Investigation of the RF Performance of n-JL (Junctionless) CombFETwith Ambient Temperature Variations

Technical Summary: Investigation of the RF Performance of n-JL (Junctionless) CombFET with Ambient Temperature Variations

Problem Statement
As transistor dimensions enter the deeply scaled regime (sub-5-nm), achieving strong electrostatic control is critical to mitigate short-channel effects. While Gate-All-Around (GAA) architectures offer superior electrostatic integrity, the drive current requirements for future 6G systems often necessitate vertically stacked channels. However, increasing the number of stacked channels exacerbates parasitic capacitance and process complexity. To address this, the CombFET architecture integrates Fin-like channels with vertically stacked GAA channels to enhance current drivability and RF performance without increasing the layout footprint. Despite these advantages, devices operating in high-performance environments face significant thermal stress. Ambient temperature variations critically influence electrothermal stability, carrier mobility, leakage currents, and RF figures of merit. While temperature effects have been analyzed for FinFETs and JL-GAA structures, investigations into the thermal sensitivity of n-JL-CombFETs remain limited, creating a gap in understanding their robustness for next-generation high-frequency applications.

Methodology
This study employs a numerical investigation using well-calibrated 3D TCAD simulations (Synopsys TCAD SDE) to analyze the impact of ambient temperature on a 14 nm channel-length n-Junctionless (JL)-CombFET. The simulation framework incorporates:

• Device Structure: A comb-shaped channel with a single fin and multiple bridges, surrounded by a gate stack.
• Physical Models: Carrier transport is modeled using the hydrodynamic approach to capture energy flux and non-local effects. Quantum confinement is included via the eQuantum potential for bridge thicknesses below 10 nm. Mobility variations are handled by the PhuMob model, while the oldSlotboom model corrects carrier density in highly doped regions. Shockley-Read-Hall (SRH) recombination is accounted for.
• Calibration: The models are calibrated against experimental results from existing literature (Ref. [17]) by adjusting Phumob parameters (mumax, mumin), work function, and trapped charges.
• Variables: The study evaluates RF metrics across a temperature range of 280 K to 380 K. The fin gap ($F_g$) is varied in four steps from 5 nm to 22 nm to determine optimal performance under different thermal stress conditions. Device parameters follow IRDS guidelines, with a channel length ($L_g$) of 14 nm and doping concentrations of $1 \times 10^{19} \text{ cm}^{-3}$.

Key Results
The simulation results demonstrate a consistent degradation in RF performance as ambient temperature increases from 280 K to 380 K:

• DC Characteristics: In the ON-state, the peak drain current ($I_d$) decreases from $5.5 \times 10^{-5}$ A to $4.4 \times 10^{-5}$ A (approx. 15% reduction), and peak transconductance ($g_m$) drops by approximately 18%. This is attributed to dominant phonon scattering reducing carrier mobility. Conversely, OFF-state leakage increases due to enhanced thermal carrier generation.
• Zero Temperature Coefficient (ZTC): A ZTC point, where temperature-induced threshold voltage shifts and mobility degradation compensate each other, was extracted for all fin gaps. The optimal ZTC performance for both $I_d$ and $g_m$ was observed at a fin gap of 10 nm.
• Capacitance and Gain: The maximum gate capacitance ($C_{gg\_max}$) decreases by 13–19% with rising temperature. The Trans Generation Factor (TGF), representing transconductance efficiency, declines by nearly 28% (from ~40 $V^{-1}$ to ~29 $V^{-1}$).
• RF Figures of Merit:
  • Cut-off Frequency ($f_T$): $f_T$ drops by approximately 11.4% (from 350 GHz to 310 GHz) as temperature rises. The maximum $f_T$ for a 5 nm fin gap decreases from ~345 MHz to ~330 MHz, while a 22 nm gap shows a more significant drop from ~165 MHz to ~145 MHz.
  • Maximum Oscillation Frequency ($f_{max}$): $f_{max}$ degrades from ~600 GHz to ~530 GHz at 280 K and 380 K respectively for a 5 nm fin gap.
  • Transconductance Frequency Product (TFP): The peak TFP reduces by roughly 21% (from ~2900 GHz/V to ~2300 GHz/V) over the temperature range.
  • Maximum Available Gain (MAG): MAG decreases from ~70 dB to ~65 dB in the low-frequency regime, with the 0 dB crossing occurring at lower frequencies for higher temperatures.

Significance and Claims
The paper claims that the n-JL-CombFET architecture preserves electrostatic integrity even under thermal stress, making it a robust candidate for high-frequency 6G applications. However, the study underscores that performance is highly sensitive to ambient temperature, primarily driven by carrier mobility degradation.

The primary contribution of this work is the identification of the fin gap as a tunable parameter to optimize device performance under varying thermal conditions. Specifically, the authors conclude that a fin gap of 10 nm yields the highest ZTC performance, allowing for the optimization of RF performance across different ambient temperatures. The findings highlight the necessity of temperature-aware design strategies in monolithic 3D integration to mitigate the adverse effects of elevated operating temperatures on RF efficiency. While the device shows promise under controlled thermal conditions, its suitability for high-temperature environments is limited without such structural optimizations.