Electron Transport Mechanism for Ohmic Contact to GaN/AlGaN/GaN Heterostructure Field-Effect Transistors

The contact resistance of Mo-based ohmic contact was investigated for undoped GaN/AlGaN/GaN heterostructure field-effect transistors having different GaN cap layer thickness (t
_cap
) and AlN mole fraction (x
_Al
) values. Measured specific contact resistivity (ρ
_c
) exhibited a drastic increase by a factor of 10
³
by increasing t
_cap
from 2 to 5 nm, and a minimum ρ
_c
value was 3.2×10
^-7
Ωcm
²
at T=300 K. We have developed a model of the ohmic contact assuming an n-type interfacial layer (donor density: N
_d
, thickness: t
_d
). Theoretical calculations suggested that the drastic increase of ρ
_c
by increasing t
_cap
is due to td being between 2 and 5 nm. Also, the present model optimizing t
_d
and N
_d
values successfully explained measured trends of ρ
_c
with respect to t
_cap
, x
_Al
, and temperature.