【Semiconductor】 ⏱️ 08-08. What Is NBTI? — How MOSFETs Degrade Over Time”

topics: [“Semiconductor”, “NBTI”, “Reliability”, “MOSFET”, “BSIM4”]

⏱️ Introduction

In the previous articles, we have examined initial MOSFET characteristics through:

• 🧪 TCAD for physical understanding
• 📐 BSIM4 for compact modeling
• 🔌 SPICE for electrical characterization

However, in real silicon devices, there is an unavoidable reality:

A device that works well today may not behave the same way five years later.

One of the most representative causes of this behavior is
👉 NBTI (Negative Bias Temperature Instability).

NBTI is a fundamental reliability issue that explains how
MOSFETs gradually degrade over time.

🔥 What Is NBTI?

NBTI mainly affects pMOSFETs when the following conditions coexist:

• Negative gate bias
• Elevated temperature
• Long operating time

It is therefore classified as a
time-dependent reliability degradation mechanism.

Its most distinctive feature is:

The threshold voltage $V_t$ shifts as a function of time.

🧠 What Is Happening Physically?

Under NBTI stress conditions, near the gate-oxide / silicon interface:

• Interface states are generated
• Trap states become activated and permanently charged

As a result:

• Even with the same applied $V_g$
• The same channel charge density cannot be formed

👉 The MOSFET becomes harder to turn on

This manifests as reduced on-current and increased delay.

📉 Impact on V–I Characteristics

NBTI effects appear directly in DC characteristics.

Typical observations include:

• A rightward shift of the $V_g$–$I_d$ curve
• Degradation of subthreshold behavior
• Reduced $I_d$ at the same $V_g$

All of these are consequences of:

Threshold voltage shift ($\Delta V_{th}$)

📐 How BSIM4 Treats NBTI

In BSIM4, NBTI is represented as:

• Degradation modeled through parameter variation
• Time-dependent $V_t$ shift
• Comparison of characteristics before and after degradation

A critical limitation must be noted:

BSIM4 alone does not directly solve time evolution

For this reason, SemiDevKit adopts a hybrid approach:

• 🧪 SPICE: Accurate measurement at $t = 0$
• 🧮 Python: Time-dependent degradation modeling for $t > 0$

🧰 NBTI Analysis with SemiDevKit

The following module is used:

• BSIM4 Analyzer Reliability
  https://samizo-aitl.github.io/SemiDevKit/bsim/bsim4_analyzer_reliability/

This framework provides fully automated:

• Initial VG–ID extraction
• Threshold voltage extraction using gmmax and constant-current methods
• Application of time-dependent degradation models
• Reconstruction of degraded device characteristics

🔬 NBTI Analysis Flow

t = 0
 ├─ VG–ID sweep
 │     ├→ Vtg0 (gmmax method)
 │     └→ Vtc0 (constant-current method)
 ├─ DC extraction
 │     └→ Idlin0 / Idsat0

t > 0
 ├─ Apply ΔVth(t) model
 ├─ Apply ΔId(t) model
 ├─ Reconstruct Vtg1 / Vtc1 / Idlin1 / Idsat1

→ Export CSV results
→ Generate degradation plots
→ Overlay VG–ID curves

🚀 Execution Example

cd bsim/bsim4_analyzer_reliability/run
python run_nbti_pmos.py

📊 Example Results

■ PMOS NBTI: Vg–Id Degradation (Linear Scale)

👉 Clear rightward shift of the Vg–Id curve
👉 Reduced drain current at the same gate voltage

■ PMOS NBTI: ΔVtg vs. Stress Time

👉 Degradation follows a power-law time dependence
👉 Early-stage degradation is dominant

⚠️ Why NBTI Matters

NBTI leads to:

• Reduced operating frequency
• Loss of timing margin
• Long-term reliability failure

It is especially critical for:

• Low-voltage operation
• SRAM and low-$V_{dd}$ logic
• Long-lifetime products (automotive, industrial)

👉 NBTI is a hard constraint, not an optional consideration.

🔗 TCAD / BSIM / SPICE Relationship

NBTI fits naturally into the same conceptual chain:

• TCAD: Physical processes at the interface
• BSIM4: Parameterized degradation models
• SPICE: Circuit-level impact assessment

👉 Reliability is also a “physics → model → circuit” problem.

📝 Summary

• ⏱️ NBTI is a time-dependent reliability degradation mechanism
• 🟦 It primarily affects pMOSFETs
• 📉 It appears as a threshold voltage shift
• 🧪 BSIM4 + SPICE + Python enable clear visualization

MOSFETs are no longer evaluated by:

“Does it work?”

but by:

“How long does it keep working?”

Next Article 👉

09: What Is HCI? — Why High Electric Fields Destroy MOSFETs