Field-effect sensors have been applied extensively to numerous biomedical applications. To develop biosensor arrays in large scale, integration with signal-processing circuits on a single chip is crucial for avoiding wiring complexity and reducing noise interference. This paper proposes and compares two CMOS-compatible processes that allow open-gate, field-effect transistors (OGFETs) to be fabricated at the die level. The polygates of transistors are removed to maximize the transconductance. The CMOS compatibility further facilitates the monolithic integration with circuitry. Based on images and electrical measurements taken at different stages of the post-CMOS processes, a more feasible and reliable process is identified. The robustness of the fabricated OGFETs against the micromachining process and against moisture is further examined and discussed. Finally, the capability of the OGFETs in detecting ion concentrations, biomolecules, and electrophysiological signals is demonstrated.
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.
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