Dynamic RDS(on) Effects in GaN Devices Explained: Current Collapse, Trapping, Measurement and Reduction

GaN Power Electronics Masterclass – Part 40

This lesson is part of the Complete GaN Power Electronics Masterclass.

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Dynamic R_DS(on) Effects in GaN Devices: Current Collapse, Trapping, Measurement and Reduction

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Table of Contents

• Introduction
• What is Dynamic R_DS(on)?
• Static vs Dynamic R_DS(on)
• Why Dynamic R_DS(on) Occurs in GaN Devices
• Current Collapse Explained
• Surface Trapping Effects
• Buffer Trapping Effects
• Impact on Power Converter Performance
• Effect on Efficiency and Thermal Stress
• Measurement Techniques
• Double Pulse Test for Dynamic R_DS(on)
• Factors Affecting Dynamic R_DS(on)
• Reduction Techniques
• GaN vs Silicon vs SiC Comparison
• Applications
• Future Trends
• Frequently Asked Questions
• Conclusion

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Introduction

Gallium Nitride High Electron Mobility Transistors are widely used in modern power electronics because they offer high switching speed, low gate charge, low output capacitance, and excellent power density. However, GaN devices also have unique reliability and performance challenges that must be understood carefully. One of the most important among them is Dynamic R_DS(on). Dynamic R_DS(on) refers to the temporary increase in ON-state resistance after the device has experienced high-voltage OFF-state stress or fast switching operation. This effect is mainly caused by charge trapping in the surface, buffer, or interface regions of the GaN device. In practical converters, dynamic R_DS(on) can increase conduction loss, raise junction temperature, reduce efficiency, distort current waveforms, and affect long-term reliability. Therefore, it is a critical parameter for GaN-based fast chargers, AI data center power supplies, EV onboard chargers, telecom converters, renewable energy systems, and MHz-class DC-DC converters.

Key Takeaway Dynamic R_DS(on) is the temporary increase in ON resistance after high-voltage switching stress. It is mainly caused by trapped charges and is closely related to current collapse in GaN HEMTs.

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What is Dynamic R_DS(on)?

R_DS(on) is the drain-to-source resistance of a transistor when it is fully turned ON. In an ideal device, this resistance would remain constant during operation. In a real GaN HEMT, R_DS(on) can increase temporarily after high-voltage OFF-state operation. This temporary increase is called dynamic R_DS(on).

Device OFF at High Voltage

↓

Charges Become Trapped

↓

Device Turns ON

↓

2DEG Channel Partially Depleted

↓

RDS(on) Temporarily Increases

↓

Trapped Charges Slowly Release

↓

RDS(on) Recovers

The effect may last from nanoseconds to milliseconds depending on trap type, temperature, voltage stress, and device structure.

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Static vs Dynamic R_DS(on)

Parameter	Static RDS(on)	Dynamic RDS(on)
Measurement Condition	Steady-state low-voltage condition	Measured after high-voltage switching stress
Main Cause	Channel resistance, contacts, package resistance	Charge trapping and current collapse
Datasheet Availability	Usually specified directly	Often shown through special test curves or application notes
Effect on Converter	Determines normal conduction loss	Increases real operating conduction loss
Time Dependence	Nearly constant for fixed conditions	Strongly time-dependent

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Why Dynamic R_DS(on) Occurs in GaN Devices

Dynamic R_DS(on) occurs because GaN HEMTs are sensitive to charge trapping. During high-voltage OFF-state operation, strong electric fields exist near the gate edge, drain access region, surface, and buffer layer. These fields can inject or trap electrons in defect states. When the device turns ON again, trapped charges locally deplete the 2DEG channel. Because the channel has fewer available electrons, its resistance increases temporarily.

• High electric field creates carrier trapping.
• Surface and buffer traps store charge.
• Trapped charge modifies local electric field.
• The 2DEG channel becomes partially depleted.
• ON resistance increases temporarily.

