High-electron mobility transistor (HEMT) metal oxide-semiconductor high electron mobility transistor (MOSHEMT)-based sensors, utilizing wide bandgap semiconductors, have garnered considerable attention, as they offer a high sensitivity owing to their increased electron mobility, impressive frequency response, low noise levels, and minimal power consumption [1–5]. In biosensing applications, AlGaN/GaN MOSHEMT biosensors play a pivotal role in detecting alterations in surface charge resulting from biomolecular binding or chemical interactions due to the two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface [6–8]. MOSHEMT biosensors are characterized by their sensitivity, selectivity, and limits of detection, which are determined by variations in electrical parameters resulting from physical or chemical changes at the gate sensing area of the devices upon binding of biological molecules [9–12]. Several research groups have elucidated these sensing metrics using analytical techniques and physics-based models to comprehend the operational mechanism of HEMT biosensors [13–16]. These crucial electrical parameters, including drain current, threshold voltage, channel potential, channel conductance, surface potential, capacitance, transconductance, and conductance-to-current ratio, among others, aid in understanding deviations in device characterization from a neutral response [17–19].

When compared to a single-gate MOSHEMT, double-gate structures have advantages in terms of performance, scalability, and reliability [20, 21]. Double-gate structures provide better control of the gate over the channel than single-gate structures. This improves electrostatic integrity and reduces leakage current, thereby resulting in higher ON/OFF current ratios as well as enhancing device performance [22]. They also offer effective modulation of the channel due to the presence of two gates; thus, it can exhibit improved subthreshold swing for lower power consumption and enhanced efficiency. They mitigate short-channel effects (SCE) such as drain-induced barrier lowering (DIBL) and subthreshold slope (SS) degradation [23, 24]. Two gates confine the electric field more effectively, limiting its impact to SCEs and allowing for better scalability on smaller devices. However, comparing them with single-gate counterparts shows that double-gated structures generally have lower values of this capacitance [25, 26]. With the inception of double-gate devices, the trend toward analyzing multi-gate devices with alternative materials for the gate, insulator, and substrate is increasing. A DG InP/InGaAs structure was presented in [27], where TCAD simulation was performed for gate and barrier dimension variations. The proposed device exhibited improved device parameters except for SS and Ion/Ioff. A double delta-doped DG-MOSHEMT with a multi-cap layer with a thin InAlAs barrier with HfO₂ as a dielectric material finds use in the high-frequency domain. Maximum transconductance and higher cut-off frequency were reported when compared with an SG-HEMT structure [28]. Simulation of an underlap double-gate (U-DG) AlGaN/GaN MOSHEMT with HfO₂ as the gate oxide improved analog performance over a Schottky HEMT, thereby making it suitable for high-power enhanced frequency devices [29]. A symmetric U-DG GaN/AlGaN MOS-HEMT with a doped AlGaN barrier and spacer of varying width showed that the reduction in the width of the doped region leads to improved gate control as well as high ON current when examined for its effect on the analog figure of merits (FoMs) [18]. The device performance of a DG n-AlGaN/GaN MOSHEMT, where the doping concentration of the AlGaN was moderated, compared with a conventional HEMT as well as a metal-oxide-semiconductor field-effect transistor (MOSFET) device [30]. A DG underlap AlInN/GaN MOSHEMT for analog and radio frequency (RF) performance measurements using a TCAD tool shows that, through gate length and underlap length scaling, there is an increase in the device parameter values like drain current, transconductance, and frequency, but it caused a drop in the on-resistance of the simulated device. Results indicate the device’s potential for use in high-power, high-frequency applications [22]. An increase in effective barrier thickness exhibited an increase in drain current and SCE, like DIBL and SS. A reduction in delay and a negative shift in threshold voltage have also been observed. A decrease in barrier thickness was associated with an increase in gate capacitance, output resistance, transconductance, and improved cut-off and maximum oscillation frequency [31]. Currently, the trend is moving toward biomedical applicability of double-gate structures [32–34]. Biosensors have been implemented by using a cavity near the gate or by coating the gate with bio-analyte to detect the presence of specific biomolecules. A cavity near the gate impacts the oxide capacitance of the device, and this, in turn, impacts the different device characteristics like the drain current, transconductance, and threshold voltage of the device. The novelty of this work is the proposal of a quad-cavity double-gate tapered dielectric MOSHEMT proposed for biosensing applications. The tapered dielectric offers more binding surface for the biomolecules in the cavity. The proposed device is compared with its counterpart, conventional MOSHEMT biosensors. A tapered dielectric component offers increased area for biomolecule binding than its non-tapered counterpart. The increased number of cavity under the gate (CUG; quad CUG) structures allows more biomolecule binding sites under the gate, hence increased parameter variation probability. This article is subdivided into the following sections: Section 2 presents the device structure; Section 3 presents the sensitivity analysis of a quad cavity tapered dielectric of a double-gate MOSHEMT; Section 4 presents results and discussion; and conclusions are provided in Section 5.

