Kenji Ueda and Makoto Kasu
Materials Science Laboratory
Diamond is expected to be the most suitable material for high-power high-frequency
electronic devices because of its high electric breakdown field (>10
MV/cm), high carrier mobility (4500 cm2/Vs for electrons, 3800 cm2/Vs for holes), and highest thermal conductivity (22 W/cmK). Recently,
using homoepitaxial single-crystal CVD diamond, we achieved the maximum
output-power density of 2.1 W/mm at 1 GHz [1], which is high enough for
power amplifiers of the base stations in wireless communications systems.
However, the size of single-crystal CVD diamond is limited to 4 mm, which
is the size of commercially available HPHT-synthesized diamond substrates.
From the viewpoint of semiconductor-device processing, at least four-inch
wafers are needed. One possible solution to this problem is to use
high-quality polycrystalline diamond, whose maximum size is 4 inches.
Here, using a high-quality polycrystalline diamond film, we report significant
progress in fabricating FETs. The grain size of the polycrystalline
film is ～100μm, which is comparable to our FET size. Thus, the
effect of the grain boundary seems to be very small.
As shown in Fig. 1, the FETs were fabricated on the freestanding polycrystalline
diamond grown by CVD (size: 10 mm×10 mm×0.5 mm). The diamond surface
was passivated with hydrogen (H-passivation) to form a quasi two-dimensional
hole channel. The source and drain Au ohmic contacts were formed
on the H-terminated surface. Electron-beam lithography and self-alignment
techniques enabled us to form 0.1-mm-long Al Schottky gate contacts.
The DC characteristics show drain current (IDS) of 550 mA/mm at gate source voltage (VGS) of -3.5 V. The IDS is comparable to the maximum value of single-crystal CVD diamond FETs.
The DC transconductance (gm) stays high (～140 mS/mm) in a relatively wide VGS range. The transition frequency (fT) and maximum frequency of oscillation (fmax) were extracted from the frequency dependence of the
short circuit current gain (|h21|2) and the unilateral power gain (U) as shown in Fig. 2. The maximum
fmax is 120 GHz, and in a different bias condition, fT of 45 GHz was obtained. These fT and fmax values are the highest among diamond FETs [2].
[1] M. Kasu, K. Ueda, H. Ye,
Y. Yamauchi, et al., Electron. Lett. 41 (2005) 1249.
[2] K. Ueda, M. Kasu, Y. Yamauchi, et al., IEEE Electron Device Lett. 27 (2006) 570.
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