Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-01-20
2002-10-08
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S173000, C257S355000, C257S328000
Reexamination Certificate
active
06462383
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a semiconductor device for securely protecting its internal element from static electricity and a method for manufacturing the same, and more in detail to the semiconductor device including the internal element and a protection element in which electrostatic energy is discharged through the protection element for protecting the internal element and the method for manufacturing the same.
(b) Description of the Related Art
In a conventional semiconductor device, protection elements are generally connected to input and output terminals for protecting an internal element against electrostatic breakdown. That is, electrostatic energy is discharged through the protection circuit toward a grounded line (GND) or a power source line VCC) for protecting the internal element before entering the internal element.
Especially, in a bipolar transistor device, the decrease of a parasitic capacitance is essential for a high-speed operation to thereby make a collectors base junction and a base-emitter junction shallower. When the static electricity enters the internal element (bipolar transistor), electrostatic breakdown is likely to occur due to concentration of an electric field. Accordingly, high-performance protection elements have been proposed heretofore.
JP-A-4(1992)-22163 describes a semiconductor device including a conventional protection element. An example of a circuit construction of the conventional semiconductor device (first conventional example) is shown in
FIG. 1 and a
vertical cross section of the semiconductor device is shown in FIG.
2
. The first conventional example employs a reverse discharge as well as a forward discharge of a P-N junction by using a breakdown phenomenon of the P-N junction.
As shown in
FIG. 1
, the emitter and the base of a bipolar transistor are connected and base current starts to flow when a breakdown voltage is applied between the base and the collector by entering static electricity in the first conventional example. The bipolar transistor is driven for operation by the breakdown current flowing through the base resistance as a trigger. Due to the amplification factor (hfe) of the bipolar transistor as high as to 50 to 150, the discharge path is switched from the base-collector path to the collector-emitter path by the function of the transistor. The bipolar operation completes the discharge at a moment for effectively acting as the protection element. Since a plenty of current flows at a moment, the size of the protection element is required to be larger than the internal transistor for preventing the destruction of the protection element itself.
As shown in
FIG. 2
, the conventional semiconductor device includes an N-type epitaxial layer
102
overlying a P-type silicon substrate
101
, and an N-type embedded layer
103
between the P-type silicon substrate
101
and the N-type epitaxial layer
102
. The N-type epitaxial layer
102
on the N-type embedded layer
103
functions as the intrinsic collector region of the bipolar transistor.
A field oxide film
110
is formed by selectively replacing the region other than a diffused region with an oxide film. A P-type dielectric layer
104
underlying the field oxide film
110
functions as an isolation layer for surrounding the side surface of the transistor and has a depth reaching to the surface of the silicon substrate
101
.
A base layer
105
is formed as a P-type layer overlying the N-type embedded layer
103
, and an N-type emitter layer
106
is formed in the base layer
105
.
A heavily doped N-type collector layer
107
is formed between a collector electrode
111
and the N-type embedded layer
103
, and a voltage is applied to the N-type embedded layer
103
through the N-type collector layer
107
.
A silicon oxide film
108
functions as a dielectric film covering the N-type epitaxial layer
102
, and the respective via holes are formed in the silicon oxide film
108
corresponding to the base layer
105
, the N-type emitter layer
106
and the N-type collector layer
107
.
An aluminum electrode
109
is disposed on the via holes to connect a base electrode and an emitter electrode.
In the bipolar transistor as the protection element in the conventional example 1, the breakdown voltage is determined by a base-collector breakdown voltage which is not largely different from that of the internal transistor (the transistors of the protection element and the internal element have substantially the same structure).
The selective discharge is generally designed to occur by employing the protection element having a long discharge path (or a large size). However, when the base-collector breakdown voltage of the internal transistor becomes lower than that of the protection element due to variation of manufacturing conditions, the discharge of the internal transistor starts earlier than that of the protection element, and the internal transistor is disadvantageously destroyed.
A circuit diagram of a second conventional example which achieves the improvement of the above disadvantage is shown in
FIG. 3. A
sectional view of a semiconductor in
FIG. 3
corresponding to
FIG. 2
is omitted because only electrodes to be connected are modified.
In the circuit shown in
FIG. 3
, the collector and the emitter are reversed from an ordinary circuit, wherein and the collector and the base are connected. A voltage is applied between the emitter and the base, and when a breakdown starts therebetween, a reverse operation of the bipolar transistor is triggered by the breakdown current flowing through a base resistance. Since its amplification factor (reverse hfe) is about 1 which is not high, the current flowing in the discharge path is divided into the breakdown current and the current flowing between the emitter and the collector. The division of the current in the second conventional example shown in
FIG. 3
more rapidly completes the discharge.
Further, since the emitter-base breakdown voltage of the protection element is less than half the base-collector breakdown voltage of the internal transistor, the first discharge advantageously starts in the protection element.
However, the reduction of the thickness of the emitter layer in the second conventional example for a high-speed operation increases the density of the breakdown current flowing between the emitter electrode and the base electrode, thereby causing a destruction.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a semiconductor device which has a sufficient durability against the destruction and can protect an internal element. Another aspect of the present invention is to provide a method for manufacturing such a semiconductor device.
The present invention provides, in a first aspect thereof, a semiconductor device comprising: a semiconductor substrate having a first conductivity-type; a first embedded layer having a second conductivity-type and formed in the semiconductor substrate; a second embedded layer having a second conductivity-type and formed in the semiconductor substrate, the first embedded layer having a depth larger than a depth of the second embedded layer; an internal bipolar transistor having an emitter, a base and a collector which is formed as the first embedded layer; and a protection bipolar transistor having an emitter, a base and a collector which is formed as the second embedded layer, the base and the emitter of the protection transistor being connected together. In the first aspect of the present invention, the first and the second embedded layers may be modified as long as current is likely to flow in the second embedded layer.
The present invention provides, in a second aspect thereof, a method for manufacturing a semiconductor device including the steps of: forming a first embedded layer having a second conductivity-type and a second embedded layer having the second conductivity-type overlying a semiconductor substrate having a first conductivity-type; forming heavily doped c
Lee Eddie
Lee Eugene
NEC Corporation
Young & Thompson
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