
119GaN-HEMT Scaling Technologies for High-Frequency
gate HEMTs—which causes a negative threshold voltage (V
th
) shift, a decreased
transconductance (g
m
), an increased drain-induced barrier lowering (DIBL), and
an increased output conductance (g
d
). Increased g
d
degrades both f
T
and f
max
as can
be seen in the above equations. The short-channel effect is characterized by the
channel aspect ratio (L
g
/d, where d is the gate-to-channel distance), and L
g
/d ≥ 5
is typically required to prevent a severe V
th
shift and g
m
degradation (Figure 4.8)
[36,37]. Spontaneous and piezoelectric polarization in GaN/AlGaN heterostruc-
tures induces a two-dimensional ...