratio (primarily larger W) leads to increased drain current (ID) for a given VGS, meaning the transistor can drive more current and thus switch faster or drive larger loads.
● Parasitic Capacitance: A larger W also increases the transistor's internal capacitances, which in turn increases the dynamic power consumption and contributes to signal delays.
● Trade-offs: Designers constantly optimize the W/L ratio to balance speed, power, and area requirements for specific applications.

EDA tools are specialized software suites used to design, simulate, verify, and lay out integrated circuits. They automate complex processes, significantly improving efficiency and accuracy.
● Schematic Editor: A graphical interface for drawing circuits using symbolic components. It translates the schematic into a textual netlist.
● SPICE Simulator: A powerful engine that takes the netlist and device models (mathematical descriptions of transistors for a specific fabrication process) as input. It then numerically solves the circuit equations to predict voltages and currents, performing various analyses like:
○ DC Analysis: Calculates the steady-state operating point (voltages/currents at a fixed input) or sweeps a DC input to show how the circuit responds.
○ Transient Analysis: Calculates circuit behavior over time, showing waveforms of voltages and currents, critical for dynamic analysis (delays, switching).
● Waveform Viewer: A post-processing tool to visualize and measure quantities from simulation results.