RF LDMOS on partial SOI substrate

Details RF LDMOS on partial SOI substrate RF LDMOS on partial SOI substrate

2002-07-01

2003-12-23

Wilson, Allan R. (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S347000, C257S408000

Reexamination Certificate

active

06667516

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the general field of MOSFETs with particular reference to LDD MOSFETs and with added emphasis on improving power and frequency behavior.
BACKGROUND OF THE INVENTION
In its most general form, a field effect transistor (FET), lasted at an active region, consists of a semiconductor substrate (usually silicon) on which is grown a thin layer of insulating oxide (SiO
2
). A conducting layer (a metal or heavily doped polysilicon) called the gate electrode is deposited on top of the oxide. Two heavily doped regions called the source and the drain are formed in the substrate on either side of the gate. The source-to-drain electrodes are equivalent to two PN junctions back to back. This region between the source and drain regions is called the channel region. The gate electrode can control the flow of current from source to drain by varying the amount of charge present in the channel region.
When power is not a concern, the most economic layout for FETs is for source, gate, and drain to all lie in the same plane. When the device is required to operate at high power, means must be found for dissipating the generated heat, particularly at the source. To accomplish this, the design illustrated in
FIG. 1
has been widely adopted in the industry. In this design, connection to the source is made though lower area
11
a
which occupies the entire bottom of the device, where it can be directly connected to a heat sink. Lower area
11
a
is connected to source
11
b
through sinker
12
. Both
11
a
and
12
are of P+ silicon because P− region
18
n
needs to be grounded and metallic shorting bar
13
is provided in order to connect
11
b
to
12
. The remainder of the device is of a standard nature. Gate
14
controls the current flow in the body of the device
18
, across channel region
15
, into the drain which is made up of an inner, lightly doped section
16
and an outer, heavily doped section
17
.
FIG. 2
shows the equivalent circuit of the design seen in FIG.
1
. In addition to the series resistances R
s
and R
d
associated with the source and drain respectively, three parasitic capacitances can also be seen. These are the source-gate capacitance C
gs
, the drain-gate capacitance C
dg
, and the source-drain capacitance C
ds
. Of these, C
ds
is the largest and most important in terms of determining frequency response of the device.
Unfortunately, C
ds
is large in designs of the type shown in
FIG. 1
because of the relatively thin depletion layer that forms at the N+/P− interface. One approach that has been used to overcome this problem has been the design illustrated in FIG.
3
. Here, dielectric layer
33
is inserted between the source, drain and channel regions
11
b
,
16
/
17
, and
15
, respectively. This ensures that the magnitude of C
ds
will be determined by the thickness of
33
rather than by any depletion layers. While this approach is effective in greatly reducing C
ds
, it has the unfortunate side effect of blocking the flow of heat from the source area
11
b
down to heat sinking area
11
a
. Thus, devices of the type shown in
FIG. 3
are generally limited to operating at low power levels.
A routine search of the prior art was performed but no references that teach the exact processes and structures of the present invention were discovered. Several references of interest were, however, encountered along the way. For example, in U.S. Pat. No. 5,554,546, Malhi shows a “partial SOI” LDMOS with oxide under the channel, drain and source. In U.S. Pat. No. 5,930,642, Moore et al. describe a LDMOS with oxide under the channel. In U.S. Pat. No. 5,650,354, Himi et al. show a SIO LDMOS without oxide under the Tx. Pein (U.S. Pat. No. 5,382,818), Pein (U.S. Pat. No. 5,378,912), and Yamaguchi et al. (U.S. Pat. No. 5,777,365) all show various LDMOS devices with different oxide layer configurations.
SUMMARY OF THE INVENTION
It has been an object of the present invention to provide a LDMOSFET design that has both good high performance characteristics and good power dissipation.
Another object of the invention has been to provide a method for manufacturing said improved LDMOSFET.
These objects have been achieved by using a partial SOI (silicon on insulator) approach. In the prior art LDMOSFET devices capable of handling high power have been made by locating the source contact on the bottom surface of the device, allowing for good heat sinking, with connection to the source region being made through a sinker, but this structure has poor high frequency characteristics. Also in the prior art, good high frequency performance has been achieved by introducing a dielectric layer immediately below the source/drain structure (SOI) but this structure has poor power handling capabilities. The present invention achieves both good high frequency behavior as well as good power capability in the same device. Instead of inserting a dielectric layer over the entire cross-section of the device, the dielectric layer is limited to being below the heavily doped section of the drain with a small amount of overlap of the lightly doped Section. The structure is described in detail together with a process for manufacturing it.


REFERENCES:
patent: 5378912 (1995-01-01), Pein
patent: 5382818 (1995-01-01), Pein
patent: 5463241 (1995-10-01), Kubo
patent: 5554546 (1996-09-01), Malhi
patent: 5650354 (1997-07-01), Himi et al.
patent: 5770881 (1998-06-01), Pelella et al.
patent: 5777365 (1998-07-01), Yamaguchi et al.
patent: 5930642 (1999-07-01), Moore et al.
patent: 6500704 (2002-12-01), Hirano et al.
patent: 2002002012 (2002-01-01), None

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