Localized Latching in GaN Multichannel Transistors Enables Sub-60 mV/Decade Switching and Enhanced RF Linearity

There is an increasing demand for compact, high-power, and highly linear RF transistors continues to escalate next-generation wireless communication and radar systems. Gallium nitride (GaN)-based devices, particularly high electron mobility transistors (HEMTs), have emerged as the cornerstone of this technological evolution. Their intrinsic properties—wide bandgap, high breakdown voltage, and superior electron transport—make them uniquely suited to endure the harsh electrical stresses imposed by high-frequency and high-power environments. Yet, even with these advantages, there remain fundamental obstacles that constrain the full exploitation of GaN in advanced RF circuitry. Chief among these is the need to enhance power density without compromising linearity or reliability, a balancing act that has proven difficult as devices are pushed closer to their material limits. One promising route to resolving this impasse lies in the use of multichannel architectures. Among these, the superlattice castellated field-effect transistor (SLCFET) stands out. Unlike traditional single-channel GaN HEMTs, the SLCFET incorporates vertically stacked two-dimensional electron gas (2DEG) channels formed via an AlGaN/GaN superlattice structure. This enables a dramatic increase in charge carrier availability without enlarging the device footprint. By combining this dense channel configuration with a three-dimensional fin geometry and wrap-around gate control, SLCFETs offer a compelling path toward ultra-compact, high-power RF transistors. However, the complexity of this structure introduces new variables—particularly fabrication-induced variation in fin dimensions—that can influence device behavior in subtle but significant ways.

New research paper published in Nature Electronics and led by Professor Martin Kuball from the University of Bristol and conducted by Dr. Akhil Kumar, Stefano Dalcanale, Michael Uren, James Pomeroy, Dr. Matthew  Smith alongside Justin Parke and Robert Howell from the Northrop Grumman Mission Systems in the United States, the researchers developed a multichannel GaN SLCFET that exhibits a latch-induced subthreshold slope below the 60 mV/decade Boltzmann limit. They demonstrated that localized impact ionization in the widest fins initiates reversible latching, leading to enhanced transconductance linearity without degrading device reliability. This architecture leverages naturally occurring fin-width variations to improve RF performance, offering a novel, scalable approach for high-power applications.

The researchers began by fabricating multifinger SLCFETs incorporating ten stacked AlGaN/GaN superlattice periods, each forming a distinct 2DEG channel. These channels were electrostatically modulated through conformally coated SiN dielectric and wrap-around gate metals on densely packed nanoscale fins. Through initial current–voltage (I–V) sweeps, they observed a peculiar two-phase subthreshold behavior—one segment with a gradual 80 mV/decade slope and another where the current surged abruptly with a slope well below the thermionic limit. Notably, transconductance (gm) plots revealed multiple shoulders, which hinted at nonuniform channel activation that could not be explained by standard GaN HEMT physics.

To trace the origin of these irregularities, the team utilized scanning electron microscopy (SEM) and found significant variation in fin widths across the device, a byproduct of fabrication processes. They then turned to 3D simulations using Silvaco Atlas to model how fin-width dispersion impacts device performance. Simulating fins with a ±20% width deviation around the mean, they reproduced the same subthreshold gm shoulders observed experimentally. This confirmed that wider fins, due to their more negative threshold voltages, entered conduction earlier, effectively initiating a cascade of channel activation at distinct gate voltages. However, the most intriguing discovery came when they performed electroluminescence (EL) microscopy during subthreshold I–V sweeps at high drain bias. As the gate voltage was stepped gradually from −12 V to −10.5 V, faint but highly localized EL spots emerged, aligned with abrupt increases in drain current. These hot spots—originating near the gate-drain edge—provided direct visual evidence of impact ionization occurring in individual fins. Crucially, as the sweep continued, additional EL spots appeared, suggesting that more fins were sequentially entering a latched, high-conduction state. Afteerward, the team conducted repeated bidirectional gate sweeps under high-field conditions. The result was both surprising and reassuring: the sharp switching behavior remained stable over 90 minutes with no signs of degradation. Gate current profiles further supported a recoverable process, showing a bell-shaped peak consistent with hole generation and subsequent release. Simulated band diagrams reinforced the hypothesis that holes temporarily accumulated at a barrier formed between GaN and residual native oxides at the fin-dielectric interface, altering the local threshold voltage. The authors compared transconductance before and after latching and demonstrated a broadened gm profile and a marked reduction in second-order nonlinearity (g″m). These outcomes strongly indicated that the latching effect, rather than undermining performance, actually enhanced the device’s RF linearity—a rare instance where a seemingly erratic behavior turned out to be a hidden asset.

In conclusion, Professor Martin Kuball and colleagues successfully revealed that under controlled conditions, it could produce ultrasteep subthreshold slopes and improve linearity in high-frequency applications. This challenges prevailing assumptions in device engineering, particularly for GaN, where the high bandgap has long been thought to preclude such behavior without structural degradation or reliability risks. Indeed, the discovery that a single wide fin—formed unintentionally during standard lithography—can initiate reversible latching through localized impact ionization represents a turning point. It underscores the importance of embracing rather than eliminating geometrical nonuniformities, provided their influence can be accurately understood and modeled. This paradigm shift could open new pathways for device tuning by harnessing naturally occurring variability, rather than expending resources to eliminate it entirely. From a practical standpoint, the study carries direct implications for next-generation RF amplifier design. The demonstrated reduction in second-order transconductance nonlinearity (g″m) confirms that latching contributes to a broader, flatter gm profile—a key requirement for signal linearity and spectral purity in communication systems. This gain in linearity, coupled with the absence of permanent degradation under prolonged high-field operation, makes SLCFETs a compelling platform for both military and commercial RF electronics. Moreover, the methodical combination of electroluminescent imaging, device modeling, and thermal analysis provided a blueprint for characterizing emerging effects in complex transistor structures. The ability to visualize and attribute localized emission to specific physical features—down to individual fins—adds a layer of diagnostic precision rarely seen in wide bandgap device research. This opens up possibilities for finer-grained control over switching dynamics in multichannel designs, particularly in environments where heat dissipation and voltage stress are limiting factors. Perhaps most critically, this study suggests that steep-slope switching in GaN need not rely on exotic materials or negative capacitance schemes. It can instead arise naturally from internal carrier dynamics when certain physical thresholds are crossed. That this transition is both recoverable and non-destructive makes it even more valuable. In doing so, the work not only advances the frontier of GaN transistor physics but also offers a realistic and manufacturable path to enhancing RF performance without compromising device longevity.