In standard vertical SJ devices the doping of the - and
-column
of the drift region must be balanced exactly. Most of the previous
works assume that the charge of the
-column
is equal to that
of the
-column
. The BV depends on the critical electric
field
of the device and the
length of the
- and
-columns.
In the SOI-LDMOSFETs a large portion
of the voltage is supported by the buried oxide layer and
the charge of the -body affects the RESURF condition significantly.
Unlike in conventional RESURF devices, three-dimensional
RESURF phenomena can be seen in this structure.
,
,
and the charge
of the p body depletion region should be
balanced. Assuming that all columns are completely depleted before
breakdown, the charges and BV are given by
![]() |
(4.1) |
![]() |
(4.2) |
![]() |
(4.3) |
where
is the length of the
- and
-columns, respectively.
The BV depends both on the critical electric field
and the column length.
Figure 4.26 shows the -column doping
dependence on the BV
of the SJ SOI-LDMOSFETs, and the doping of the
-column is lower than that of the
-column.
For the SJ SOI-LDMOSFET with an
-column doping
of 9.9
, a maximum BV of 124V is obtained
at
6.5
. With the
of
6.0
, a maximum BV of 127V is
obtained at
2.5
.
These results demonstrate that the charge of the
-body strongly
affects the charge balance condition of the SJ SOI-LDMOSFETs.
Figure 4.27 shows the
dependence on the electric field
strength near the surface of the device
along the
- and
-column junction.
![]() |
![]() |
At the gate edge a high
electric field can be seen with a low
of 5.5
, and if the
is increased
to the value of 9.0
, a high
electric field is moved toward the drain edge. The optimum electric
field strength distribution is obtained
with the
of 7.0
. It proves that the
optimum RESURF condition can be obtained
with
much lower than
.
A similar result can be seen for the lateral trench gate SJ
SOI-LDMOSFET (dashed line in Figure 4.26). With an -column doping
of
6.0
and a width
2
of 1.0
m,
the maximum BV is 120V at
6.0
.
Even with a 2 times larger
-column width than that of the
-column
the optimum doping
is the same as
in this case.
Figure 4.28 shows the almost uniformly distributed potential lines of a
lateral trench gate SJ SOI-LDMOSFET at the drain voltage
of 120V.
One can clearly see that most of the potential drops over the buried oxide layer.
Curved potential lines at the top surface of the device by
the lateral depletion along the
- and
-column junction are also visible.
The BV of SJ devices strongly depends on the charge balance condition.
As has been shown in Figure 4.26, the BV decreases abruptly with decreasing
.
In practical manufacturing it is difficult to achieve perfect charge balance.
Generally, it is assumed that the doping can be controlled within
10% of the
nominal charge [32].
Figure 4.29 shows the sensitivity of the
charge imbalance on the BV. By proper choosing the -column doping
(near the value of the maximum breakdown region in Figure 4.26)
the relations between the BV and the charge imbalance can be seen clearly.
In this figure
of 7.0
(SJ SOI-LDMOSFET with
of 9.9
), 3.0
(SJ SOI-LDMOSFET
with
of 6.0
),
and 6.0
(lateral trench gate SJ SOI-LDMOSFET with
of
6.0
) are used as reference values, respectively.
As shown in Figure 4.29, this sensitivity (slope of the line) is reduced if the
doping of the drift region is lowered. The drastically reduced sensitivity can be seen
in the SJ SOI-LDMOSFET with a doping concentration
of
6.0
(dotted line).
The reduced BV (110V) with the
change from
20% to
20% is
over 90% of the reference value (120V at zero charge imbalance).
![]() |
However, this results in an increasing on-resistance, which can be overcome
by increasing the -column width together with the lateral trench gate.
Then it is possible to lower the doping of the drift region without degrading
the on-resistance. The reduced BV (104V) of the lateral trench gate
SJ SOI-LDMOSFET with the
change from 0% to
20% is about 87% of
the reference value (120 V). Compared to the BV reduction (88 V) of the
standard SJ SOI-LDMOSFET with
= 9.9
,
the sensitivity of the BV to the charge imbalance is reduced in the
proposed structure.
SJ LDMOSFET on SOI | Lateral trench gate SJ SOI-LDMOSFET | |
![]() |
9.9 ![]() ![]() ![]() |
6.0 ![]() ![]() ![]() |
![]() |
7.0 ![]() ![]() ![]() |
6.0 ![]() ![]() ![]() |
![]() |
2.03m![]() ![]() |
1.79m![]() ![]() |
BV | 117V | 120V |
Figure 4.30 and Table 4.3 show the results of the on-state characteristics
of a conventional, a SJ SOI-LDMOSFET, and a lateral trench gate SJ SOI-LDMOSFET.
From this figure it is clear that the lateral trench gate SJ
SOI-LDMOSFET has superior current handling capability compared to the others.
of this device is 1.79m
at
12V
and
0.5V. It is about 60% of the corresponding
value
of a conventional 120V SOI-LDMOSFET (about 3.0m
).
Even the doping of the drift region is reduced by increasing the width
of the
-column
is lower than that of the SJ SOI-LDMOSFET with
a much higher
-column doping up to 9.9
.
Jong-Mun Park 2004-10-28