III-Nitride Field-Effect Transistors with Capacitively-Coupled Contacts

We demonstrate novel wide bandgap AlGaN/GaN heterostructure field-effect transistor design using capacitively-coupled electrodes (C3HFET). The source, gate and drain of C3HFET have capacitive coupling with the 2D-channel. Both Schottky and insulated gate (C3MOSHFET) device implementations have been realized. The devices do not require annealed ohmic contacts and can be fabricated using gate alignment-free technology. Besides obvious advantages in the fabrication technology, the C3HFETs and C3MOSHFETs have low effective RF contact resistance (below 0.5 Qmm) and are capable of handling much higher RF powers when used as RF control devices, such as power modulators, switches, attenuators etc. To the best of our knowledge this is first report of this type of wide bandgap high-power RF devices. To date nearly all the III-N high frequency electronic devices are based on 2D electron gas (2DEG) channel at the AlGaN-GaN heterointerface with high-temperature annealed source-drain ohmic contacts. The annealing temperatures (typically over 850 C) degrade AlGaN-GaN heterojunction, generate trapping centers, and may significantly reduce the device reliability. There exists a broad class of RF control devices, which operation does not require the DC bias. These are RF switches, attenuators, modulators, power limiters etc. For this device types we explore a new design approach using capacitive coupling between the metal electrodes and the high-density 2DEG at the AlGaN/GaN interface. Novel C3HFET consists of three metal electrodes, source, gate and drain, deposited on top of AlGaNGaN structure. The 25 nm thick A1025Ga075N barrier layer was grown by MOCVD over 1.5 pim thick undoped GaN buffer layer on SiC substrate. The C3MOSHFET has an additional 10 nm thick SiO2 layer under the metal electrodes. No annealing was used in device fabrication. The length of the source and drain electrodes, L= 5 ptm; the gate length LG=1 ptm. The source-drain spacing is LDs=5jm. Total device width W= 2 x 125 ptm. High-frequency characteristics of C3devices were measured using HP 851 OC network analyzer. Above the contact cut-off frequency of around 1 GHz, the insertion loss is the same or lower than that of a regular HFET or MOSHFET with annealed contacts. The effective contact resistance extracted from the S-parameters was below 0.5 Q-mm. The power-handling capability of novel C3 transistors was characterized at 10 GHz. The most significant large-signal distortions in RF control devices manifest themselves as an increase in the subthreshold currents due to channel potential modulation; normally, this increase in the sub-threshold current impose the most critical limitation on the maximum operating RF powers. As a result, the typical maximum operating (switching) RF powers do not exceed 1W for GaAs devices and around 20 W for the AlGaN/GaN based HFETs. We have found that for the novel AlGaN/GaN C3 HFETs and C3MOSHFETs the maximum RF powers were around 10 times higher than those of HFET and MOSHFET devices with ohmic contacts. The highest maximum powers were achieved with the insulated gate C3MOSHFETs. Our estimations show that for 1 mm wide C3MOSHFETs the maximum RF powers are as high as 100 W. To the best of our knowledge these are the highest RF power densities ever achieved with the field-effect transistors. Further experiments and simulations showed that these record high switching powers are due to the self-modulation if the 2DEG density under the capacitively-coupled contacts by the large-amplitude RF input signals.