Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-06-01
2001-07-10
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S344000, C257S408000
Reexamination Certificate
active
06259136
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a high-breakdown-voltage semiconductor device.
In general, a high-breakdown-voltage semiconductor device used in a high voltage driving circuit or the like and a low-breakdown-voltage semiconductor device used in a low voltage driving circuit or the like are formed on the same substrate, constituting a power IC. Such a kind of power IC is known widely and used in various applications. Generally, at the output stage of the power IC, a high-breakdown-voltage MOSFET is used as a high-breakdown-voltage semiconductor device. The high-breakdown-voltage MOSFET requires a low ON-resistance.
FIG. 1
is a cross-sectional view of an element structure of the high-breakdown-voltage MOSFET. In the high-breakdown-voltage MOSFET, a p-type body layer
2
is selectively formed on a surface of a p-type semiconductor substrate
1
having a high resistance. An n-type source layer
3
is selectively formed in a surface of the p-type body layer
2
.
An n-type offset layer
4
having a high resistance is formed in that region of the surface of the p-type semiconductor substrate
1
which differs from the region of the surface thereof in which the p-type body layer
2
is formed. A gate electrode
8
is formed on that region of the p-type body
2
which is located between the n-type source layer
3
and the n-type offset layer
4
, and that region of the offset layer
4
which is adjacent to the above region of the p-type body
2
, with a gate insulating film
6
and a field oxide film
7
interposed between the gate electrode
8
and the above regions of the p-type body
2
and offset layer
4
.
In the high-breakdown-voltage MOSFET, an n-type drain layer
5
is formed in a surface of an offset layer
4
, and thus the offset layer
4
serves as a so-called resurf (reduced surface field) layer. The resurf layer can keep the breakdown voltage of the semiconductor device at a high value, and at the same time, restrict the ON-resistance to a low value.
FIG. 2
shows drain voltage/drain current characteristic curves in relation to gate voltages V
G
of 0V (OFF state) to 5V with respect to the high-breakdown-voltage MOSFET.
As seen from
FIG. 2
, the high-breakdown-voltage MOSFET having the above structure can achieve a high breakdown voltage when the gate voltage V
G
is low, i.e., it is about 1V or less, and the gate is in the OFF state. However, it cannot achieve a high breakdown voltage when the gate voltage V
G
is more than 1V, and the gate is in the ON state.
To be more specific, in the above high-breakdown-voltage MOSFET, equipotential lines are present at a high density on the drain side in the surface of the n-type offset layer
4
, and an electric field in that end portion of the n-type drain layer
5
which is opposite to the source layer
3
has a high intensity, due to a drain current flowing through the element when the gate is in the ON state. In other words, part of the positive space charge of the n-type offset layer
4
is neutralized by the charge of electrons moving through the n-type offset layer
4
. Consequently, the n-type layer
4
does not act as the resurf layer, lowering the breakdown voltage. This problem becomes more remarkable when the gate voltage V
G
is 3V or more which is ½ or more of the rated gate voltage.
In such a manner, the breakdown voltage of the above high-breakdown-voltage MOSFET is low when the gate is in the ON state. Thus, the high-breakdown-voltage MOSFET cannot be used in an analog circuit in which the drain is directly connected to a power source, and the gate is biased.
When a drain current I
D
per 1 cm of a channel width is I
D
, the amount of charge of electrons is q (=1.6×10
−19
C: coulomb), and the drift speed of electrons is &ugr;
drift
(=8×10
6
cm/s), the negative charge of the n-type offset layer
4
is I
D
/(q·&ugr;
drift
)cm
−2
. In addition, the gate width is a length of the gate which is measured in a direction perpendicular to the-cross-section of the element structure of the conventional high-breakdown-voltage MOSFET which is shown in FIG.
1
. Hereinafter, it is referred to as a channel width.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a high-breakdown-voltage semiconductor device which can achieve a high breakdown voltage, both when the gate is in the ON state, and when it is in the OFF state.
The first subject matter of the present invention resides in that an offset layer is formed to have a two-layer structure, i.e., it is divided into a first offset layer and a second offset layer the dosage of which is higher than that of the first offset layer, such that the first offset layer is closer to the source side than the second offset layer, and the second offset layer is closer to the drain side than the first offset layer.
The second subject matter of the present invention having the above structure resides in that, even if part of the charge of the first offset layer extending to the source side is neutralized by a drain current which flows through an element having a low resistance when the gate is in the ON state, the charge of the second offset layer closer to the drain side than the first offset layer is made to remain, and the second offset layer is made to serve as a resurf layer. By virtue of these features, the entire element achieves a low ON resistance and, simultaneously, a high breakdown voltage, both when it is in the ON state and when it is in the OFF state.
In order to attain the above object, a high-breakdown-voltage semiconductor device according to the first aspect of the present invention comprises:
a semiconductor substrate;
a body layer of a first conductivity type selectively formed on a region of a surface of the semiconductor substrate;
a source layer of a second conductivity type selectively formed in a surface of the body layer;
a first offset layer of the second conductivity type selectively formed on a region of the surface of the semiconductor substrate which differs from the region of the surface of the semiconductor substrate on which the body layer is formed;
a second offset layer of the second conductivity type formed on at least a surface region of the first offset layer;
a drain layer of the second conductivity type selectively formed in a surface of the second offset layer;
a source electrode formed to contact a surface of the body layer and a surface of the source layer;
a drain electrode formed on a surface of the drain layer;
an insulating film formed on a region of the semiconductor substrate which is located between the source electrode and the drain electrode; and
a gate electrode formed on at least that region of the body layer which is located between the source layer and the first offset layer, with the insulating film interposed between the gate electrode and the an region of the body layer,
wherein when mobility of carriers in a channel of an element is &mgr;[cm
2
V
−1
s
−1
], a dielectric constant of the gate insulating film is &egr; [F cm
−1
], a thickness of the gate insulating film is d[cm], a channel length is L[cm], a threshold voltage is V
T
[V], and a rated gate voltage is V
G
[V], a drain current I
D
per 1 cm of a channel width is given by:
I
D
=(&mgr;·&egr;)·(
V
G
/2
−V
T
)/(4
Ld
)
and when an amount of charge electrons is q[C], and a drift speed of carriers is &ugr;
drift
[cm s
−1
], a dosage n
2
of the second offset layer is expressed by the following formula:
n
2
≧I
D
/(
q·&ugr;
drift
)[cm
−2
].
In the above high-breakdown-voltage semiconductor device, the impurity concentration of the second offset layer is higher than the impurity concentration of the first offset layer.
A high-breakdown-voltage semiconductor device according to the second aspect of the present invention comprises:
a semiconductor substrate;
a body layer of a first conductivity type selectively formed on a region of a sur
Kawaguchi Yusuke
Kinoshita Kozo
Nakagawa Akio
Clark Sheila V.
Kabushiki Kaisha Toshiba
Ostrolenk, Spivak, McClelland, Maier & Neustadt, P.C.
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