Microwave Performance of Diamond Field-Effect Transistor
Physical Science Laboratory
At present the data transfer rate in communications is increasing very rapidly. Therefore, electronic devices that can operate at higher frequencies with higher output power are eagerly needed. Diamond has the highest thermal conductivity among materials and therefore offers the highest heat dissipation efficiency during high-power operation. Diamond exhibits a very high breakdown electric field, which means diamond devices can operate at extremely high voltage. These properties are important for high-output-power devices. In addition, the carriers in diamond have a high mobility and a high saturation drift velocity, which make high-frequency and high-speed operation possible. In fact, from device figures of merit calculated from the physical properties of high-frequency high-power devices diamond is the best among semiconductors and can therefore be called “ultimate semiconductor.” However, until now, the growth of high-quality diamond thin film had been impossible because of the formation of numerous crystalline defects and impurities during growth.
We have developed a CVD growth technology for high-quality diamond thin films with a low density of defects and impurities  and, using those thin films, have, in collaboration with the University of Ulm, Germany, fabricated diamond field-effect transistors (FET) , as shown in Fig. 1. In the FET, a quasi two-dimensional hole channel formed as a result of hydrogen surface termination, and we formed a 0.2 μm-long short gate contact using electron-beam lithography and self-aligned technology. As shown in the frequency dependence of power gains (Fig. 2), the FET exhibited the transition frequency (fT) for |h21|2 of 25 GHz, and the maximum oscillation frequency (fMAX) for the maximum available gain (MAG) and unilateral power gain (U) of 63 and 81 GHz, respectively. These values are the highest ever for diamond, and show the first amplification in the millimeter-wave range.
The power characteristics of the diamond FET for A-class operation at 1 GHz showed a linear power gain of 14 dB for a wide input power range, and the maximum output power (PMAX) of 0.35 W/mm. The PMAX is the highest ever for diamond, and will be increased by one order by optimizing the device structure. The microwave noise measurements of the FET showed that the minimum noise figure (Fmin) was only 0.72 dB at 3 GHz. This Fmin is better than that of Si MOSFET and comparable with those of p-GaAs HEMT and n-GaN HEMT with the same gate length. This means that a diamond FET is very promising for microwave receivers as well as transmitters.
 M. Kasu, M. Kubovic, et al., Diamond and Related Materials 13 (2004) 226.  M. Kubovic, M. Kasu, et al., to be published in Diamond and Related Materials (2004).
Fig. 1. Overview of diamond FETs.
Fig. 2. Frequency dependence of power gains.
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