III–V FET channel designs for high current densities and thin inversion layers

III–V FETs are being developed for potential application in 0.3–3 THz systems and VLSI. To increase bandwidth, we must increase the drive current I
d
= qn
s
v
inj
W
g
per unit gate width W
g
, requiring both high sheet carrier concentrations n
s
and high injection velocities v
inj
. Present III–V NFETs restrict control region transport to the single isotropic Γ band minimum. As the gate dielectric is thinned, I
d
becomes limited by the effective mass m*, and is only increased by using materials with increased m* and hence increased transit times.
¹
The deep wavefunction also makes Γ -valley transport in low-m*materials unsuitable for < 22-nm gate length (L
g
) FETs. Yet, the L-valleys in many III–V materials
²
have very low transverse m
t
and very high longitudinal mass m
1
. L-valley bound state energies depend upon orientation, and the directions of confinement, growth, and transport can be chosen to selectively populate valleys having low mass in the transport direction
^3,4
. The high perpendicular mass permits placement of multiple quantum wells spaced by a few nm, or population of multiple states of a thicker well spaced by ∼10–100 meV. Using combinations of Γ and L valleys, n
s
can be increased, m* kept low, and vertical confinement improved, key requirements for <20-nm L
g
III–V FETs.