Deblina Sarkar and Kaustav Banerjee. Appl. Phys. Lett. 100, 143108 (2012)

Abstract

 Tunnel field-effect-transistor (TFET) based biosensor is proposed, and it is shown that they can surpass by several orders, the performance of those based on conventional FET (CFET) and hence, can potentially revolutionize the biosensing applications. Analytical formula is derived for the sensitivity and response time to provide physical insights in terms of material bandgap and operation regime of the TFET biosensor for achieving optimal results. At the same time, rigorous numerical simulations have been performed in order to obtain accurate values of sensitivity for both biomolecule and pH sensing operations. The time dependent response of the biosensors has also been discussed through analytical and numerical solutions. It is shown that while the CFET biosensors suffer from fundamental limitations on the maximum sensitivity and minimum detection time achievable, TFET biosensors, with their fundamentally different current injection mechanism in the form of band-to-band tunneling, can overcome such limitations and lead to over four orders of magnitude higher sensitivity and over an order of magnitude lower response time.

Biosensors based on field-effect-transistors (FETs)1–6 have attracted a lot of attention in recent times, due to their advantages of label-free electrical detection, small size and weight, low-cost mass production, and possibility of on-chip integration of both sensor and measurement systems. The principle behind electrical detection using FET biosensor is based on the gating effect of the charged biomolecules on the semiconductor, which can be monitored directly by the change in electrical properties such as current, conductance, etc. For sensing purpose, the dielectric/oxide layer on the semiconductor is functionalized with specific receptors for capturing the desired target biomolecules. Sensitivity is a critical parameter for gauging the performance of the biosensors. Improved sensitivity is desired for detection of biomolecules at low concentration and reduction of detection time. However, the conventional FET (CFET) based biosensors suffer from theoretical limitations on the maximum achievable sensitivity and minimum detection time. We propose and show that the tunnel field-effect-transistors (TFETs)7–11 employing a fundamentally different current injection mechanism from the source to the channel in the form of band-to-band tunneling12 can overcome these limitations and lead to substantially higher sensitivity while retaining all other advantages of CFET sensors. The schematic diagram of a nanowire based TFET biosensor and its working principle is shown in Fig. 1. Before the attachment of biomolecules to the sensor surface, the tunneling barrier between source and channel is high (Fig. 2(a)), and hence, the current in TFET is low. After biomolecule-receptor conjugation, due to the charges present in the biomolecules (positive charge is assumed here), the bands in the channel bend down, leading to a decrease in the tunneling barrier (Fig. 2(b)) and, hence, increase in the tunneling current. Thus, the biomolecules can be detected by monitoring the change in current through the TFET biosensor device.