Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-07-30
2004-03-23
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S228000, C257S229000, C257S341000
Reexamination Certificate
active
06710403
ABSTRACT:
BACKGROUND OF THE INVENTION
Power MOSFETs (metal oxide field effect transistors) are well known in the semiconductor industry. Two types of known power MOSFET cell structures are shown in
FIGS. 1 and 2
.
FIG. 1
is a cross section view of a conventional vertically-conducting UMOS structure, and
FIG. 2
is a cross section view of a power MOSFET with source trenches and planar gate structure.
In
FIG. 1
, gates
110
a,b
are formed in gate trenches
113
a,b
extending from the top surface through body region
106
, and terminating in n-type epitaxial region
104
. Vertical channels are formed between source regions
114
a,b
and epitaxial region
104
along the sidewalls of gate trenches
113
a,b
. Although this structure has a relatively low on-resistance and enables high packing density due the vertical gate structure, it suffers from high input capacitance (i.e., high gate to source and gate to drain capacitance) due to the long channel length and the large number of gates of a highly packed device.
In
FIG. 2
, a conventional double-diffused MOS (DMOS) planar surface structure is combined with source trenches
213
a,b
. Source trenches
213
a,b
extend from the top surface into epitaxial region
204
, and are filled with conductive material
216
a,b
(e.g., polysilicon). Conductive material
216
a,b
are insulated from epitaxial region
204
and body regions
206
a,b
by a layer of insulating material
212
a,b
, but are electrically connected to the body/source regions through the top metal layer
218
. A maximum forward blocking voltage, hereinafter referred to as “the breakdown voltage”, is determined by the avalanche breakdown voltage of the reverse-biased body-drain junction. Source trenches
213
,
b
help achieve a higher breakdown voltage by causing the electric field to spread deeper into epitaxial region
204
. This structure however suffers from the same horizontal limitations (e.g., packing density and JFET resistance) as conventional planar DMOS structures.
Thus, there is a need for a power MOSFET structure which, among other advantages and features, exhibits low input capacitance, high breakdown voltage, improved packing density, and low on-resistance.
BRIEF SUMMARY OF THE INVENTION
In accordance with an embodiment of the present invention, a MOSFET includes a first semiconductor region of a first conductivity type, a gate trench which extends into the first semiconductor region, and a source trench which extends into the first semiconductor region. The source trench is laterally spaced from the gate trench.
In one embodiment, the source trench extends deeper into the first semiconductor region than the gate trench.
In another embodiment, the MOSFET further includes a body region and a source region. The body region is in the first semiconductor region between the source trench and the gate trench. The body region is of opposite conductivity type as the first semiconductor region. The source region is in the body region such that a channel is formed in the body region along a sidewall of the gate trench. The source region is of the same conductivity type as the first semiconductor region.
In accordance with another embodiment of the present invention, a method of forming a MOSFET is as follows. A first semiconductor region of a first conductivity type is formed. A gate trench extending into the first semiconductor region is formed. A source trench extending into the first semiconductor region is formed. The source trench is laterally spaced from the gate trench.
In one embodiment, a body region is formed in the first semiconductor region between the source trench and the gate trench. The body region is of opposite conductivity type as the first semiconductor region. A source region is formed in the body region such that a channel is formed in the body region along a sidewall of the gate trench. The source region is of the same conductivity type as the first semiconductor region.
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paten
Fairchild Semiconductor Corporation
Townsend and Townsend / and Crew LLP
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