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