Abstract-A pH sensor fabricated on a single chip by an unmodified, commercial 0.6-m CMOS process is presented. The sensor comprises a circuit for making differential measurements between an ion-sensitive field-effect transistor (ISFET) and a reference FET (REFET). The ISFET has a floating-gate structure and uses the silicon nitride passivation layer as a pH-sensitive insulator. As fabricated, it has a large threshold voltage that is postulated to be caused by a trapped charge on the floating gate. Ultraviolet radiation and bulk-substrate biasing is used to permanently modify the threshold voltage so that the ISFET can be used in a battery-operated circuit. A novel post-processing method using a single layer of photoresist is used to define the sensing areas and to provide robust encapsulation for the chip. The complete circuit, operating from a single 3-V supply, provides an output voltage proportional to pH and can be powered down when not required.Index Terms-CMOS analog circuit, encapsulation, ion-sensitive field-effect transistor (ISFET), pH, system-on-chip (SoC).

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Single electron tunnel junctions have been formed in ultrathin silicon-on-insulator material by electron beam lithography and dry etching. Clear Coulomb blockade effects have been observed at 300 mK in the current-voltage characteristics as a function of the voltage applied to a control sidegate. The effects are not smeared out by thermal fluctuations until temperatures greater than 4 K are reached.

Abstract-This paper describes the design and implementation of a system-on-chip digital pH meter, for use in a wireless capsule application. The system is organized around an 8-bit microcontroller, designed to be functionally identical to the Motorola 6805. The analog subsystem contains a floating-electrode ISFET, which is fully compatible with a commercial CMOS process. On-chip programmable voltage references and multiplexors permit flexibility with the minimum of external connections. The chip is designed in a modular fashion to facilitate verification and component re-use. The single-chip pH meter can be directly connected to a personal computer, and gives a response of 37 bits/pH, within an operating range of 7 pH units.

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