【Semiconductor】📏 08-07. Visualizing MOSFET L/W Scaling and Short-Channel Effects with SPICE

topics: [“Semiconductor”, “MOSFET”, “Scaling”, “Short-Channel Effects”, “BSIM4”]

🚀 Introduction

MOSFETs are often described as devices where

“smaller means faster.”

However, in real silicon, shrinking a device also reveals another face:

• ⚠️ Reduced controllability
• ⚠️ Increased leakage current
• ⚠️ Enhanced parameter variability

In this article, using BSIM4 models and SPICE simulations, we perform
L/W dimensional scaling analysis to understand:

• Why device characteristics change when dimensions shrink
• Where textbook intuition breaks down in real devices

All observations are based on actual simulation results.

📐 What Is Dimensional Scaling?

MOSFET dimensional scaling examines the impact of changing:

• Channel length: $L$
• Device width: $W$

For an ideal long-channel MOSFET, one might expect:

• $I_d \propto W$
• $I_d \propto 1/L$

However, real devices deviate significantly from this ideal behavior.

The root cause of this deviation is known as
👉 Short-Channel Effects (SCE).

🧰 Analysis Environment (SemiDevKit)

This article uses the following DIM analysis module:

• BSIM4_ANALYZER_DIM
  https://samizo-aitl.github.io/SemiDevKit/bsim/bsim4_analyzer_dim/

Key Features

• Python + ngspice
• BSIM4 (130 nm educational model)
• Fully automated flow:
  • L/W-dependent model generation
  • Netlist creation
  • SPICE execution
  • CSV / PNG output

👉 Enables hands-free comparison of dimensional dependence.

🔬 Channel Length ($L$) Scaling

How to Run

cd bsim/bsim4_analyzer_dim/run
python run_vg_dim.py
python run_vd_dim.py

This analysis varies the channel length $L$ step by step and compares:

• Vg–Id (transfer characteristics)
• Vd–Id (output characteristics)

⚠️ What Happens When $L$ Becomes Shorter?

From the simulation results, we observe:

• 📉 Threshold voltage roll-off
• 📉 Reduced output resistance
• ⚡ DIBL (Drain-Induced Barrier Lowering)
• ⚠️ Degraded current saturation

These phenomena collectively define
👉 Short-Channel Effects (SCE).

■ Vg–Id (L Sweep, NMOS)

👉 Shorter $L$ lowers $V_{th}$ and increases leakage
👉 $g_m$ enhancement and loss of gate control occur simultaneously

■ Vd–Id (L Sweep, NMOS)

👉 Weaker saturation and stronger $V_d$ dependence
👉 A classic signature of DIBL

📏 Device Width ($W$) Scaling

How to Run

cd bsim/bsim4_analyzer_dim/run
python run_vg_dim.py
python run_vd_dim.py

(Only the fixed-width condition is changed.)

🤔 Intuition vs. Reality in $W$ Scaling

Intuitively, one might expect:

• Increasing $W$ → proportional increase in current

In practice, however, the following effects become significant:

• Parasitic resistance ($R_{dsw}$)
• Parasitic capacitance ($C_{gg}$)
• Narrow-width effects

👉 Increasing width does not solve everything
👉 This is a critical consideration in standard-cell design

🧠 The Role of BSIM4

BSIM4 explicitly includes:

• Threshold voltage roll-off
• DIBL
• Mobility degradation
• Narrow-width effects

as dimension-dependent parameters.

As a result, SPICE simulations alone can reveal:

“What breaks when the device is scaled.”

This makes BSIM4 an effective bridge between device physics and circuit design.

🔗 Relationship with TCAD

In TCAD simulations, short-channel scaling shows:

• Distorted electrostatic potential
• Drain electric field penetrating into the channel

BSIM4 DIM analysis:

• Compresses these spatial effects
• Into observable I–V characteristics

👉 BSIM4 acts as a bridge between TCAD and circuit-level design.

💡 The True Value of DIM Analysis

DIM analysis is particularly valuable for:

• SRAM β-ratio design
• Standard-cell drive-strength tuning
• Performance vs. leakage trade-off studies

👉 It reveals how far scaling can be pushed before functionality degrades.

📝 Summary

• 📏 L/W scaling strongly alters MOSFET characteristics
• ⚠️ Shorter $L$ introduces short-channel effects
• 📐 Increasing $W$ is not a simple linear solution
• 🧠 BSIM4 + SPICE enable realistic device behavior analysis

To understand MOSFETs as real devices rather than equations,
DIM analysis is an indispensable tool.

Next Article 👉

08: What Is NBTI? — How MOSFETs Degrade Over Time