Semiconductor device with improved breakdown voltage characteristics

A semiconductor device includes a semiconductor body (1, 2) with an island-shaped region (3) adjoining the surface, in which a contact pad (6) is provided above the island-shaped region (3) and separated therefrom by an insulating layer (5). The island-shaped region (3) forms a pn-junction (34) with an adjoining isolating region (4). According to the invention, the device is provided with regions (40, 41) for increasing the breakdown voltage of the pn-junction (34).

BACKGROUND OF THE INVENTION 
The invention relates to a semiconductor device comprising a semiconductor 
body with an island-shaped region of a first conductivity type adjoining 
the surface and surrounded by an isolating region of a second, opposite 
conductivity type with which the island-shaped region forms a pn-junction, 
the island-shaped region being covered by an insulating layer on which a 
contact pad is provided. 
Such a device is known from U.S. Pat. No. 3,812,521. This reference 
describes a device in which the contact pad is separated from a p-type 
island-shaped region by a silicon oxide layer. The p-type island-shaped 
region lies in an island-shaped n-type isolating region with which it 
forms a pn-junction. In the known device, the n-type isolating region is 
in its turn surrounded by a p-type isolating region which is in electrical 
contact with the substrate. 
It was found in practice that a device of the kind described frequently 
gives rise to failures in applications for higher voltages. The known 
device, therefore, has the disadvantage that it cannot be reliably 
utilized for higher voltages. 
SUMMARY OF THE INVENTION 
The invention has for its object inter alia to counteract this disadvantage 
by providing a device of the kind described in the opening paragraph which 
can be reliably used at higher voltages. 
According to the invention, a device of the kind described in the opening 
paragraph is for this purpose characterized in that the device is provided 
with means for increasing the breakdown voltage of the pn-junction between 
the island-shaped region and the adjoining semiconductor region. 
The invention is based on the recognition that, when a short-circuit arises 
between the contact pad and the island-shaped region, the voltage at the 
contact pad is applied fully across the pn-junction between the 
island-shaped region and the isolating region, so that the pn-junction 
must be able to withstand the voltage offered at the contact pad. 
Such a short-circuit may be due, for example, to fastening of a metal wire 
to the contact pad. Pressure is usually exerted on the contact pad during 
this operation, so that stresses are introduced into the subjacent 
insulating layer, and small cracks may arise. A short-circuit between the 
contact pad and the island-shaped region is possible through such a crack. 
This problem arises especially when the insulating layer between the 
contact pad and the island-shaped region is comparatively resilient, for 
example, in that the layer was not grown but deposited by means of 
gas-phase deposition, which is a usual technique in semiconductor 
technology for providing a layer. 
The breakdown voltage increasing means in the device according to the 
invention ensure that the pn-junction between the island-shaped region and 
the isolating region is able to withstand the voltage which is present at 
the contact pad during operation. As a result, the device according to the 
invention will still continue to function reliably, even in the case of 
any short-circuit in the insulating layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The device of FIG. 1 comprises a semiconductor body with a p-type substrate 
of silicon and a superimposed n-type epitaxial layer 2 with an average 
phosphorus concentration of 6.10.sup.14 cm.sup.-3. The epitaxial layer 2 
comprises a number of island-shaped regions 3 which are each surrounded by 
a p-type isolating region 4 with an average boron concentration of 
approximately 5.10.sup.19 cm.sup.-3 which extends from the surface to the 
substrate 1. The isolating region may be formed, for example, through a 
combination of diffusion from the surface and from a buried layer. 
On the surface there is an approximately 2.5 .mu.m thick silicon oxide 
layer of which approximately 0.1 .mu.m was grown thermally on the surface, 
but the remaining portion was provided by gas-phase deposition (CVD). A 
contact pad 6 of aluminum with dimensions of approximately 100.times.100 
.mu.m.sup.2 is provided on the oxide layer 5 in the location of an 
island-shaped region 3. The contact pad 6 is connected to a switching 
element situated elsewhere in the semiconductor body by means of an 
aluminum conductor track outside the plane of the drawing. 
