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.

Further enhancement in sensitivity is explored using technology computer-aided (TCAD) device simulator tool (Silvaco ATLAS) by integrating with different device architectures. First we model the baseline hetero-ISFET. The 2-D materials were modeled using their material parameters and 3-D equivalents of their density of states. Amorphous hafnium oxide (HfO₂) was used as the dielectric. The mobile ions in the electrolyte were modeled as charge carriers in an intrinsic semiconductor, with its effective density of states varying as a function of pH. The simulation model was calibrated with the experimental device (at pH = 7), as shown in Fig. 2(a). The transfer characteristics of the back-gate at different pH and fixed V_FG (= 0 V) for the simulated device is shown in Fig. 2(b). We note that the sensitivity from simulations is lower than the experimental device. This is likely due to non-ideal and 2-D material specific factors which are not accounted in simulations. Nevertheless, the simulated device also shows super-Nernstian sensitivity (Fig. 2(b), right axis), validating the model. Hence, the calibrated TCAD model is used as the baseline for further studies.

Next, an NC-FE layer (aluminum-doped HfO₂) was added to the top fluid-gate stack⁶. We have used a ferroelectric-metal-insulator-semiconductor (FMIS) stack for the proposed NC-hetero-ISFET. Fig. 3(a) shows the new top-gate stack with the FMI layer, which replaces the top-gate stack in the earlier schematic. The fluid-gate charge (Q_FG), and drain current (I_D) as a function of V_FG (V_BG = 0 V), were obtained from the TCAD simulations. The 1-D Landau–Khalatnikov (L-K) equations were used to model the voltage across the FE layer (V_FE = 2αQ_FG+4βQ³_FG = V^'_FG - V_int; where V^'_FG is the newly computed fluid-gate bias and V_int is the internal node voltage)⁷. The calculated V_int (for fixed V^'_FG) is coupled back into the ATLAS simulator to extract voltage sensitivity (S_V) by sweeping V_BG at different pH values.

The fluid-gate transfer curve of the proposed NC-hetero-ISFET, in Fig. 3(b), clearly shows a steeper sub-threshold slope and higher ON current than the baseline device. The corresponding FE layer parameters are shown in Table 1. These improved fluid-gate characteristics contribute to an increased voltage-sensitivity (S_V) when V_BG is applied. The transfer characteristic (I_D v/s V_BG, at fixed V^'_FG) of the NC-hetero-ISFET at different pH values is shown in Fig. 3(c), along with the voltage sensitivity. Further, in Fig. 3(d), we compare the peak S_V obtained at different V^'_FG. There is an improvement in voltage sensitivity (as much as ~ 100 mV/pH) over the baseline device when NC is introduced. The results pave the way for highly sensitive super-Nernstian ISFETs by combining 2-D heterostructure with NC effect.

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