Active solid-state devices (e.g. – transistors – solid-state diode – With specified shape of pn junction
Reexamination Certificate
2000-12-05
2002-10-15
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
With specified shape of pn junction
C257S475000
Reexamination Certificate
active
06465874
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor devices such as semiconductor rectifying devices for electric power use.
BACKGROUND
Semiconductor rectifying devices such as diodes are used in various semiconductor apparatuses, such as inverters. The semiconductor rectifying devices are used widely from small- or medium-capacity semiconductor apparatuses, which have a breakdown voltage of about 600 V or less, to high-capacity semiconductor apparatuses, which have a breakdown voltage of about 2.5 kV or more. Recently, switching devices, represented by IGBT's (insulated gate bipolar transistors), that work at high frequencies with low switching losses have been developed and are used in the high-breakdown-voltage and high-capacity field. Especially in the field of high-capacity use, GTO's (gate turnoff thyristors) have been replaced by IGBT's. In association with this replacement, it has been required for the diodes to exhibit high-speed recovery characteristics. In addition, it has also been required for the diodes to operate at high frequencies with low switching loss and to exhibit soft recovery characteristics for reducing EMI noises caused by the diodes operating in power electronics instruments.
A p-i-n diode, that is, a representative semiconductor rectifying device used widely in these days, typically includes an n-type drift layer (i-layer) for securing a high breakdown voltage between a p
+
-type anode layer contacting an anode electrode and an n
+
-type cathode layer contacting a cathode electrode. The specific resistance of the n
−
-type drift layer is higher than the specific resistance of the anode layer and the specific resistance of the cathode layer.
FIG. 4
is a cross-sectional view of a conventional p-i-n diode. Referring to
FIG. 4
, the conventional p-i-n diode includes an n
−
-type drift layer
2
with high specific resistance, an n
+
-type cathode layer
3
contacting a cathode electrode
5
on a major surface of n
−
-type drift layer
2
and a p
+
-type anode layer
1
contacting an anode electrode
4
on another major surface of n
−
-type drift layer
2
.
While the diode is switching from the ON-state to the OFF-state (during the reverse recovery process), a high transient current (hereinafter referred to as a “reverse recovery current”) flows in the opposite direction through the diode. When a reverse recovery current flows, an electric loss (hereinafter referred to as a “reverse recovery loss”) larger than the electric loss caused in the steady state is caused in the diode. There is a great need for a diode capable of reducing the reverse recovery loss. The reverse recovery process causes an electrical burden heavier than that in the steady state. As the steady state current or the reverse blocking voltage is increased, the electrical burden increases, which tends to cause a breakdown of the diode. To provide a highly reliable diode for electric power use, it would be highly advantageous to increase capability of a diode to withstand reverse recovery such that it is higher than its capability to withstand the rated current.
The minority carrier lifetime is controlled, in these days, by using heavy metal diffusion or electron beam irradiation to improve the reverse recovery characteristics and the reverse recovery withstanding capability of a diode. Since the total carrier concentration in the steady state is reduced by shortening the carrier lifetime, the carrier concentration swept out during the reverse recovery process by the expansion of the space charge region, the reverse recovery period, the peak reverse recovery current and the reverse recovery electric charges are reduced. Since the high electric field caused by the holes flowing through the space charge region is relaxed by reducing the hole concentration, the electrical burden is reduced and the reverse recovery withstanding capability is thereby improved. A merged p-i-n/Schottky diode (hereinafter referred to as an “MPS”), which reduces the injection efficiency of the minority carriers for improving the reverse recovery characteristics, has been developed (cf. U.S. Pat. No. 4,641,174 issued Feb. 3, 1987).
FIG. 5
is a cross-sectional view of a conventional MPS diode. Referring to
FIG. 5
, the conventional MPS diode includes an n
−
-type drift layer
2
with high specific resistance, an n
+
-type cathode layer
3
contacting a cathode electrode
5
on a major surface of n
−
-type drift layer
2
, a p
+
-type anode layer
1
contacting an anode electrode
4
in a portion of another major surface of n
−
-type drift layer
2
, and a Schottky junction
7
formed between n
−
-type drift layer
2
and anode electrode
4
in parallel to p
+
-type anode layer
1
.
Japanese Unexamined Laid Open Patent Application H05-218389 discloses a parallel arrangement of a Schottky junction and a p-i-n structure that facilitates reducing the carrier concentration without controlling the carrier lifetime, thus reducing the peak reverse recovery current, reducing the reverse recovery electric charges, and improving the reverse recovery withstanding capability.
One important parameter involved in addition to the reverse recovery characteristics is the temperature coefficient of the forward voltage in the ON-state of the diode. The temperature coefficient of the forward voltage indicates whether the forward voltage at a high temperature is higher or lower, around the rated current, than the forward voltage at the room temperature. The temperature coefficient is positive when the forward voltage at a high temperature is higher than the forward voltage at room temperature, and is negative when the forward voltage at a high temperature is lower than the forward voltage at room temperature. Preferably, the temperature coefficient of the forward voltage is positive.
The current balance in the diode chip, between the diode chips and the diode modules, explains why the positive temperature coefficient of the forward voltage is preferable. When a current localizes in a specific region of a diode chip, the temperature of the specific region rises locally. When the temperature coefficient of the forward voltage is positive, the resistance of the specific portion rises with increasing temperature, relaxing the current localization. When the temperature coefficient of the forward voltage is negative, the resistance of the specific region lowers with increasing temperature, causing further current localization. An imbalance is thus caused in the current flowing through a diode chip, between the currents flowing through diode chips or between the currents flowing through diode modules. At present, multiple diode chips or multiple diode modules are operated, in many cases, in unison for high breakdown voltage use or for high capacity use. Therefore, the temperature coefficient of the forward voltage is preferably positive to achieve well-balanced operations of the diode chip, the diode chips and the diode modules.
The reverse recovery characteristics and the capability of the p-i-n diode to withstand reverse recovery is improved by reducing the impurity concentration in the anode layer. Reducing the concentration of impurities in the anode layer is effective to suppress the peak reverse recovery current. However, the integral impurity concentration in the p
+
-type anode layer should be maintained at a level of 3×10
12
cm
−2
or more to obtain a certain breakdown voltage. Therefore, the integral impurity concentration in the p
+
-type anode layer should not be reduced below 3×10
12
cm
−2
. Moreover, reducing the integral impurity concentration in the anode layer has the disadvantage of increasing the forward voltage when a high current (about 500 A cm
−2
or higher) flows during the application of forward bias voltage. This problem is caused by the reduced impurity concentration, since the minority carriers injected are thus reduced. Although it is necessary to control t
Fuji Electric & Co., Ltd.
Nelms David
Nguyen Thinh
Rossi & Associates
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