A double-gate MOSHEMT features two gate electrodes on a thin semiconductor layer, forming a dual-gate structure. The thin semiconductor layer is typically composed of a high-mobility material, as seen in Figure 1a. The gates are separated by a nanometer-scale thickness, allowing for precise control of the channel’s charge density. These devices operate based on charge modulation in the channel region between the two gates by applying different voltages to the two gates, which enables enhanced gate control over the device, leading to improved performance compared to traditional single-gate transistors. The experiment used a double-gate MOSHEMT (DG-MH) with a 100   n m gate length, with a 1 0   n m HfO₂ as the insulator material under each gate, having two 18   n m ternary AlGaN as the barrier confining a binary III-V compound 180   n m GaN as the channel. The models used for simulation were calibrated with the results of the literature [18], as presented in Figure 1b. The calibration was obtained with a root mean square (RMS) value of 0.078, and the average % deviation between the values is calculated to be 2.6%.

Using a Cogenda Visual TCAD 2D simulator, we analyzed the performance of a double-gate tapered dielectric AlGaN/GaN MOSHEMT with a quad cavity under the gate toward potential biosensing applications.

Keeping the device dimensions similar, a cavity under the gate (CUG) was incorporated in the DG-MH to explore the device’s applicability toward biosensing applications. Figure 2 presents the cross-section of the proposed AlGaN/GaN double-gate double-CUG-MOSHEMT (DG-DCUG-MH) and double-gate quad cavity under the gate tapered dielectric MOSHEMT (DG-QC-TD-MH) (DG-QCUG-TD-MH). The device has a double gate of aluminum with length L g of 100   n m , respectively. The top and bottom oxides each have a thickness h o x ) of 10   n m . The DG-DCUG-MH has two CUGs ( C U G 1   a n d   C U G 2 ), while the DG-QCUG-TD-MH has four CUGs ( C U G t l , C U G t r , C U G b l , a n d   C U G b r ). The length of each CUG in DG-DCUG-MH is 50 nm. The DG-QCUG-TD-MH is composed of a tapered ( L t a p , l   a n d   L t a p , r ) and a non-tapered ( L c u g , l   a n d   L c u g , r ) section, as mentioned in Table 1. Each cavity has a height h c u g of 8   n m in the current investigation. Two A l x G a 1 − x N barriers with an Al mole fraction ( x = 0.31 ) have been considered with a thickness ( t b of 18   n m [18] over the GaN channel with thickness t c h . The source and drain form ohmic-type contacts.

Biomolecules have different shapes and are associated with different dielectric constants. The DG-QCUG-TD-MH can act as a biosensor if we modulate the dielectric constants of the CUG regions to emulate the presence of neutral biomolecules. Any surface modification near the gate affects the 2DEG of the MOSHEMT and can be used to study the variation in parameters due to the presence of biomolecules that can be further used to perform the sensitivity analysis. Possible fabrication steps of a tapered cavity were presented in our previous work [35, 36]. Because 20° tapered cavities have been fabricated [37] with precise photolithography and meticulous etching, a 60° tapered CUG may also be feasible. However, fabrication of these nanocavities comes with different fabrication challenges. Process variability can significantly impact achieving consistent nanometer-scale dimensions and the desired tapered geometry, thereby affecting the performance of the biosensor. Both the fabrication process and the subsequent bioassay functionalization can result in surface roughness, which can affect the performance. High-resolution techniques like electron beam lithography and focused ion beam milling are expensive and slow, limiting their use for mass production. Integrating the tapered nanostructures into a complete, functional biosensor system requires complex multi-step processes.

The top gate section is shown in Figure 3. The capacitance in each region is calculated as follows. The middle layers constitute the top and bottom barrier layers and the channel.

The gate region is subdivided as follows: R e g i o n   I , V I I : 0 < x < L 1 , 0 < y < h c u g , R e g i o n   I I , V I I I : L 1 < x < L 2 , 0 < y < h c u g , R e g i o n   I I I , I X : L 2 < x < L 3 , 0 < y < t o x , I I I   , R e g i o n   I V , X : L 3 < x < L 4 , 0 < y < h c u g , R e g i o n   V , X I : L 4 < x < Lg , 0 < y < h c u g , R e g i o n   V I , X I I : 0 < x < Lg , h c u g < y < t o x , V I .

