Transistor

Details Transistor Transistor

2000-08-17

2004-03-09

Eckert, George (Department: 2815)

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

Field effect device

Having insulated electrode

C257S342000, C257S339000, C257S328000, C257S262000, C257S263000, C257S139000, C257S302000, C257S333000

Reexamination Certificate

active

06703665

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the art of a field-effect transistor such as a MOSFET, an IGBT, or the like.
DESCRIPTION OF THE RELATED ART
One conventional MOSFET will be described below with reference to
FIGS. 39 and 40
of the accompanying drawings.
As shown in
FIG. 39
, a conventional MOSFET
101
disclosed in literature comprises a drain layer
105
of single crystal of silicon and doped with a high concentration of an N
+
-type impurity, and an N
−
-type conductive layer
106
deposited on the drain layer
105
by epitaxial growth. The conductive layer
106
includes base regions
112
formed by diffusing a P-type impurity from the surface thereof.
Each of the base legions
112
includes a ring-shaped source region
114
formed by diffusing an N-type impurity from the surface thereof. A channel region
115
lies between the outer end of the base region
112
and the outer peripheral edge of the source region
114
.
The base region
112
, the source region
114
, and the channel region
115
make up one rectangular cell
117
. The MOSFET
101
has a number of cells
117
that are arranged regularly in a grid-like pattern.
FIG. 40
shows the layout of the cells
117
of the MOSFET
101
.
A gate insulating film
121
in the form of a silicon oxide film is disposed on the channel regions
115
of adjacent two of the cells
117
and the surface of the conductive layer
106
between those two cells
117
. A gate electrode film
131
is disposed on the gate insulating film
121
.
The base region
112
has a surface exposed inside of the ring shaped source region
114
. An inter layer insulation film
122
is disposed on the gate electrode film
131
.
Reference numeral
132
represents a part of the source electrode film deposited on the surface of the source region
114
and the base region
112
and a part deposited on the interlayer insulation film
122
. Those two parts are connected each other.
The source electrode film also has a part deposited on the surface of gate electrode film
131
and is insulated from the part of the source electrode film deposited on the surface of the source region
114
and base region
112
and the part deposited on the interlayer insulation film
122
.
The MOSFET
101
also has a protective film
135
disposed on the source electrode films
132
. The protective film
135
and the interlayer insulation films
122
are patterned to expose portions of the source electrode films
132
and also portions of the thin metal film connected to the gate electrode films
131
.
A drain electrode
133
is disposed on the surface of the drain layer
105
remotely from the conductive layer
106
. The drain electrode
133
, the exposed portions of the source electrode films
132
, and the exposed portions of the thin metal film connected to the gate electrode films
131
are connected to respective external terminals which are connected to an electric circuit for operating the MOSFET
101
.
To operate the MOSFET
101
, the source electrode films
132
are placed on a ground potential, and a positive voltage is applied to the drain electrode
133
. When a gate voltage (positive voltage) equal to or higher than a threshold voltage is then applied to the gate electrode films
131
, an N-type inverted layer is formed on the surface of the P-type channel region
115
of each cell
117
, and the source region
114
and the conductive layer
106
are connected to each other by the inverted layer, so that a current flows from the drain electrode
133
to the source electrode films
132
.
When a voltage, e.g., a ground potential, lower than the threshold voltage is thereafter applied to the gate electrode films
131
, the inverted layer is eliminated, and the base regions
112
and the conductive layer
106
are reverse-biased, so that no current flows between the drain electrode
133
and the source electrode films
132
.
Therefore, the drain electrode
133
and the source electrode films
132
can be connected to each other or disconnected from each other by controlling the voltage applied to the gate electrode films
131
. The MOSFET
101
is widely used as a high-speed switch in power electric circuits such as power supply circuits, motor control circuits, etc.
While the drain electrode
133
and the source electrode films
132
are being disconnected from each other, a large voltage may be applied between the drain electrode
133
and the source electrode films
132
.
Since the base regions
112
including the channel regions
115
and the conductive layer
106
are reverse-biased while the drain electrode
133
and the source electrode films
132
are being disconnected from each other, the withstand voltage, i.e. the avalanche breakdown voltage, of the MOSFET
101
is determined by the withstand voltage of the PN junction between the base regions
112
and the conductive layer
106
.
PN junctions are classified into a planar junction, a cylindrical junction, and a spherical junction according to the shape of a diffusion layer of higher concentration. It is known that the planar junction has a highest withstand voltage and the spherical junction has a lowest withstand voltage.
In the MOSFET
101
composed of the many cells
117
, the planar junction is formed at the bottom of each of the cells
117
. However, since the cells
117
are polygonal, e.g., rectangular, in shape, the cylindrical junction is necessarily formed at the sides of each of the cells
117
and the spherical junction is necessarily formed at the top of each of the cells
117
. The overall withstand voltage is determined by the withstand voltage at the top of each of the cells
117
.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a transistor having a high withstand voltage.
Another object of the present invention is to provide a transistor having a low conduction resistance.
To achieve the above objects, there is provided in accordance with the present invention a transistor comprising a semiconductor substrate having a drain layer of a first conductivity type and a withstand voltage region of a second conductivity type disposed on the drain layer, a conductive region of the first conductivity type formed by an impurity partly diffused into the semiconductor substrate from the side of the withstand voltage region side, the conductive region layer of the first conductivity type having a bottom connected to the drain, a base region of the second conductivity type formed by an impurity partly diffused into the semiconductor substrate from the side of the withstand voltage region side, a source region of the first conductivity type formed in the base region, a gate insulating film having a central region positioned on the base region, an end positioned on the conductive region, and an opposite end positioned on the source region, a gate electrode film disposed on the gate insulating film, a channel region positioned between the source region and the conductive region and including a surface of the base region below the gate insulating film, a source electrode electrically connected to the source region and the base region, and a drain electrode electrically connected to the drain layer.
The base region has a surface concentration higher than the surface concentration of the withstand voltage region.
The conductive region has a surface concentration higher than the surface concentration of the withstand voltage region.
The base region has a surface concentration higher than the surface concentration of the conductive region.
The conductive region has a surface surrounded by a region having a conductivity type opposite to the conductivity type of the conductive region.
The base region is diffused from a surface of the withstand voltage region and a surface of the conductive region, and the bottom of the base region has a part in contact with the withstand voltage region and a part in contact with the conductive region.
The base region has a portion positioned within the conductive region and serving as the channel region.
T

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