Abstract
Ion-sensitive, field-effect transistors (ISFET) have been useful biosensors in many applications. However, the signal-to-noise ratio of the ISFET is limited by its intrinsic, low-frequency noise. This paper presents an ISFET capable of utilizing lateral-bipolar conduction to reduce low-frequency noise. With a particular layout design, the conduction efficiency is further enhanced. Moreover, the ISFET is compatible with the standard CMOS technology. All materials above the gate-oxide are removed by simple, die-level post-CMOS process, allowing ions to modulate the lateral-bipolar current directly. By varying the gate-to-bulk voltage, the operation mode of the ISFET is controlled effectively, so is the noise performance measured and compared. Finally, the biasing conditions preferable for different low-noise applications are identified. Under the identified biasing condition, the signal-to-noise ratio of the ISFET as a pH sensor is proved to be improved by more than five times.
Highlights
Field-effect transistors have been employed as biosensors to detect ion concentration, biomolecules, neural activity, etc. [1,2,3,4,5,6]
Based on the post-CMOS process proposed in [23], this paper presents a CMOS-compatible ISFET
The white line segments within the circle corresponded to the gate oxide of the ISFET, and the light-red regions to the source diffusion covered by a thicker oxide
Summary
Field-effect transistors have been employed as biosensors to detect ion concentration, biomolecules, neural activity, etc. [1,2,3,4,5,6]. The low-frequency noise of field-effect sensors has to be further reduced for recording weak biomedical signals such as neural activity, which could be less than tens of micro-volts in magnitude. To avoid complex post-CMOS processing, most CMOS-compatible, field-effect sensors employ the passivation layer (silicon nitride/silicon oxynitride) as the surface material, and using a floating gate formed by metals to couple the potential changes at the sensory surface [13,14,15]. Based on the post-CMOS process proposed in [23], this paper presents a CMOS-compatible ISFET able to operate both in the metal-oxide-semiconductor (MOS) mode and in the lateral-bipolar junction transistor (LBJT) mode. All materials above the gate-oxide of the ISFET are removed by the die-level, post-CMOS process, allowing the ions to modulate the drain current directly, so as to enhance the sensitivity.
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