Diamond FET with Maximum Frequency of Oscillation of 120 GHz

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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 `100ƒÊm, 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.

Fig. 1. Schematic cross-section of a polycrystalline diamond FET.
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@Fig. 2. RF gain plot of FET.

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