Fabrication of insulated gate bipolar devices

Details Fabrication of insulated gate bipolar devices Fabrication of insulated gate bipolar devices

1999-07-16

2001-08-07

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Making regenerative-type switching device

Having field effect structure

Reexamination Certificate

active

06271061

ABSTRACT:

TECHNICAL FIELD
The present invention is directed to devices of the type that include insulated gate bipolar devices, commonly known as IGBTs, and modulated-conductivity MOS power devices.
More particularly, the invention relates to IGBT type devices having enhanced features of rugged construction, conductivity modulation, and speed.
BACKGROUND OF THE INVENTION
As is well known, from a functional standpoint, an IGBT type device is considered to be the equivalent of a MOSFET transistor driving a bipolar power transistor, for instance as shown in
FIG. 1
of this application which is reproduced from Klaus Rischmuller's publication “PCIM90 EUROPE”. Power Conversion Conference, June '90.
A description of the basics of the operation of an IGBT type of structure is given in U.S. Pat. No. 4,495,493.
A different embodiment which brings out other features of the same IGBT type device is described in U.S. Pat. No. 5,073,511.
Problems related to IGBT type devices are dealt with in Italian Patent 1241049.
From the above-referenced prior art, it can be evinced that the following are among the advantages afforded by an IGBT type device:
ruggedness, speed or relative fall time, and
conductivity modulation or relative output resistance.
Also known is that conductivity modulation is dependent on the injection of minority carriers from the P+ substrate layer, designated
10
in
FIG. 1
, into a resistive epitaxial layer
20
of the N− type, also shown in FIG.
1
.
This lowers the device output resistance accordingly.
It follows that a reduction in conductivity modulation, resulting from a reduction in the gain of transistor
60
which is a parasitic PNP transistor intrinsic to the IGBT type device and shown in
FIG. 1
, will in turn result in increased output resistance of the device.
Also known is that an IGBT type device has an output resistance, or VCEsat directly tied to it, which increases with the breakdown voltage of the device.
Also known is the relationship of breakdown voltage to the thickness of the epitaxial layer and its dopant concentration. This can be expressed as follows:
BV=[q C X**
2]/[2
e]
  (1)
where: BV is the breakdown voltage; C is the dopant concentration in the epitaxial layer; X is the actual thickness of the epitaxial layer, i.e., the overall thickness less the thickness of the deep body layer P+; q is the electron charge; and e is the dielectric constant of silicon.
Increasing the conductivity modulation of an IGBT type device and concurrently decreasing its output resistance provides a better IGBT type device.
One viable prior approach to increasing conductivity modulation used to be a decreased doping of an N+ layer
21
which corresponds to the base of the PNP parasitic transistor
60
, shown in
FIG. 1
, so as to turn it into a high gain transistor.
However, this involves, as the skilled one will recognize, an increase in the re-combination time of the minority carriers, and this is a variable which is inversely proportional to the device “speed”.
Accordingly, this first solution is only practicable on condition that a compromise can be struck, between the increase in conductivity modulation and corresponding increase in turn-off time, which represents an improvement over the basic device.
The ruggedness of a device is defined as the device ability not to destroy itself when, during a turnoff under an inductive load, it is called upon to dissipate the power related to the product of voltage by current at the crossing point of the two curves, as shown in FIG.
2
.
Also referring to
FIG. 1
, the parasitic thyristor of the structure are made up of the two transistors
60
, of the PNP type, and
61
, of the NPN type.
The triggering of this parasitic thyristor at a given current value (called the latch-up current) restricts the safe range of the device.
The gain of the transistor
60
is resolutive for the control of the parasitic thyristor triggering, which in turn provides a measure of the IGBT type device ruggedness.
As previously mentioned, gain is controlled by acting on the equivalent of the base of the PNP transistor
60
that is the layer
20
shown in FIG.
1
.
It follows that, to improve the ruggedness of the IGBT type device, a transistor
60
with low gain will be aimed at, e.g., by increasing the dopant dose to the N+ layer
21
.
However, this manipulation of the dopant dose will act in an inversely proportional manner on the output resistance for the reasons given above in connection with conductivity modulation.
It is evident, therefore, that any improvement of the device ruggedness would be at the expense of the other significant variable, namely conductivity modulation, and vice versa.
There is another known method of acting on the output resistance, speed and ruggedness variables without altering the breakdown voltage value.
This consists selecting the thickness of the N− layer
20
.
It will be recalled that it depends on breakdown voltage according to relation (1).
The thickness of the layer
20
is known, from the physics of semiconductor electronic devices, specifically of IGBTs, to be directly proportional to the re-combination time of the minority carriers.
It follows that that thickness is inversely proportional to the device speed.
In addition, by the same laws of physics, the thickness of the layer
20
is tied in a directly proportional manner to the output resistance of the device.
In light of the foregoing, it can be evinced that the conductivity modulation can be improved, or the output resistance lowered, by decreasing the thickness of the layer
20
, to concurrently improve the device speed as well.
Based on relation (1), this would result in a decreased breakdown voltage.
Also according to relation (1), it would be possible to keep the same breakdown voltage by suitably reducing the thickness of the layer
20
, while increasing its conductivity by a raise in the dopant dose.
While being in many ways advantageous, the last-mentioned solution has a drawback in that it implies a considerable reduction of the resultant device ruggedness due to the strong electric field which is present at the interface between the N− layer
20
and N+ layer
21
.
This is also shown by the following relation (2), which expresses the electric field in the semiconductor material as a function of the thickness of the N− layer
20
:
ⅆ
E
ⅆ
X
=
q
·
C
e
(
2
)
It is evinced from this relation that a decreased thickness and concurrently increased conductivity for the layer
20
is unproposable because this would seriously impair the device ruggedness.
SUMMARY OF THE INVENTION
An object of this invention is to provide an insulated gate bipolar transistor type device with such functional and structural features as to yield improved conductivity modulation and ruggedness for the same breakdown voltage, thereby overcoming the limitations and/or drawbacks mentioned above in connection with the prior art. According to principles of the present invention, the object is obtained by a semiconductor power device of the IGBT type comprising a semiconductor substrate having a first type of conductivity and an overlying epitaxial layer having a second type of conductivity, which is opposite to the first, and whose junction to the substrate constitutes the base/emitter junction of the bipolar transistor. The layer adjacent the junction comprises a layer of a semiconductor material of said second type but having a higher dopant concentration than that of the epitaxial layer. The epitaxial layer with conductivity of the second type has at least two zones with increasing concentration, namely a first zone being adjacent to the junction layer and having a higher dopant concentration and a second zone being adjacent to the first zone and having a lower concentration.
More particularly, the invention contemplates the provision of an additional layer having a concentration intermediate those of the N− and N+ layers.
This additional layer is located between a lay

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