Sensors zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA and Actuators B, 6 ( 1992) 61-65 61 An application of carbon-type semiconductors for the construction of a humidity-sensitive diode Jerzy P. Lukaszewicz Institute of Chemistry, Nicholas Copernicus University, ul. Gagarina 7, 87-100 Toruri (Poland) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO Abstract This paper describes an application of carbon-type semiconductors for the construction of humidity-sensitive diodes. The sensing diodes consist of two semiconducting layers with different electronic properties deposited onto a quartz substrate by means of the spray pyrolysis method. The main electronic properties of carbons, i.e., electrical conductivity, and the type of majority current carriers were estimated by a suitable heat treatment. This resulted in the construction of humidity-sensitive diodes exhibiting a typical voltage-current diode-like behaviour in the presence of moisture. Some water adsorption and electronic properties of single carbon layers are also presented and discussed in terms of the amplification effect of the diode junction. Introduction Recently, many organic or inorganic materials have been utilized for the construction of humidity sensors based on different principles. Capacitive- type humidity sensors containing a polypyrrole adsorptive layer seem to be the most developed humidity-sensitive elements frequently applied in numerous commercial electronic hygrometers [ 11. However, some other materials and sensor struc- tures are under investigation, such as perovskite- type oxides [ 21. The application of diode-structured elements led to moisture-sensitive sensors, despite the fact that the materials the sensors consisted of (i.e., LaCr03) did not change their electronic prop- erties significantly upon humidity variation when tested separately at room temperature [2]. The potential barrier existing at the diode junction and the change of its height drastically affect the con- duction properties of the whole diode-type element zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC 12, 31. Carbon-type materials are well known as efficient water adsorbents, but they also exhibit reversible adsorption of water and other polar molecules at room temperature [4]. Moreover, nu- merous carbons exhibit semiconducting properties dependent on their preparation method. By select- ing a suitable raw material and carbonization pro- cedure, some electronic properties of carbons may be driven to yield either n-type or p-type semicon- ductors. For example, carbons obtained from polyfurfuryl alcohol at temperatures above 900 “C (long-term heating) usually show n-type semicon- ductivity, while those prepared at lower tempera- tures (550-600 “C) from the same raw material behave like p-type semiconductors [S]. Thus, the carbonization procedure is a useful instrument for creating carbon semiconductors suitable for a fur- ther application as gas-sensitive electronic ele- ments such as resistors and particularly diodes, for which the ‘amplification effect’ of the potential barrier is supposed to occur. Experimental To obtain thin carbon layers, the spray pyrolysis method was applied. Partially polycondensated furfuryl alcohol was sprayed from a precise atomiser onto a hot (400-450 “C) quartz sub- strate. Small drops of furfuryl alcohol underwent simultaneous polycondensation and thermal de- composition in contact with the hot substrate, yielding a black non-conductive layer of thermally decomposed polymer. Then, the polymer-covered substrate was placed in a quartz tube, carefully outgassed and heated up to the required tempera- ture of 800 “C. This was sufficient to produce a carbon layer of relatively high electrical conductivity (called HT (high-temperature) carbon). Using a suitable screen and repeating the above described spraying, 0925-4005/92/$5.00 @ 1992 - Elsevier Sequoia. All rights reserved