What Is Current Slump?

Current slump, also called current collapse or dynamic on-resistance degradation, is a reliability phenomenon in wide-bandgap semiconductor transistors, particularly gallium nitride (GaN) high-electron-mobility transistors (HEMTs), in which the drain current drops significantly below the value predicted by static device characterization when the transistor transitions from an off-state blocking condition back to on-state conduction. The reduction in current can persist for microseconds to milliseconds before recovering, degrading the efficiency and power output of RF amplifiers and power converters. Current slump is a major reliability concern in GaN-based power electronics and has driven substantial research in materials science, device physics, and passivation engineering since the 1990s.

The phenomenon arises because wide-bandgap semiconductors such as GaN contain a high density of deep-level trap states associated with crystal defects, surface states, and buffer impurities. When a transistor is held at high drain voltage in the off state, these traps capture electrons from the two-dimensional electron gas (2DEG) channel. Upon turn-on, the captured charge acts as a virtual gate that partially depletes the channel, reducing the available drain current until the trapped electrons are thermally emitted and the channel recovers.

Physical Mechanisms and Charge Trapping

The dominant physical mechanism in AlGaN/GaN HEMTs is the capture of electrons at surface donor states on the AlGaN layer between the gate and drain. High electric field in this region during off-state operation drives electron injection from the gate edge into surface traps through tunneling or thermionic emission. The resulting negative surface charge reduces the 2DEG carrier density beneath it, increasing the effective on-resistance and reducing the saturation current. Buffer trap states, arising from threading dislocations and intentionally incorporated iron or carbon acceptors used to achieve substrate isolation, contribute a secondary trapping mechanism. Research on trapping phenomena and degradation in GaN power HEMTs published in Materials Science in Semiconductor Processing provides a systematic classification of surface and buffer trap signatures and their impact on current collapse magnitude.

Characterization and Measurement

Current slump is quantified by comparing static (DC) and pulsed current-voltage characteristics. In pulsed I-V measurement, the gate and drain are biased to a high-field quiescent condition that fills traps, then rapidly switched to various on-state bias points while measuring drain current before thermal recovery can occur. The ratio of pulsed to static drain current at a given bias defines the collapse factor. A collapse factor of 0.7, for example, means the device delivers only 70% of its expected current after a high-voltage off-state stress. Analysis of pulsed I-V curves and power slump in field-plate GaN FETs demonstrates how field-plate geometry modifies the electric field distribution and affects collapse behavior in power transistors. Time-resolved measurement and deep-level transient spectroscopy (DLTS) are used to identify trap activation energies and emission time constants.

Mitigation Techniques

The most effective mitigation strategy for surface-related current slump is silicon nitride (SiN) passivation deposited directly on the AlGaN surface. By filling surface donor states with a dielectric material, passivation reduces the density of trappable states and the magnitude of the associated current collapse. Field plates, which redistribute the electric field away from the gate edge toward the drain, reduce the field-induced trap filling rate and simultaneously improve breakdown voltage. GaN-based power device physics and reliability analysis published in the Journal of Applied Physics reviews how combined passivation and field-plate engineering has reduced current collapse to acceptable levels for commercial power switching applications.

Applications

Current slump is a design constraint in a wide range of disciplines, including:

• GaN RF power amplifiers for base stations and radar systems
• Wide-bandgap power switches in electric vehicle inverters
• High-voltage DC-DC converters for data center power delivery
• Space electronics requiring radiation-tolerant power transistors
• Reliability qualification and screening of compound semiconductor devices