Fundamental limitations of conventional-FET biosensors: Quantum-mechanical-tunneling to the rescue

Electrical detection of biomolecules using Field-Effect-Transistors (FETs) [1-5] is very attractive, since it is label-free, inexpensive, allows scalability and on-chip integration of both sensor and measurement systems. Nanostructured FETs, especially nanowires have gained special importance due to their high electrostatic control and large surface-to-volume ratio. In order to configure the FET as a biosensor (Fig. 1(a)), the dielectric/oxide layer on the semiconductor is functionalized with specific receptors. These receptors capture the desired target biomolecules (a process called conjugation), which due to their charge produce gating effect on the semiconductor, thus changing its electrical properties such as current, conductance etc. Thus it is intuitive, that greater the response of the FET to the gating effect, higher will be its sensitivity where sensitivity can be defined as the ratio of change in current due to biomolecule conjugation to the initial current (before conjugation). While the highest response to gating effect can be obtained in the subthreshold region, the conventional FETs (CFET) suffer severely due to the theoretical limitation on the minimum achievable Subthreshold Swing (SS) of [KBT/q ln(10)] due to the Boltzmann tyranny (Fig. 1(b)) effect where KB is the Boltzmann constant and T is the temperature. This also poses fundamental limitations on the sensitivity and response time of CFET based biosensors [6]. In recent times, Tunnel- FETs have attracted a lot of attention for low power digital applications [7]-[17], due to their ability to overcome the fundamental limitation in SS (60 mV/decade) of CFETs. Recently, it has been shown that the superior subthreshold behavior of TFETs can be leveraged to achieve highly efficient biosensors [6]. This is possible, thanks to the fundamentally different current injection mechanism in TFETs in the form of band-to-band tunneling [17]. The working principle of TFET biosensors is illustrated in Fig. 1c.