Investigation on the MOSFET with underlap structure for bio sensor application

Abstract

In this work, an underlap channel-embedded field-effect transistor (FET) is proposed for label-free biomolecule detection. The device was fabricated by standard CMOS process, except for the gate-to-source/drain (S/D) alignment offset. The fabricated underlap device has a channel length of $10\microm$, a channel width of $20\microm$, and various underlap lengths from 100nm to 1400nm. Specifically, silica binding protein fused with avian influenza surface antigen (SBP-AIa) and avian influenza antibody (anti-AI) were designed as a receptor molecule and a target material, respectively. After the immobilization of SBP-AIa, the drain current increased slightly in comparison to that in the initial state. The small increment of the drain current can be explained by the presence of an additional fringing field due to the immobilized SBP-AIa, of which the thickness is approximately 3nm on the underlap region. However, by binding of the anti-AI molecules with the SBP-AIa at the underlap region, the drain current decreased considerably in all various underlap devices. The abrupt drop of the drain current is attributed to the negative charges of the anti-AI, and the decrement of the drain current becomes larger as the underlap length increases. To conform that the dramatic current reduction and the threshold voltage changes are due to the specific binding of SBP-AIa and anti-AI on the underlap region, a set of control experiments were carried out. As a result, only the binding of SBP-AIa and anti-AI effectively modulate the characteristics of underlap device and device also shows high specificity for detecting anti-AI biomolecule. Moreover, the drain current was returned back to its initial value after recovery test to break the binding sites of the biomolecules. With the merits of a simple fabrication process, complementary metal-oxide-semiconductor compatibility, and enhanced sensitivity, the underlap FET could be a promising candidate for a chip-based biosensor. Furth...