Publication

Background

Ion-sensitive field-effect transistors (ISFETs) are quite popular as compact, low-cost biosensors with fast response time and label-free detection¹. They can be used as pH sensors or functionalized for complex biomolecule detection. The voltage sensitivity (S_v) in classical ISFETs is fundamentally limited to 59 mV/pH (Nernst limit). Surpassing the Nernst limit requires complex device architectures or novel transport phenomena. Sensitivity beyond the Nernst limit can be achieved using specific device architectures such as dual gate ISFETs², negative capacitance ISFETs (NC-ISFET)³, tunnel ISFETs⁴, etc. Compatible architectures can be combined for further enhancements in sensitivity.

First, we experimentally demonstrate a super-Nernstian hetero-ISFET that uses 2-D WSe₂/MoS₂ heterostructure in a double-gated configuration⁵. The schematic of the device structure is shown in Fig. 1(a) along with its dimensions. The fluid gate to the pH solution is biased at V_FG = 0 V and the voltage sensitivity (S_V) is extracted by applying bias to the back-gate (V_BG). Fig. 1(b) shows the variation of drain current for change in V_BG at different pH. The voltage sensitivity is also included in the same graph. The device uses charge screening due to the interface traps and inversion charges at the hetero-interface to modulate the back-gate transconductance (g_mb), thereby allowing super sensitivity.

Background

We propose a nanoscale, highly sensitive and label-free biosensor based on negative capacitance gateall-around tunnel field-effect transistor (NC-GAA-TFET). NC-GAA-TFETs provide steeper, sub-60 mV/dec subthreshold swing (SS) and higher drive current compared with the conventional gate-all-around tunnel field-effect transistor (GAA-TFETs). The combination of differential voltage amplification, due to the negative capacitance (NC) of the gate ferroelectric, and the quantum mechanical band-toband tunneling (BTBT) effect leads to a significant boost in current sensitivity (SI) compared with the state-of-theart field-effect transistor (FET)-based sensing devices. 1-D Landau–Khalatnikov (L-K) equations are solved numerically and integrated with 3-D technology computer-aided design (TCAD) simulations to evaluate the sensor performance. The results show that the proposed sensor can achieve SS down to 27 mV/dec. The subthermal SS can be maintained over five decades of current leading to reduced power consumption in the weak inversion region and achieving SI as high as ∼10^6 and ∼10^5 for detecting biomolecules and pH changes, respectively. In addition, NC-GAA-TFETs provide ∼7× higher signal-to-noise ratio (SNR) than their baseline counterparts which make NC-GAA-TFETs promising candidates for low noise and ultrasensitive biosensing platforms.

Background

Negative capacitance (NC) effect in nanowire (NW) and nanosheet (NS) field effect transistors (FETs) provide the much-needed voltage scaling in future technology nodes. Here, we present a comparative analysis on the performance of NC-NWFETs and NC-NSFETs through fully calibrated, three-dimensional computer aided design (TCAD) simulations. In addition to single channel NC-NSFETs and NC-NWFETs, those, with vertically stacked NSs and NWs, have been examined for the same layout footprint (LF). Results show that NC-NSFETs can achieve lower subthreshold swing (SS) and higher ON-current (ION) than NC-NWFET of comparable device dimensions. However, NC-NWFETs show slightly higher ION/IOFF ratio. Negative differential resistance (NDR) is found to be more pronounced in NC-NSFET, enabling these devices to attain a stronger drain-induced-barrier-rising (DIBR) and steeper SS for gate lengths as small as 10 nm. The results presented here can, therefore, provide useful insights for performance optimization of NC-NWFETs and NC-NSFETs, in ultra-scaled and high-density logic applications, for 7 nm and beyond technology nodes.