Active solid-state devices (e.g. – transistors – solid-state diode – Including region containing crystal damage
Reexamination Certificate
2000-02-14
2003-08-05
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Including region containing crystal damage
C257S612000, C257S577000, C438S328000
Reexamination Certificate
active
06603189
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device and manufacturing method thereof, and, in particular, relates to a suitable technique for improving recovery characteristic of diodes.
2. Discussion of the Background
FIG. 17
is a cross section illustrating a basic structure of a diode. In
FIG. 17
, on a surface of an N substrate
4
P comprising an N+ layer
1
P and N− layer
2
P, an anode region
3
P is formed by diffusing P type impurities. An anode electrode
5
P is formed on a surface of the anode region
3
P, and a cathode electrode
6
P is formed on a rear surface of the N substrate
4
P. Operation of the diode is described as follows.
In the structure of
FIG. 17
, a predetermined anode voltage VAK (forward bias) is applied between the anode electrode
5
P and cathode electrode
6
P and, when the anode voltage exceeds a certain threshold value (~0.6 V), holes are injected from the anode electrode
5
P into the N− layer
2
P, and thus the diode conducts. When a predetermined anode voltage VKA (reverse bias) is applied between the cathode electrode
6
P and anode electrode
5
P, no current flows in the diode until the reverse bias VKA reaches a breakdown voltage of a PN junction comprising the anode region
3
P and N− layer
2
P. The mentioned situation is shown in FIG.
18
. Note that
FIG. 18
is also employed in a description of preferred embodiments of the present invention, together with FIG.
19
.
Meanwhile, characteristics obtained when an anode voltage applied to a diode is changed from forward bias to reverse bias is called reverse recovery characteristic (recovery characteristic), and it is known that the reverse recovery characteristic shows transition of current with time (transient response). In
FIG. 19
, symbol Irr denotes a peak value of current (recovery current) Ir that flows in the reverse direction, symbol Trr denotes time required until the current Ir flown in the reverse direction dissipates, and symbol If denotes a current value at the time of forward bias.
In the reverse recovery characteristic, there has been a desire for one in which the magnitude of the peak current Irr of the recovery current is small and current IR flowing in the reverse direction dissipates slowly. That is, with time T
1
, T
2
determined as shown in
FIG. 19
, it is defined that recovery characteristic is hard when T
1
>T
2
, and recovery characteristic is soft when T
1
<T
2
. When a diode is used in a combination with a main switching element such as IGBT, if recovery characteristic is hard, heat generation is caused due to the occurrence of surge voltage and switching loss. Therefore, to avoid these, there is a desire for characteristics having a low loss and soft recovery (a reduction in the transition of current Ir with time dIr/dt). Hereinafter, the peak value Irr of the recovery current Ir is called “recovery peak current.”
As to the transient response of the current Ir following in the reverse direction, the following points are found out by the recent researches and studies. That is, {circle around (1)} the recovery peak current Irr depends upon the carrier density of a semiconductor region in the vicinity of an anode electrode, and the recovery peak current Irr decreases as the carrier density decreases. In addition, {circle around (2)} it is known that the mentioned dissipation time Trr depends upon the carrier density of a semiconductor region in the vicinity of a cathode electrode, and the dissipation time Trr extends as the carrier density of the cathode region increases.
In view of these research results, a large number of structures for improving the reverse recovery characteristic have been proposed conventionally.
(I) In the first place, there is a technique which is, for example, disclosed in Japanese Patent Unexamined Publication No. P08-46221A, and pointed out as a conventional technique in Mitsubishi Denki Giho and Precedings of National Convention of IEEJ, Industrial Application Section, 1995 (No. 136, p79), both of which are described later. That is one in which a heavy metal represented by platinum serving as a lifetime killer, is doped and diffused from the anode electrode side, so that the lifetime of an N type layer in the vicinity of a PN junction part is controlled such as to be short. Especially, with this technique, the diffusion of platinum can be controlled such that the lifetime of the carriers in the N type layer on the cathode electrode side is longer than the lifetime of the surrounding of the PN junction part on the anode side. It is therefore possible to increase the carrier density on the cathode side, thereby increasing the mentioned dissipation time Trr.
Even with the first conventional technique, however, the problem remains in the point that it is not easy to control more shortly the lifetime of the carriers in the N type layer on the anode side, including the problems of homogeneity and reproducibility.
(II) A second conventional technique is one which is disclosed in Patent Publication No. 59(1984)-49714. A cross-sectional structure of a diode to which this conventional technique is applied is shown in FIG.
20
. In
FIG. 20
, the same symbol as in
FIG. 17
denotes the same. With the second conventional technique, an anode region
3
P to be formed on a surface is formed partially for suppressing the injection of holes from the anode regions
3
P, thereby the carrier density in the surrounding region of an anode electrode
5
P is lowered to reduce a recovery peak current Irr.
In this structure, however, there newly arises the problem that, although the carrier density in the vicinity of the anode is controlled by the partially formed anode regions
3
P and clearances W therebetween, a breakdown voltage lowers when the clearance W is too large, and thus this point constitutes obstruction to a sufficient control of the carrier density in the vicinity of the anode.
(III) As a diode structure employing a third conventional technique, there is one which is disclosed in Mitsubishi Denki Giho Vol. 67. No. 9. 1993, PP94-97. Although this is basically the same in structure as that shown in
FIG. 17
, it aims to improve reverse recovery characteristic by changing the structure of an anode region to be formed on a surface. That is, in the technique of the Technical Report, by reducing thickness of an anode region
3
P shown in
FIG. 17
, and reducing the surface concentration of the anode region, injection of holes from the anode region
3
P is suppressed and thus the carrier density in the vicinity of the anode is lowered to reduce a recovery peak current Irr. It is reported that, with this technique, the recovery peak current Irr is reduced by about 40%, and a slope dIr/dt at the time of recovery is reduced to about ½.
With this structure, it is however necessary to set thickness and concentration of the anode region
3
P to a certain degree of value in order to ensure a breakdown voltage, and there are limitations in reducing thickness and concentration of the anode region. Therefore, as in the case with the second conventional technique (II), there remains the problem that the carrier in the vicinity of the anode cannot be controlled sufficiently.
(IV) In addition, as a diode structure to which a fourth conventional technique is applied, there is one which is disclosed in Precedings of National Convention of IEEJ, Industrial Application Section, 1995, PP79-80. This structure is shown in FIG.
21
. In
FIG. 21
, the same symbol as in
FIG. 17
denotes the same. Symbol
2
PP in
FIG. 21
shows a region that is damaged by proton implantation. In this conventional technique, in order to reduce loss at the time of recovery in a pin structure, an irradiation of electron beam is conducted in place of the mentioned platinum doping, to reduce the lifetime in the carriers in an n layer and increase a dissipation time Trr. Further, the lifetime of an n-layer is controlled locally by proton implantation, so that the carrier density in the vicinity of the anod
Dickey Thomas L.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tran Minh Loan
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