Depletion- and enhancement-mode modulation-doped field-effect transistors for ultrahigh-speed applications: An electrochemical fabrication technology

This paper is devoted to an electrochemical-etching-based technology for fabricating high-performance MODFET's for high-speed applications. The electrochemical etching in the gate openings is induced by the exposure of the Ni surface metal on the ohmic electrodes. It results in very slender gate-recess grooves, which are desirable for high-speed MODFET's because of the resulting achievable small gate-to-channel separation and low parasitic resistance. The technology is easy to implement, and is effective for enhancing the aspect ratio. Good control of aspect ratio is essential for achieving excellent device performance and limiting deleterious short-channel effects. Successful vertical scaling, together with minimization of gate length by well-established electron-beam lithography using fullerene-incorporated electron-beam resist, leads to the realization of both optimal D- and E-mode MODFET's with ultrahigh extrinsic transconductance values and current gain cut-off frequencies. Fully passivated 0.07-μm D-MODFET's with 2.25 S/mm extrinsic transconductance and current gain cut-off frequency exceeding 300 GHz have been successful fabricated. In addition, 0.03-μm E-MODFET's with 2 S/mm transconductance and 300 GHz current gain cut-off frequency have been demonstrated. This electrochemical-etching-based technology provides both high-performance D- and E-MODFET's and, therefore, opens up the possibility to achieve ultrahigh-speed IC's based on DCFL configurations.