pH sensors based on wide bandgap semiconductors

Wide bandgap semiconductors like diamond and GaN are privileged materials for chemical sensing because of their high stability to many chemical agents. Indeed, the acidity variation of chemical solutions from pH=0 to pH=14 will modify the surface potential by /spl Delta//spl phi//spl ap/0.84 eV (59.3 mV/pH at 20/spl deg/C), which is only a small part of the bandgap of these materials. Therefore, such sensors can operate without surface passivation like Si-based sensors and no degradation/hysteresis behaviour is expected even in harsh environments and at high temperatures. However, the second requisite is a unpinned surface potential. The operation of the chemically controlled semiconductor sensors presented here is based on the FET mode of operation, i.e. on modulation of the conductivity of the device channel by an electrochemical potential at the liquid-semiconductor interface forming a liquid gate. It is known that an unpinned surface state can be obtained on p-type diamond provided that the surface is terminated with hydrogen atoms (Kawarada, 1996). In this case, the barrier heights of Schottky contacts exhibit a strong dependence on the metal work function. A similar behaviour of Schottky barrier heights (SBH) is also observed on as-grown n-type GaN layers. These results imply that the channels of both diamond and GaN can be depleted depending on the redox potential of aqueous solutions. In this paper, the results of first pH sensors on diamond and GaN are described.