To protect the device, the entire assembly is coated with a protective top 
layer 7 of silicon nitride in which a window is provided at the area of 
the contact pad 6. A contact wire may be fastened on the contact pad 6 in 
the window in order to connect the device to a lead of a lead frame (not 
drawn). It is alternatively possible that only a small metal elevation is 
provided on the contact pad, a so-called bump, whereby the device can be 
mounted directly on a lead frame or a tape. 
In all these cases, pressure is exerted on the contact pad 6, so that 
stresses are introduced into the subjacent insulating layer 5. As a result 
of these, cracks may arise which may cause a short-circuit between the 
contact pad 6 and the island-shaped region 3. In that case the voltage 
offered at the contact pad is applied fully across the pn-junction 34 
between the island-shaped region 3 and the surrounding isolating region 4. 
This problem arises especially when the insulating layer was provided by 
means of a deposition technique, as in the present case. In contrast to 
grown layers, deposited layers are usually comparatively resilient, so 
that the layer will quickly break if pressure is exerted on it locally. 
To counteract undesirable consequences of such a short-circuit, the device 
according to the invention is provided with means for increasing the 
breakdown voltage of the pn-junction 34 between the island-shaped region 3 
and the isolating region 4. In this example, the breakdown voltage 
increasing means comprise a lateral extension 40 of the isolating region 4 
adjoining the surface with a comparatively low doping concentration of 
approximately 5.10.sup.16 cm.sup.-3. The extension 40 extends 
approximately 30 .mu.m into the island-shaped region 3, maintaining a 
distance l of approximately 20 .mu.m from the contact pad 6. 
In practice, a pn-junction such as the pn-junction 34 surrounding the 
island-shaped region 3 and situated transverse to the surface is found to 
be most susceptible to breakdown near the surface. To increase the 
breakdown voltage of the pn-junction 34, therefore, it is especially 
necessary to reduce the electric field around the pn-junction 34. The 
extension 40 accomplishes this. As a result of this extension, the 
depletion region which surrounds the pn-junction 34 during operation is 
considerably wider near the surface, so that the electric field is 
weakened. Without such an extension the pn-junction 34 in this example 
would not be able to withstand voltages above 170 V; with the extension 
voltages up to 300 V can be accommodated without problems. 
The device of FIG. 2 largely corresponds to that of FIG. 1, the difference 
being that in this second embodiment of the device according to the 
invention the breakdown voltage increasing means comprise not only a 
lateral extension 40 of the isolating region 4, but also a number of 
p-type zones 41 adjoining the surface. The p-type zones 41 lie in the 
island-shaped region 3, forming narrow rings therein which entirely 
surround the contact pad. The zones have a width b of approximately 6 
.mu.m and can be applied in the same process step as the extension 40. 
The interspacing .lambda. between the zones 41 and between the zones 41 and 
the extension 40 is approximately 6 .mu.m, sufficiently small to ensure 
that during operation the depletion regions around the surface zones 41 
and around the pn-junction 34 mutually overlap. As a result, the depletion 
region around the pn-junction 34 is further widened at the surface, so 
that the electric field strength in the depletion region drops still 
further. This results in a further increase in the breakdown voltage of 
the pn-junction 34. In the device shown here, the pn-junction 34 was found 
to resist voltages of more than 1400 V. 
Although the invention was described with reference to only two 
embodiments, it will be apparent that the invention is by no means 
restricted to the embodiments given. Many more variations are possible for 
those skilled in the art within the scope of the invention. Thus, in the 
examples given, the conductivity types may all be simultaneously replaced 
by the opposite conductivity types. In addition, those skilled in the art, 
within the scope of their skills, are aware of alternative breakdown 
voltage increasing measures which may be taken in the device according to 
the invention. Thus, for example, the so-called RESURF principle may be 
used in suitable locations in the device for further increasing the 
breakdown voltage of the pn-junction.