Assuming that the CUGs are fully filled, the capacitance of each region can be calculated as follows: C I , t o p = C I , b o t = ϵ 0 κ c u g A L 1 h c u g L g , (1a) C I I , t o p = C I I , b o t = ϵ 0 κ o x κ c u g A L 2 − L 1 h c u g L g κ c u g − κ o x ln ⁡ κ c u g κ o x , (1b) C I I I , t o p = C I I I , b o t = ϵ 0 κ o x A L 3 − L 2 t o x , I I I L g , (1c) C I V , t o p = C I V , b o t = ϵ 0 κ o x κ c u g A L 4 − L 3 h c u g L g κ o x − κ c u g ln ⁡ κ o x κ c u g , (1d) C V , t o p = C V , b o t = ϵ 0 κ c u g A L g − L 4 h c u g L g , (1e) C V I , t o p = C V I , b o t = ϵ 0 κ o x A t o x , V I . (1f)

Here, ϵ 0 is the permittivity of free space, κ c u g is the relative permittivity of the oxide, A is the gate area, L g is gate length, and W is the channel width. L 1 , L 2 , L 3 , L 4 are sections of the gate length , h c u g is the height of the CUG, and t o x , I I I and t o x , V I are the thicknesses of the oxide in region III and region VI.

The total capacitance associated with Regions I–V ( C o x , I − V ) is calculated as C o x , I − V , t o p = C o x , I − V , b o t = ∑ i = I V C i . (2)

Using Equation 2, the total oxide capacitance ( C o x , T o t , t o p ) is expressed as C o x , T o t , t o p = C o x , T o t , b o t = 1 C o x , I − V , t o p + 1 C V I , t o p − 1 . (3)

The MOSHEMT capacitance for the top region is assumed to be equal to the capacitance of the bottom region which is expressed in Equation 3 as C M O S H E M T , t o p = C M O S H E M T , b o t = C b , C o x , T o t , t o p C o x , T o t , t o p + C b , (4) where A stands for the area of the individual region, and C b , t o p = C b , b o t = ϵ 0 ϵ b t b are the capacitances of the barrier layers. Here, ϵ b is the relative permittivity of the barrier layers, and t b specifies the thickness of the barrier layers, in Equations 3, 4, the subscript on the right-hand side may be replaced by to depict the capacitance of the bottom layer.

The capacitance terms C M O S H E M T , t o p and C M O S H E M T , b o t are connected in series, so the total MOSHEMT capacitance C M O S H E M T , T o t can be expressed as C M O S H E M T , T o t = C M O S H E M T , t o p C M O S H E M T , b o t C M O S H E M T , t o p + C M O S H E M T , b o t . (5)

The MOSHEMT capacitance derived in Equation 5 are used to perform the sensitivity analysis, as it impacts the drain current, which in turn affects other parameters like transconductance and output conductance. The presence of a neutral biomolecule in the CUG will change the surface potential, which impacts the sheet carrier density and therefore the drain current. The sensitivity analysis uses Equation 6 to determine the variations in the different sensing parameters P , such as the on current I O N , off current I O F F , threshold voltage V t , transconductance ( g m , drain current I d s , and output conductance ( g d s both when the CUG has no molecule and when biomolecules enter the CUG [17]. S P = Δ P P N M , (6) where Δ P = P N M − P B M , S P represents sensitivity, Δ P represents the difference in the sensing parameters due to the biomolecule hybridization in the cavity, P N M is the sensing parameter of the device with no molecule N M in the CUG, and P B M is the sensing parameter of the device with a biomolecule B M in the cavity (neutral). The sensitivity analysis is based on the variation of the parameters of the device due to the presence of biomolecules rather than the value of the variation.

This section is divided into three sub-sections. The first sub-section discusses the sensing metrics for a DG-DCUG-MH, while the next sub-section delves into different studies for the DG-QCUG-TD-MH. The last part of this section makes a comparison between different MOSHEMT structures with tapered and non-tapered dielectric with single to quad CUG structures.

This article delves into the applicability of double-gate structures for biosensing. To analyze the performance of the device in the nano regime, a gate length of 100 nm is used to obtain improved results for various metrics. Below 50 nm, as L g shrinks, the source and drain depletion regions begin to overlap, weakening gate control. SCE performance degrades, and DIBL and SS increase. A DG-MH structure with a CUG as a biosensor leverages the benefits of a double-gate structure and the high electron mobility properties of the semiconductor material to achieve sensitive and accurate biomolecule detection. The compact nature of the double-gate design allows for miniaturization, making it suitable for portable and point-of-care diagnostic applications.