Akira Takahashi1
Waseda univ1
abstract<br/>We have fabricated a high frequency 2DHG diamond MOSFET with air-bridge and multi-finger structure to increase the gate width. DC and RF performance of the double-finger devices and multi-finger devices were compared and investigated. As a result, the multi-finger devices did not degrade the current density due to the increase of gate width (W<sub>GT</sub>).<br/>1. Introduction<br/>We fabricated 2DHG diamond MOSFET which W<sub>G</sub>=1 mm using a multi-finger structure. High output power performance is critical for the RF amplifier applications. It is valid for high output to increase actual current by extending the gate width. The device with extended W<sub>G</sub> may deteriorate output current because of self-heating, and influence characteristics of devices caused by increase of the gate resistance. In this work, 2DHG MOSFETs with multi-finger structure were fabricated on a diamond substrate, and its DC and RF characteristics were evaluated and compared with those of the double-finger structure.<br/>2. Device information<br/>Devices were fabricated on a (001) diamond substrate. After the boron-doped layer deposited, source and drain electrodes are deposited Ti/Pt/Au (20nm/20nm/90nm). After H-termination and isolation, deposited Al<sub>2</sub>O<sub>3</sub> as gate insulator. After that, 100nm of Al was deposited as the gate. The gate length was fixed L<sub>G</sub>=0.5µm and the source-drain length L<sub>SD</sub> was fixed 4.0 µm. the gate-drain length L<sub>GD</sub> defined as 2.5µm. The air-bridge over the gate fingers is made of Au (1µm) after resist application.<br/>3. Result and Discussion<br/>We compared DC characteristics of double-finger structure and multi-finger structure. The result of double-finger device with W<sub>GT</sub>=25µm×2 at V<sub>GS</sub>=-24V and V<sub>DS</sub>=-40V, the maximum drain current density I<sub>Dmax</sub> was 428mA/mm. The result of multi-finger device by air-bridge with W<sub>GT</sub>=25µm×6 at V<sub>GS</sub>=-24V and V<sub>DS</sub>=-40V, the maximum drain current density I<sub>Dmax</sub> was 433mA/mm. Since the drain current densities are almost the same in the double-finger and multi-finger devices the actual current value of multi-finger is almost three times higher than that of double-finger. In multi-finger, the current density is not degraded by gate resistance or heat, because W<sub>GU</sub> (length of one finger) remains the same even when W<sub>GT</sub> is increased.<br/>The frequency at MSG and MAG convert, defined f<sub>k</sub>, is decreased with increasing gate width. But it is confirmed that the rate of decreasing f<sub>k</sub> with the multi-finger device is much smaller than that of double-finger device. Comparing the maximum oscillation frequency (f<sub>max</sub>) with W<sub>GT</sub>=1000µm, double-finger device indicates 4.8GHz and multi-finger device indicates 9.0 GHz. The reason why high f<sub>max</sub> is obtained with multi-finger is that the gate resistance becomes smaller and the decrease in f<sub>k</sub> is smaller, because the W<sub>GU</sub> is maintained. These results indicate that the multi-finger structure, in which the gate length (W<sub>GT</sub>) can be increased while keeping the length of one finger (W<sub>GU</sub>) short, can suppress the decrease in f<sub>max</sub> and improve the high-frequency performance.<br/>3.Conclusion<br/>In this work, we fabricated 2DHG diamond MOSFETs which W<sub>GT</sub> = 1mm with multi-finger structure on (001) diamond substrate for RF application and evaluated DC and RF performance. It was also confirmed that the introduction of the multi-finger structure did not cause deterioration in current density as the gate width expanded in association with the increase of gate fingers. In the RF characteristics evaluation, it was found that the multi-finger structure has a smaller rate of decrease in f<sub>k</sub> with an increase in gate width, and the maximum oscillation frequency is 9.0 GHz at W<sub>GT</sub>＝100 µm×10 whereas 4.8 GHz at W<sub>GT</sub>=500 µm×2. The multi-finger structure makes it possible to fabricate diamond devices in densely integrated structure with much smaller W<sub>S</sub> (width of Source electrode between gate fingers). It improves high-frequency characteristics. Because of high thermal conductivity, the device integration is the advantage of diamond compared with GaN.