RESPONSIVITY AND NOISE IN ILLUSTRATIVE SOLID-STATE CHEMICAL SENSORS

Chemical sensors provide for a quantification of the presence of gaseous or liquid ambients at external or internal interfaces of electronic or ionic materials and devices designed for this purpose. The relationship between the pertinent electrical. parameter, usually a potential, current, or conductance, and the chemical input defines the responsivity, R(t) or its Fourier transform R(i omega). From this the signal-to-noise ratio SIN, and the concepts of 'apparent-noise-equivalent signal' (ANES) and (real) 'noise-equivalent-signal' (NES) are easily deduced, providing a bandwidth corresponding to the time response of the device can be defined. Next we dwell on the nature of the response signal as occurring in various classes of devices. We briefly discuss four types: (i) chemFETs, based on the operation of surface-potential modulation in MOSFETs or MOS capacitors or on Schottky-barrier modulation (MESFETs); (ii) current devices such as MIS structures or multiple barrier structures in which intergranular barrier heights, which limit the conductance, are modulated by electron or hole capture, as in polysilicon or PbS films; (iii) pressed powder oxides or thin-film materials, in which the conductance is either of a percolation nature (porous films like amorphous hydrogenated silicon) or is due solely to surface conductance; (iv) membrane sensors, such as those based on superionic conductors like the beta- and beta''-aluminas, available in recent years. The main causes for noise in these four categories are indicated and formal expressions for the NES are given. In the third part of this paper, a brief summary of the most important noise phenomena is presented, including thermal noise, generation-recombination noise, 1/f noise, and diffusion noise. The manifestation of these causes of noise in quasi-homogeneous devices, like categories (iii) and (iv) above, is straightforward. For nonlinear electronic devices, such as FETs and multiple-barrier conductors, standard device theory is necessary to convert the noise sources to noise effects in the terminal electrical parameters that are modulated by the chemical ambient.