Schottky diode gas sensors

An ozone treatment (10 minutes at room temperature) of the HF-etched SiC surface before the metallization step was introduced as a very convenient processing step to produce Schottky diode gas sensors with an increased stability and reproducibility. The use of spectroscopic ellipsometry analysis and also photoelectron spectroscopy using synchrotron radiation showed that an oxide, 1-nm in thickness, was formed by the ozone exposure [74, 75]. The oxide was also found to be close to stochiometric SiO in composition. This thin oxide increased the stability of the SiC Schottky diodes considerably, without the need for any further interfacial layer such as Ta or TaSi which have been frequently used. Schottky diodes employing a porous Pt gate electrode and the ozone-produced interfacial layer have been successfully operated in both diesel exhausts and flue gases [76, 77]. 

Weidemann et al. found that wet etching of a GaN surface before Pd deposition also produced an interfacial oxide, which increased the hydrogen sensitivity by approximately a factor of 50 [14], They concluded that comparing device parameters between different GaN Schottky diode gas sensors requires a defined standard treatment of the GaN surface to introduce a controlled interfacial oxide. 

Matsuo, K., Negoro, N., Kotani, J., Hashizume, T. and Hasegawa, H. (2005) Pt Schottky diode gas sensors formed on GaN and AlGaN/GaN heterostructure Applied Surface Science, 244(1-4), 273-276. 

Chen, L.-Y., Hunter, G.W., Neudeck, P.G., Bansal, G., Petit, J.B., Knight, D., Liu, C.-C. Wu, Q.H. (1996). Electronic and interfacial properties of Pd/6H-SiC Schottky diode gas sensors. Proceedings of the Third International High Temperature Electronics Conference, Albuquerque, NM... 

Among the semiconductor hydrogen sensors, Schottky diodes are preferably selected as gas sensors due to its much simpler fabrication procedures and electronic circuitry required for operation. The early Schottky-diode hydrogen sensors often used silicon as substrate because high-quality Si is commercially available and its cost is very low. Silicon is the second most abundant element in the world and is the dominant semiconductor used in the solid-state electronics industry. The properties of silicon have been widely studied and the silicon technology is the most mature and advanced among all semiconductor technologies. 

The hydrogen sensitivity of palladinm-oxide-semiconductor (Pd-MOS) strnctnres was first reported hy Lnndstrom et al. in 1975 [61]. A variety of devices can he nsed as field-effect chemical sensor devices (Fignre 2.6) and these are introdnced in this section. The simplest electronic devices are capacitors and Schottky diodes. SiC chemical gas sensors based on these devices have been under development for several years. Capacitor devices with a platinum catalytic layer were presented in 1992 [62], and Schottky diodes with palladium gates the same year [63]. In 1999 gas sensors based on FET devices were presented [64, 65]. There are also a few publications where p-n junctions have been tested as gas sensor devices [66, 67]. 

SiC capacitor sensors have demonstrated gas-sensitivity to gases such as hydrogen and hydrocarbons [21, 46, 68] up to a maximum temperature of 1,000°C [1, 68]. Devices that can be operated both as MOS capacitors (reverse bias) and as Schottky diodes at temperatures greater than 490°C have also been demonstrated (see Section 2.4.2) [10]. These devices showed sensitivity to combustible gases such as propane, propylene, and CO and were tested at temperatures up to 640°C. 

It was demonstrated that reproducible gas-sensitive silicon Schottky sensors could be produced after terminating the silicon surface with an oxide layer [71, 72]. This interfacial oxide layer permits the device to function as a sensor, but also as a diode, as the charge carriers can tunnel through the insulating layer. The layer made the Schottky diode behave like a tunneling diode, and the ideality factor could be voltage-dependent [73]. 

Schottky diode sensors based on other wide bandgap materials have also been investigated, as previously mentioned. GaN Schottky diodes processed on either the Ga or N face have been examined by Schalwig et al. [11,21]. A Pt/GaN Schottky diode with a barrier height of 1-eV has been shown to reversibly transform into an ohmic contact through exposure to [94]. Kokobun et al. have also investigated Pt-GaN Schottky diodes as hydrogen sensors up to 600°C [15]. 

Schottky-Barrier Diode and Metal-Oxide-Semiconductor Capacitor Gas Sensors Comparison and Performance... 

Schottky-barrier diode and metal-oxide-semiconductor (MOS) capacitor gas sensors have established themselves as extremely sensitive, versatile solid state sensors. 

Schottky diodes are the simplest sensor type to fabricate and can be operated in two different modes. In this section we cover their use for gas sensing. 

S. J. and Ren, F. (2009a) Hydrogen sensing of N-polar and Ga-polar GaN Schottky diodes . Sensors and Actuators B-Chemical, 142(1), 175-178. 

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