L_g= 19 nm In_0.8Ga_0.2As composite-channel HEMTs with f_T= 738 GHz and f_max= 492 GHz

We present Lg = 19 nm In0.8Ga0.2As composite-channel high-electron mobility transistors (HEMTs) with outstanding DC and high-frequency characteristics. We adopted a composite-channel design with an In0.8Ga0.2As core layer that led to superior carrier transport properties. The device with Lg = 19 nm displayed an excellent combination of RON = 271 Ω-μm, gm_max = 2.5 mS/μm and fT/fmax = 738/492 GHz. To understand the physical origin of such an excellent combination of DC and RF responses, we analyzed the effective mobility (μn_eff) and delay time for both long- and short-Lg devices, revealing a very high μn_eff value of 13, 200 cm2/V•s and an average velocity under the gate (vavg) of 6.2 × 107 cm/s. We also studied the impact of the gate-to-source spacing (LGS) and the electrostatic integrity of the device, finding that a reduction of LGS less than 0.6 μm was of little use in improving gm_max and fT. Additionally, the intrinsic output conductance (go_int) had an important impact on fT in short-Lg HEMTs.