Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device – With switching speed enhancement means
Reexamination Certificate
2000-02-01
2001-04-17
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
With switching speed enhancement means
C257S610000, C257S611000
Reexamination Certificate
active
06218683
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a diode suitable for a free wheel diode to be used together with a high breakdown voltage switching element such as an IGBT (Insulated Gate Bipolar Transistor), a GCT (Gate Commutated Turn-off Thyristor) or the like, or suitable for a high breakdown voltage clamp diode or the like.
BACKGROUND ART
FIGS. 36 and 37
are a sectional front view and a plan view which show a conventional diode as the background of the present invention, respectively.
FIG. 36
is a sectional view taken along the line E—E in
FIG. 37. A
diode
151
comprises, as a main part, a semiconductor substrate
80
using silicon as a base material. The semiconductor substrate
80
has a P layer
81
, an N
−
layer
82
and an N
+
layer
83
provided sequentially from an upper main surface to a lower main surface.
An anode electrode
84
is connected to the upper main surface of the semiconductor substrate
80
, that is, an exposed surface of the P layer
81
, and a cathode electrode
85
is connected to the lower main surface of the semiconductor substrate
80
, that is, an exposed surface of the N
+
layer
83
. These electrodes
84
and
85
are formed of an electrically conductive metal. Furthermore, life time killers which are crystal defects for promoting the annihilation of carriers as the recombination centers of the carriers are introduced into the semiconductor substrate
80
. Thereby the life time of the carrier is controlled.
FIG. 38
is a graph showing a profile of a density of the life time killers introduced into the semiconductor substrate
80
. In the conventional diode
151
, two kinds of profiles have been known. In a conventional example 1 represented by a curve Pr
1
shown in
FIG. 38
, the life time killers are uniformly introduced over the whole semiconductor substrate
80
. Accordingly, the life time of the N
−
layer
82
is uniformly controlled.
On the other hand, in a conventional example 2 corresponding to a curve Pr
2
, the life time killers are selectively introduced into a region of the N
−
layer
82
adjacent to a junction interface between the N
−
layer
82
and the P layer
81
. Consequently, the life time of the region adjacent to the junction interface between the N
−
layer
82
and the P layer
81
is locally controlled to be short. A diode corresponding to the conventional example
2
is a device which has been disclosed in the International Conference PCIM '97 (International POWER CONVERSION '97 CONFERENCE NURNBERG, GERMANY Jun. 10-12, 1997).
Immediately after operating conditions are instantaneously changed by the switching operation of an external circuit from a state in which a current flows to a diode in a forward direction to a state in which a reverse bias is applied, a reverse current transiently flows to the diode.
FIG. 39
is a graph showing a waveform of a current flowing in the diode in the transient state in relation to both the conventional examples 1 and 2. At a time t0, when a switching operation is performed from a state in which a forward current I
F
steadily flows to a state in which a reverse bias is applied, a current starts to be decreased. The current is continuously decreased to have a negative value in a short time. In other words, a reverse current (minus current) flows to the diode.
Even if the switching operation is performed to apply a reverse bias, a depletion layer is not immediately formed in a PN junction between the P layer
81
and the N
−
layer
82
due to excess carriers remaining in the vicinity of the PN junction. For this reason, the diode is transiently brought into a conductive state. As a result, the reverse current flows. An increase rate of the reverse current in an initial stage, that is, (an absolute value of) a current decrease rate represented by di/dt in
FIG. 39
is defined by the magnitude of an inductance acting as a load in the external circuit. If the inductance is increased, the current decrease rate di/dt is increased. Correspondingly, the reverse current is rapidly increased.
In a process in which the reverse current is increased, a depletion layer is generated at a time tl. The depletion layer is formed in the PN junction as shown in
FIG. 40. A
front
92
of a depletion layer
91
advances toward the N
+
layer
83
with the passage of a time. Consequently, the depletion layer
91
is enlarged to cover the whole N
−
layer
82
shortly.
Returning to
FIG. 39
, as the depletion layer
91
is generated and grows, a reverse voltage v is generated at the time t1 between the anode electrode
84
and the cathode electrode
85
, and then the reverse voltage v is increased to shortly converge on a value of a reverse bias applied from the outside. More specifically, when the depletion layer
91
grows, a reverse voltage blocking capability which is the original function of the diode is recovered.
FIG. 39
typically shows only the reverse voltage v related to the conventional example
2
.
When the reverse voltage v is increased, the reverse current gradually reduces the speed of the increase, and shortly reaches a peak and is then decreased. As the current decrease rate di/dt is increased, the peak is increased. A value of the peak is referred to as a reverse recovery current I
rr
, and is one of parameters for evaluating a reverse recovery characteristic in the diode. The reverse current converges on zero while continuing the decrease. Thus, a transient state, that is, a reverse recovery operation comes to an end, and a steady state in which the reverse voltage v is equal to the reverse bias and the reverse current does not flow is realized.
As the parameter for evaluating the reverse recovery characteristic, an attenuation rate of the reverse recovery current, a di/dt capability and a reverse recovery loss have been known in addition to the above-mentioned reverse recovery current I
rr
.
The attenuation rate of the reverse recovery current is defined as a rate of convergence on zero after the reverse current passes through a peak in the graph of FIG.
39
. The di/dt capability is a maximum value of the current decrease rate di/dt which can be applied without causing a damage on the diode. Moreover, the reverse recovery loss is a magnitude of a loss caused on the diode in the process of the reverse recovery operation.
If the reverse recovery current I
rr
is smaller, it is possible to resist a greater current decrease rate di/dt. Accordingly, a simple relationship is established between the reverse recovery current I
rr
and the current decrease rate di/dt. Moreover, the reverse recovery loss is equivalent to a time integral of a product of the reverse current and the reverse voltage v in the graph of FIG.
39
. Accordingly, if the magnitude of the reverse recovery current I
rr
is smaller and the attenuation of the reverse recovery current is performed more quickly, the reverse recovery loss is more reduced. As a matter of course, it is desirable that the magnitude of the reverse recovery current I, should be smaller, the attenuation of the reverse recovery current should be performed more quickly and the di/dt capability should be larger. Furthermore, it is desirable that the reverse recovery loss should be as small as possible.
In the diode according to the conventional example 1, the life time killers are introduced over the whole semiconductor substrate
80
. Therefore, the attenuation of the reverse recovery current is performed quickly as shown in the curve Pr
1
of FIG.
39
. Consequently, there is an advantage that the reverse recovery loss is small. However, the magnitude of the reverse recovery current I
rr
is large. As a result, there has been a problem in that the di/dt capability is small. In addition, there has been a problem in that a forward voltage acting as a significant parameter for evaluating a forward characteristic is high.
In the diode according to the conventional example 2, the life time killers are locally introduced in the vicinity of the PN junction at a higher density
Koga Shinji
Morishita Kazuhiro
Satoh Katsumi
Meier Stephen D.
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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