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Current Collapse Explained

Current collapse is the reduction of drain current caused by charge trapping in GaN HEMTs. It is closely related to dynamic R_DS(on). If the same gate and drain voltage are applied but the measured drain current is lower after stress, current collapse has occurred.

Before Stress:

Normal 2DEG Density

↓

High Drain Current

↓

Low RDS(on)


After High-Voltage Stress:

Trapped Charges

↓

Reduced 2DEG Density

↓

Lower Drain Current

↓

Higher Dynamic RDS(on)

In power converters, current collapse appears as higher conduction loss after switching transitions.

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Surface Trapping Effects

Surface traps are defect states located at or near the surface between the gate and drain. These traps can capture electrons during high-voltage operation. The trapped electrons create a negative charge that reduces the electron density in the 2DEG channel below the surface.

Common Causes of Surface Traps

• Surface contamination.
• Plasma etch damage.
• Dangling bonds.
• Poor passivation quality.
• High electric field near gate edge.

Reduction Methods

• High-quality SiN passivation.
• Improved surface cleaning.
• Optimized field plate design.
• Low-damage etching processes.
• Better dielectric interface control.

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Buffer Trapping Effects

Buffer traps exist inside the GaN buffer layer or transition layers. These traps are often introduced intentionally or unintentionally during epitaxial growth. A highly resistive buffer helps reduce leakage current, but trap-rich buffers can increase dynamic R_DS(on).

This creates a design trade-off: the buffer must block high voltage and suppress leakage, but it must not introduce excessive charge trapping that degrades switching performance.

Engineering Insight The best GaN devices balance low leakage current and low dynamic R_DS(on). Too many buffer traps may improve blocking capability but worsen current collapse.

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Impact on Power Converter Performance

Dynamic R_DS(on) affects real converter efficiency because the ON resistance during operation may be higher than the static datasheet value.

• Increases conduction loss.
• Raises junction temperature.
• Reduces efficiency at high frequency.
• Changes current sharing in parallel devices.
• Reduces reliability margin.
• Can distort switching waveforms.
• Increases thermal stress during hard switching.

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Effect on Efficiency and Thermal Stress

Conduction loss increases when R_DS(on) increases. Even a temporary rise in resistance can cause higher average loss in high-frequency converters because switching events repeat continuously.

Dynamic RDS(on) ↑

↓

Conduction Loss ↑

↓

Junction Temperature ↑

↓

Mobility ↓

↓

RDS(on) Further Increases

↓

Thermal Stress Increases

This feedback makes dynamic R_DS(on) an important electro-thermal design parameter.

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Measurement Techniques

Dynamic R_DS(on) measurement is more difficult than static measurement because the resistance must be captured immediately after switching stress before trapped charges recover.

Method	Purpose
Pulsed I-V Measurement	Measures current collapse after voltage stress.
Double Pulse Test	Evaluates real switching behavior and dynamic resistance.
Fast Clamp Circuit	Protects measurement instrument during high voltage switching.
Oscilloscope Method	Measures VDS(on) and current during turn-on.
Temperature-Controlled Testing	Evaluates trap behavior at different temperatures.

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Double Pulse Test for Dynamic R_DS(on)

The double pulse test is widely used to evaluate GaN switching performance. For dynamic R_DS(on), the device is first exposed to high drain voltage in the OFF state, then rapidly turned ON while measuring the ON-state voltage drop.

Pulse 1:

Establish Load Current

↓

OFF-State High Voltage Stress

↓

Pulse 2:

Turn ON Device

↓

Measure VDS(on)

↓

Calculate Dynamic RDS(on)

Accurate measurement requires high-bandwidth probes, low-inductance layout, careful deskewing, and protection against high-voltage transients.

