Ultra-thin barrier (UTB) 4-nm-AlGaN/GaN normally-off high electron mobility transistors (HEMTs) having a high current gain cut-off frequency (fT) are demonstrated by the stress-engineered compressive SiN trench technology.The compressive in-situ SiN guarantees the UTB-AlGaN/GaN heterostructure can operate a high electron density of 1.27× 1013 cm-2,a high uniform sheet resistance of 312.8 Ω/□,but a negative threshold for the short-gate devices fab-ricated on it.With the lateral stress-engineering by full removing in-situ SiN in the 600-nm SiN trench,the short-gated(70 nm) devices obtain a threshold of 0.2 V,achieving the devices operating at enhancement-mode (E-mode).Meanwhile,the novel device also can operate a large current of 610 mA/mm and a high transconductance of 394 mS/mm for the E-mode devices.Most of all,a high fT/fmax of 128 GHz/255 GHz is obtained,which is the highest value among the reported E-mode AlGaN/GaN HEMTs.Besides,being together with the 211 GHz/346 GHz of fT/fmax for the D-mode HEMTs fabricated on the same materials,this design of E/D-mode with the realization of fmax over 200 GHz in this work is the first one that can be used in Q-band mixed-signal application with further optimization.And the minimized processing differ-ence between the E-and D-mode designs the addition of the SiN trench,will promise an enormous competitive advantage in the fabricating costs.