30-nm InAs PHEMTs With $f_{T} = \hbox{644}\ \hbox{GHz}$ and $f_{\max} = \hbox{681}\ \hbox{GHz}$

We present 30-nm InAs pseudomorphic HEMTs (PHEMTs) on an InP substrate with record <i>fT</i> characteristics and well-balanced <i>fT</i> and <i>f</i><sub>max</sub> values. This result was obtained by improving short-channel effects through widening of the side-recess spacing (<i>L</i><sub>side</sub>) to 150 nm, as well as reducing parasitic source and drain resistances. To compensate for an increase in <i>Rs</i> and <i>Rd</i> due to <i>L</i><sub>side</sub> widening, we optimized the ohmic contact process so as to decrease the specific ohmic contact resistance (<i>Rc</i>) to the InGaAs cap to 0.01 Ω·mm. A 30-nm InAs PHEMT with <i>t</i><sub>ins</sub> = 4 nm exhibits excellent <i>gm</i>,max of 1.9 S/mm, <i>fT</i> of 644 GHz, and <i>f</i><sub>max</sub> of 681 GHz at <i>V</i><sub>DS</sub> = 0.5 V simultaneously. To the knowledge of the authors, the obtained <i>fT</i> in this work is the highest ever reported in any FET on any material system. This is also the first demonstration of simultaneous <i>fT</i> and <i>f</i><sub>max</sub> higher than 640 GHz in any transistor technology.

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