Analysis of Dependence of Breakdown Voltage on Gate–Drain Distance in AlGaN/GaN HEMTs With High-k Passivation Layer

A 2-D analysis of OFF-state breakdown characteristics of AlGaN/GaN HEMTs with a high- k passivation layer is performed as a function of gate-to-drain distance L
_GD
. The relative permittivity of the passivation layer ε
_r
is changed from 1 to 60, and L
_GD
is changed from 1.5 to 10 μ\textm. It is shown that, in all cases with different L
_GD
, the breakdown voltage V
_br
increases as ε
_r
increases. When a deep-acceptor density in an Fe-doped buffer layer N
_DA
is 10
¹⁷
cm
^-3
and the gate length is 0.3 μm, V
_br
is determined by buffer leakage current at ε
_r
≥ 30 before impact ionization dominates. Hence, V
_br
is similar at L
_GD
= 3-10 μm, and the increase rate in V
_br
from L
_GD
= 1.5 μm is about 50% even at ε
_r
= 60. However, when N
_DA
is 2×10
¹⁷
cm
^-3
, V
_br
is determined by impact ionization of carriers even at ε
_r
≥ 30 because the buffer leakage current is reduced. V
_br
becomes about 500, 930, 1360, and 1650 V for L
_GD
= 1.5, 3, 5, and 7 μm, respectively, at ε
_r
= 60. These voltages correspond to gate-to-drain average electric fields of about 3.3, 3.1, 2.7, and 2.3 MV/cm, respectively. Particularly, for short L
_GD
, the electric field profiles between the gate and the drain are rather uniform. However, in the case of L
_GD
= 10 μm, V
_br
is about the same as that (1650 V) of L
_GD
= 7 μm, suggesting that the electric field at the drain edge of the gate becomes a critical value before the high electric field region extends to the drain enough. This may be a limitation to increase V
_br
by using a high- k passivation layer in this case. However, it can be said that, to improve V
_br
further at long L
_GD
, such as 10 μm, the combination of field plate or using a higher ε
_r
material may be effective because both of them decrease the electric field at the drain edge of the gate.