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Factors Affecting Dynamic R_DS(on)

Factor	Effect
Drain Voltage Stress	Higher voltage generally increases trapping.
Off-State Time	Longer stress time may increase trapped charge.
Temperature	Affects trapping and detrapping rates.
Surface Passivation	Better passivation reduces surface trapping.
Buffer Design	Trap density strongly affects current collapse.
Field Plate Design	Reduces electric field peaks and trapping.
Switching Frequency	Affects recovery time between cycles.
Device Technology	Different GaN structures show different dynamic behavior.

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Reduction Techniques

• Use high-quality epitaxial GaN layers.
• Optimize buffer doping and trap profile.
• Improve surface passivation.
• Use field plates to reduce electric field crowding.
• Reduce plasma etch damage.
• Optimize gate-to-drain spacing.
• Use improved dielectric interfaces.
• Control drain voltage overshoot.
• Use soft-switching topologies when possible.
• Maintain proper thermal design.

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GaN vs Silicon vs SiC Comparison

Parameter	Silicon MOSFET	SiC MOSFET	GaN HEMT
Dynamic RDS(on)	Usually minimal	Usually low	Important design concern
Main Cause	Temperature effect	Interface and oxide effects	Surface and buffer trapping
Current Collapse	Not common	Limited	Major reliability topic
High-Frequency Operation	Limited	Good	Excellent
Need for Passivation	Moderate	High	Very high

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Applications Where Dynamic R_DS(on) Matters

• AI data center power supplies.
• Electric vehicle onboard chargers.
• High-frequency synchronous buck converters.
• Point-of-load voltage regulators.
• USB-C fast chargers.
• LLC resonant converters.
• Telecommunication power supplies.
• Solar microinverters.
• Battery energy storage systems.
• Aerospace power converters.

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Future Trends

• Trap-engineered GaN buffer layers.
• Advanced passivation stacks.
• Lower dynamic R_DS(on) p-GaN devices.
• Improved real-time dynamic resistance monitoring.
• Better compact SPICE models.
• AI-assisted device reliability prediction.
• GaN-on-diamond substrates for thermal stability.
• Monolithic GaN power ICs with integrated protection.

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Frequently Asked Questions (FAQs)

What is dynamic R_DS(on)?

Dynamic R_DS(on) is the temporary increase in ON-state resistance after a GaN device experiences high-voltage switching stress.

What causes dynamic R_DS(on) in GaN HEMTs?

It is mainly caused by charge trapping in surface states, buffer layers, dielectric interfaces, and high electric field regions.

What is current collapse?

Current collapse is the reduction in drain current after stress due to trapped charge reducing the available 2DEG channel density.

How is dynamic R_DS(on) measured?

It is commonly measured using pulsed I-V testing or double pulse testing while capturing V_DS(on) immediately after high-voltage stress.

Why is dynamic R_DS(on) important?

It increases conduction loss, raises junction temperature, reduces efficiency, and affects long-term reliability in practical converters.

How can dynamic R_DS(on) be reduced?

It can be reduced through better epitaxy, optimized buffer design, high-quality passivation, field plates, reduced etch damage, soft switching, and improved thermal design.

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Conclusion

Dynamic R_DS(on) is one of the most important real-world performance and reliability issues in GaN HEMTs. It occurs when trapped charges temporarily reduce the 2DEG channel density after high-voltage stress, increasing ON resistance and conduction losses. Although GaN devices offer outstanding switching speed and power density, designers must account for dynamic R_DS(on) during device selection, testing, thermal design, and converter optimization. Proper passivation, buffer engineering, field plate design, gate-drive control, and soft-switching strategies can significantly reduce this effect. As GaN technology matures for electric vehicles, AI data centers, aerospace systems, and renewable energy converters, minimizing dynamic R_DS(on) will remain a major focus for improving efficiency, reliability, and long-term device performance.

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Continue Learning

• Transient Thermal Impedance
• Thermal Resistance in GaN Devices
• Understanding R_DS(on)
• Leakage Current in GaN HEMTs
• Double Pulse Test for GaN Devices

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