Influence of the Biomaterial Thickness in a Dielectrically Charged Modulated Fringing Field Bio-Tunnel-FET Device
Authors: Macambira Christian Nemeth , Agopian Paula Ghedini Der , Martino Joao Antonio 
Published: May 2020

The biosensor market is expected to increase more than 40% in 5 years ( 2019 to 2024) due to the versatile of these devices (1). Many research groups are studying new types and optimizations of these devices and the tunnel field-effect transistors (TFET) have attracted the interest due to the many advantages of these devices (2).
Figure 1 shows the Bio-TFET cross section which has been simulated with TCAD Sentaurus (3). The proposed device has a gate length (LG), equivalent gate oxide thickness (tox) and channel silicon thickness (tSi) of 40 nm, 1 nm and 10 nm. The drain underlap (LUD) is variable in the range of 1 to 100 nm. The uniform doping profile of the device consists of a source and drain doping concentration of 1020 atom/cm3 of boron and arsenic, respectively. An intrinsic channel with 1015 atom/cm3 of boron. The gate electrode has a workfunction of 4.7 eV. The main models incorporated in the simulation were the bandgap narrowing (BGN), Shockley Read Hall recombination (SRH) and the nonlocal band-to-band tunneling (BTBT). In the drain underlap region is deposited a biomaterial for biosensing purposes. This material has a dielectric constant (k, where ε=k*ε0) of 1 (air), 2.1 (Streptavidin), 3.57 (APTES), 8 (Anti-Iris antibody) and 10 (Isoquinoline) (4).
Figure 2 shows the Bio-TFET transfer characteristics. The ambipolar region is a parasitic tunneling current that occurs for negative gate bias (VG) for nTFET at drain to channel region. In order to decrease this effect a drain underlap (LUD) is performed at the device (5). However, when the biomaterial dielectric constant (k) increases, the ambipolar current also increases orders of magnitude, meaning that the device is very susceptible to k variation in the biomaterial region and can be used for biosensing of different types of biomolecules with a distinct k. In order to quantify the influence of change in k in the biomaterial region the equation in terms of ID: Sensitivity = ΔID/ID(Ref.) was used. Where ΔID represents the difference between the drain current of biosensing (output), e.g., the drain current with the presence of biomaterial ID(Bio) and the drain current of reference ID (Ref.) (input), e.g., drain current without biomaterial ID (k=1) . Both for a fixed value of gate voltage (VG = -2 V) in the ambipolar region. Fig.3 shows the sensitivity of the device as a function of LUD for different k values. The maximum sensitivity occurs for a LUD = 30 nm for all range of k values. The sensitivity increases for higher k as reported in (6). Another parameter that is important for sensing is the dimension of the target. Many biomolecules have different types, e.g., spherical, ellipsoidal or elongated (7). Therefore, the study of the parameter is fundamental for the functional operation of the device. In Fig.4 the sensitivity is analyzed as a function of biomaterial thickness (tBio) for a fixed value of LUD=30nm. With the rising of tBio and k the sensitivity increases, i.e., exceeding 2 orders of magnitude comparing tBio = 10nm and tBio = 30 nm for k = 10. This increment on sensitivity is due to the tunneling length lowering, as can be noticed in Fig.5. The tunneling length decrease from 5.7 nm (tBio= 10 nm) to 9.5 nm (tBio = 30 nm), both for k = 10 and fixed energy of - 1 eV. The lowering of the tunneling length increases the ambipolar current ID(Bio) that enhances the sensitivity of the device. An important consideration for the biosensing process is the charge on the biomaterial that affect the tunneling current and therefore affect the sensitivity of the device. In Fig. 6, the sensitivity is studied as in the function of positive fixed charges in the range of 1010 to 1012 cm-2. The sensitivity increases for thicker values of tBio. This is due to the higher effect of the fringing field in the region. As tBio increase the effect of fringing field increases in the biomaterial region and as consequence, the device becomes more sensible with the presence of charges.
The study of charges and biomaterial thickness in a dielectrically modulated fringing field Bio-TFET was performed in this work. The device shows high sensitivity with the increase of tBio in the range studied in this work. Although, the effect of fixed charges is more notable for thicker tBio. Then, the tBio thickness shows a high impact in the sensing of the device.

Figure 1