Active solid-state devices (e.g. – transistors – solid-state diode – Avalanche diode
Reexamination Certificate
2000-07-14
2003-04-22
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Avalanche diode
C257S604000, C257S605000, C257S606000, C257S481000, C257S104000, C257S106000, C438S379000, C438S380000, C438S979000, C438S983000
Reexamination Certificate
active
06552413
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diode which requires a high breakdown voltage and a quick reverse recovery characteristic, for example, a free—wheeling diode, a voltage clamp diode or the like which is to be used together with a high breakdown voltage power semiconductor device such as an IGBT (Insulated Gate Bipolar Transistor), a GCT (Gate Commutated Turn—off Thyristor) or the like.
2. Description of the Background Art
FIG. 8
is a view showing a sectional structure of a conventional diode. The diode comprises an N layer
501
formed in a semiconductor substrate such as silicon, a P layer
502
formed adjacently to the N layer
501
, an N
+
layer
503
formed adjacently to the N layer
501
on the opposite side of the P layer
502
and having a higher impurity concentration than the impurity concentration of the N layer
501
, an anode electrode
504
, and a cathode electrode
505
. The N
+
layer
503
is provided to increase a carrier concentration, thereby making the diode thinner. The N
+
layer
503
is not provided in the vicinity of the P layer
502
but on the cathode electrode
505
side in order to effectively increase a carrier concentration except a portion to be depleted.
When a reverse bias voltage is applied, by the instantaneous switching of an external circuit, to the diode in which a current flows forward, the magnitude of the current is reduced to 0, and furthermore, a great current flows in an opposite direction for a certain period. This is a transient phenomenon caused by the movement of minority carriers stored in the diode. A reverse current decreases with a decrease ratio comprising, as parameters, a value of a reverse bias voltage to be applied and a value of an inductance of the external circuit and continuously flows until a concentration of excess carriers in the vicinity of a PN junction is reduced to a certain concentration or less to form a depletion layer. A value obtained by integrating a product of the reverse current and the reverse bias voltage value with respect to time is an energy loss consumed during a reverse recovery operation.
In order to enhance the reverse recovery characteristic, a proton is often irradiated in the vicinity of a PN junction formed on a boundary surface of the N layer
501
and the P layer
502
. Consequently, a recombination center of a carrier is formed and a life time of the carrier is controlled to be shortened. Moreover, the diffusion of a heavy metal, the irradiation of electron beams and the like are carried out over the whole semiconductor substrate and the life time of the carrier is similarly controlled. If the life time of the carrier is shortened, the number of the minority carriers to be stored is decreased. Consequently, the value of the reverse current can be controlled.
In the conventional diode described above, the current can be reduced during the reverse recovery operation by controlling the life time of the carrier in the vicinity of the PN junction to be shortened and the energy loss can be controlled.
In the case in which a great reverse bias voltage value is applied during the reverse recovery operation, however, there is a problem in that the voltage to be applied to the diode is rapidly oscillated to easily make EMI (Electro—Magnetic Interference) noises to cause a malfunction of peripheral electric machinery and apparatus. It is supposed that the variation in the voltage is caused in the following manner.
More specifically, during the reverse recovery operation, the diode has a capacitance component comprising, as parameters, a distance of a depletion layer and the number of excess carriers and a resistance component comprising, as parameters, a value of an applied reverse bias voltage and values of a leakage current and a current flowing by the movement of the excess carriers. By adding an inductance component of an external circuit for applying a reverse bias voltage to the capacitance component and the resistance component of the diode, an LCR series circuit is formed. The capacitance component and the resistance component of the diode are changed on a time basis with an enlargement of the depletion layer. Particularly the resistance component is rapidly increased when the depletion layer is generated so that a concentration distribution of the excess carriers is changed. If the natural oscillating conditions of the LCR series circuit are met by the change in the capacitance component and the resistance component, a voltage oscillation is caused.
Moreover, when the depletion layer reaches the N
+
layer
503
, the resistance component is changed suddenly. Consequently, it is also supposed that a trigger is generated to cause a voltage oscillation.
SUMMARY OF THE INVENTION
In order to solve the above-mentioned problems, it is an object of the present invention to implement a diode in which an energy loss is controlled during a reverse recovery operation and an applied voltage is oscillated with difficulty even if a reverse bias voltage has a great value.
A first aspect of the present invention is directed to a diode comprising a semiconductor substrate having a first main surface and a second main surface opposed to the first main surface, a first semiconductor region of a first conductivity type which faces the first main surface and is formed in the semiconductor substrate, a second semiconductor region of a second conductivity type which faces the second main surface and is formed in the semiconductor substrate adjacently to the first semiconductor region, at least one third semiconductor region of the first conductivity type formed in a position on the second semiconductor region facing the second main surface that a depletion layer extended from a boundary surface of the first and second semiconductor regions with application of a reverse bias voltage does not reach, a first electrode formed on the first main surface, and a second electrode formed on the second main surface.
A second aspect of the present invention is directed to the diode according to the first aspect of the present invention, wherein the second semiconductor region has a higher impurity concentration in second main surface side than in first semiconductor region side.
A third aspect of the present invention is directed to the diode according to the first or second aspect of the present invention, wherein a diameter of a portion of the third semiconductor region which faces the second main surface is approximately 400 &mgr;m or less.
A fourth aspect of the present invention is directed to the diode according to any of the first to third aspects of the present invention, wherein a total area of a portion of the third semiconductor region which faces the second main surface is approximately ⅖ or less of an area of the second main surface.
A fifth aspect of the present invention is directed to a diode comprising a semiconductor substrate having a first main surface and a second main surface opposed to the first main surface, a first semiconductor region of a first conductivity type which faces the first main surface and is formed in the semiconductor substrate, a second semiconductor region of a second conductivity type which faces the second main surface and is formed in the semiconductor substrate adjacently to the first semiconductor region, at least one third semiconductor region of the first conductivity type which faces the second main surface and is formed in the second semiconductor region, a first electrode formed on the first main surface, and a second electrode formed on the second main surface, wherein a diameter of a portion of the third semiconductor region which faces the second main surface is approximately 400 &mgr;m or less.
A sixth aspect of the present invention is directed to a diode comprising a semiconductor substrate having a first main surface and a second main surface opposed to the first main surface, a first semiconductor region of a first conductivity type which faces the first main surface and is forme
Hirano Noritoshi
Satoh Katsumi
Kang Donghee
Loke Steven
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