Semiconductor junction gas sensors

Schottky barrier devices are promising candidates for sensor applications. The devices are simple to fabricate, obviating the need for expensive processes used in microtechnology. Both the metal and the semiconductor forming the Schottky barrier junction can be used as the chemical sensing components interacting with the gases or vapors. The material choices and combinations define the working principle, the sensitivity, and the selectivity of the sensor devices. In Schottky barrier devices, particular attention must be paid to the formation of the junction barrier between the metal and the semiconductor. In the case of organic semiconductors, there is still only limited understanding of the interface between the organic material and the metal, which makes data interpretation complicated. Experimental data show that the junction barrier turns out to be not ideal because of the presence of interface states as well as the undesired formation of an interfacial layer caused either by instability of the components in air or by chemical reaction between the meta is and semiconductors. The deviation from ideality can influence the sensor characteristics up to a complete breakdown of the ability to detect gases or vapors. Therefore, a high quality of the interface is the key factor affecting the sensing performances of the Schottky barrier devices. This certainly can increase the fabrication complexity and cost. The key difference between organic and inorganic Schottky barrier junction devices is the ability of many gases and vapors to penetrate through the organic semiconductor to the interface either to change the Schottky junction resistance or to interact with the bulk of the semiconductor, which causes a work function change of the material. In the case of inorganic Schottky barrier junction devices, gas permeation toward the metal/semiconductor interface works only for hydrogen or hydrogen-producing compounds and causes formation of a dipole layer. Organic semiconductors offer a viable alternative to conventional inorganic semiconductors for application in chemical sensors. They show sensitivities toward many gases or vapors, ranging from organic solvents to inorganic gases. Moreover, their porosity enables an ease of penetration of gases or vapors. The mechanical flexibility, environmental stability, and solution processability offer an enormous potential for applications within the field of microsensors. The macromolecular character and the high degree of flexibility in preparation make various physical and chemical properties realizable. The tunability of the sensing properties by the nature of the dopants as well as by the preparation procedures is an important benefit. Individual modification of each sensor is possible in only one step. This allows for the inexpensive fabrication of multisensing arrays, an aspect that makes sensors based on organic semiconductors suitable for commercialization. They are inherently compatible with solid-state integrated chemical sensors, because they can be readily incorporated into microfabricated structures. Additional benefits are the low device prices, the small dimensions, and low power consumption. © 2008 American Chemical Society.