I-V Curve

I-V curve (current-voltage characteristic) maps the relationship between applied voltage and resulting current — the fundamental electrical fingerprint of semiconductor devices that reveals threshold voltage, on-resistance, leakage, and device physics.

What Is I-V Curve?

• Definition: Plot of current vs. voltage for a device.
• Axes: Voltage (x-axis), Current (y-axis, often log scale).
• Purpose: Characterize device electrical behavior.

Why I-V Curves Matter?

• Device Characterization: Complete electrical description of device.
• Model Extraction: Basis for SPICE models used in circuit design.
• Process Monitoring: Detect process variations and defects.
• Failure Analysis: Identify degradation mechanisms.

Transistor I-V Regions

Linear Region: Low VDS, current proportional to VDS.
Saturation Region: High VDS, current saturates.
Subthreshold Region: Below threshold, exponential I-V.
Breakdown Region: High voltage, avalanche breakdown.

Key Parameters Extracted

Threshold Voltage (Vth): Voltage where transistor turns on.
On-Current (Ion): Drive current in saturation.
Off-Current (Ioff): Leakage current when transistor off.
Subthreshold Slope (SS): How sharply transistor turns on/off.
On-Resistance (Ron): Resistance in linear region.
Output Resistance: Slope in saturation region.
DIBL: Drain-induced barrier lowering.

Measurement Types

Id-Vg: Drain current vs. gate voltage (transfer characteristic).
Id-Vd: Drain current vs. drain voltage (output characteristic).
Ig-Vg: Gate current vs. gate voltage (gate leakage).
Log Scale: Subthreshold region visible on log plot.

What I-V Curves Reveal

Process Variations: Vth shifts indicate doping or implant issues.
Mobility: Slope in linear region reveals carrier mobility.
Series Resistance: Deviation from ideal I-V at high current.
Short Channel Effects: DIBL, velocity saturation.
Leakage Mechanisms: Subthreshold slope, gate leakage.

Applications

Model Extraction: Generate SPICE models for circuit simulation.
Process Monitoring: Track Vth, Ion, Ioff across lots.
Device Optimization: Tune process for target I-V characteristics.
Reliability Testing: Monitor I-V changes under stress.

Analysis Techniques

Linear Extrapolation: Extract Vth from linear region.
Transconductance: gm = dId/dVg reveals mobility.
Subthreshold Slope: SS = dVg/d(log Id) indicates interface quality.
DIBL Calculation: Vth shift with VDS.

I-V Curve Factors

Channel Length: Shorter channels have higher Ion, more short-channel effects.
Oxide Thickness: Thinner oxides increase drive current.
Doping: Affects Vth, subthreshold slope, junction leakage.
Temperature: Mobility decreases, leakage increases with temperature.
Stress: Mechanical stress modulates mobility and Vth.

Comparison to Models

• Overlay measured I-V with SPICE model predictions.
• Identify discrepancies in mobility, series resistance, or leakage.
• Refine models to match measured behavior.
• Validate models across process corners.

Reliability Monitoring

BTI: Vth shift under bias temperature stress.
HCI: Degradation from hot carrier injection.
TDDB: Gate leakage increase before breakdown.
NBTI/PBTI: Negative/positive bias temperature instability.

Advantages: Complete device characterization, model extraction, process monitoring, failure analysis.

Limitations: Time-consuming for full characterization, requires multiple test structures, temperature and bias dependent.

I-V curves are foundational electrical fingerprint — enabling engineers to tune process recipes, extract models, and ensure device behavior matches design requirements across all operating conditions.