Improvement of H-terminated Diamond FETs in NO2 Atmosphere
Michal Kubovic and Makoto Kasu
Materials Science Laboratory
Diamond transistors are expected to provide the best performance among semiconductors in high-frequency high-power operation because diamond has very high carrier velocity, the highest breakdown electric field strength, and the highest thermal conductivity. We reported the highest cut-off frequencies of fT of 45 GHz, fmax of 120 GHz, and the highest RF output power density of 2.1 W/mm at 1 GHz . However, the p-type doping mechanism of the hydrogen termination used in diamond transistors has not been clarified and the hole concentration has been limited at 1×1013 cm-2.
We have experimentally clarified that nitrogen dioxide (NO2) is the most beneficial p-type dopant for H-terminated diamond [2-4]. As shown in Fig. 1, when NO2 gas is adsorbed onto H-terminated diamond, the hole concentration increased greatly. With 300 ppm NO2, the maximum hole concentration reached 2.3×1014 cm-2, a value that is 20 times higher than the reported value in air. With this technology, we fabricated H-terminated diamond FETs whose drain current is 1.8 times higher due to a decrease in source resistance [Fig. 2(a)] and whose power-gain cut-off frequency (fmaxU) is 1.5 times higher [Fig. 2(b)] than those in air .
This work was partly supported by the SCOPE project of the Ministry of Internal Affairs and Communications, Japan.
 M. Kasu, K. Ueda, Y. Yamauchi et al., Diamond Relat. Mater. 16 (2007) 1010.
 M. Kubovic and M. Kasu, Appl. Phys. Express 2 (2009) 086502.
 M. Kubovic, M. Kasu, and H. Kageshima, Appl. Phys. Lett. 96 (2010) 052101.
 M. Kubovic, M. Kasu, H. Kageshima, and F. Maeda, Diamond Relat. Mater. (in press).
Fig. 1. Time evolution of hole sheet concentration
during NO2 adsorption and desorption.
Fig. 2. (a) Drain current-voltage characteristics and (b) frequency dependence of power gain (unilateral gain)
before and after NO2 adsorption.
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