Lg = 19 nm In0.8Ga0.2As composite-channel HEMTs with fT = 738 GHz and fmax = 492 GHz

We present L
_g
= 19 nm In
_0.8
Ga
_0.2
As composite-channel high-electron mobility transistors (HEMTs) with outstanding DC and high-frequency characteristics. We adopted a composite-channel design with an In
_0.8
Ga
_0.2
As core layer that led to superior carrier transport properties. The device with L
_g
= 19 nm displayed an excellent combination of R
_ON
= 271 Ω-μm, g
_{m_max}
= 2.5 mS/μm and f
_T
/f
_max
= 738/492 GHz. To understand the physical origin of such an excellent combination of DC and RF responses, we analyzed the effective mobility (μ
_{n_eff}
) and delay time for both long- and short-L
_g
devices, revealing a very high μ
_{n_eff}
value of 13,200 cm
²
/V·s and an average velocity under the gate (v
_avg
) of 6.2 × 10
⁷
cm/s. We also studied the impact of the gate-to-source spacing (L
_GS
) and the electrostatic integrity of the device, finding that a reduction of L
_GS
less than 0.6 μm was of little use in improving g
_{m_max}
and f
_T
. Additionally, the intrinsic output conductance (g
_{o_int}
) had an important impact on f
_T
in short-L
_g
HEMTs.