Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – With pn junction isolation
Reexamination Certificate
1999-06-04
2001-03-27
Saadat, Mahshid (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
With pn junction isolation
C257S106000, C257S199000, C257S481000, C257S603000
Reexamination Certificate
active
06208011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power semiconductor device of a voltage-controlled transistor, such as power MOS field effect transistor (power MOSFET) and insulating gate bipolar transistor (IGBT).
2. Description of the Prior Art
The voltage-controlled power semiconductor devices include, for example, power MOS field effect transistors and insulating gate bipolar transistors, and each of them is provided with a gate oxide as its essential constituent element.
FIG. 12
is a sectional view showing the construction of a power MOS field effect transistor of the prior art, and
FIG. 13
shows its equivalent circuit. As illustrated in
FIG. 12
, the conventional power MOS field effect transistor is, for example, formed as follows: On an n
+
substrate
1
made of n-type silicon, an n
+
epitaxial layer
3
formed by n-type silicon subjected to epitaxial growth, a p-type region
4
formed by diffusing p-type impurities in the n
+
epitaxial layer
3
and n
+
source region
5
formed by diffusing n-type impurities therein are stacked, and on these layers, a gate
7
, insulated by a gate oxide
6
made of, for example, oxide silicon, is formed and Al conductor
8
serving as a source electrode is further formed. Moreover, in the power MOS field effect transistor of the prior art as shown in
FIG. 12
, n-type polysilicon layers
9
and p-type polysilicon layers
10
alternately placed by using polycrystalline silicon (polysilicon) used at the time of forming the gate electrode so that n-number of zener diodes d
11
, d
12
, . . . , d
1
n
are formed, thereby preventing a high voltage, which might cause breakdown of the insulation of the gate oxide
6
, from being applied to the gate oxide
6
.
The power MOS field effect transistor, configured as described above, is allowed to carry out switching operations by turning ON/OFF the gate voltage. In general, since this gate oxide is a comparatively thin film, if there is any defect in the gate oxide, dielectric breakdown tends to occur at the corresponding portion, causing breakdown of the device. For this reason, in general, after devices have been manufactured, a voltage, which is higher than the actually used voltage, is applied between the gate and source, thereby breaking and eliminating (screening) those originally weak devices containing any defect in the gate oxide; thus, only the devices having sufficient reliability can be shipped. Moreover in recent years, there have been demands for devices having a low switching voltage, and in response to these demands, attempts to reduce the switching voltage have been made by making the thickness of the gate oxide thinner or reducing the diffusion concentration (impurity concentration). As the gate oxide is made thinner, the dielectric breakdown voltage of the gate oxide is lowered, with the result that the possibility of breakdown due to defects, etc. occurring upon formation of the gate oxide become higher; therefore, more effective screening is required. Moreover, besides the power MOSFETs, power semiconductor devices of the voltage-control type include insulation gate bipolar transistors (IGBT), and the IGBT is also provided with a gate insulator in the same manner as the power MOSFET and normally with zener diodes, and therefore requires effective screening during its production process.
However, in the voltage-controlled power semiconductor device, since zener diodes are generally formed therein as described above, the voltage used for screening is set as low as possible, while the voltage of protection-use zener diodes has to be set to a voltage higher than the screening voltage. For this reason, in the conventional power semiconductor devices, it is not possible to apply any voltage exceeding a predetermined value to the gate oxide, resulting in failure to carry out an effective screening.
For example, in a power semiconductor device, in the case when zener diodes, which have a voltage-current characteristic as shown in
FIG. 14
, are formed as a protection circuit, upon receipt of an excessive current such as a surge, the gate oxide is subjected to a voltage application corresponding to the withstanding voltage VBG of the zener diodes plus the value of the voltage drop. Therefore, even in the case of screening by the use of a voltage not more than the withstanding voltage VBG, there is a possibility that the device might be broken by a voltage not more than the withstanding voltage VBG even though the zener diodes are provided.
Moreover, when the withstanding voltage VGB of the zener diodes is set lower, the voltage that can be applied upon screening has to be further lowered, with the result that it becomes more difficult to eliminate the initially defective products.
SUMMARY OF THE INVENTION
That is, an object of the present invention is to provide a voltage-controlled power semiconductor device with high reliability, which allows elimination of initially defective products with ease and can sufficiently protect the gate oxide in use.
In order to solve the object described above, the present invention provides a power semiconductor device comprising a semiconductor substrate; a voltage-controlled transistor having a first electrode formed on the lower surface of the semiconductor substrate, a gate insulated by a gate oxide formed on said semiconductor substrate and a second electrode formed on the semiconductor substrate; and a zener diode formed on the upper surface of the semiconductor substrate so as to be connected between the gate and the second electrode; wherein p-type regions and n-type regions alternately formed between the zener diode and the second electrode on the semiconductor substrate, a plurality of pad electrodes on the semiconductor substrate provided with the alternate p-type regions and n-type regions so as to allow one or not less than two diodes are series connected between the zener diode and the second electrode, and the distance between the adjacent pad electrodes is set so that when the diode is subjected to a current not less than a predetermined value, the respective pad electrodes are fused so that short-circuiting occurs between the adjacent pad electrodes.
With such a construction, after production, screening can be carried out by using a voltage not less than the withstanding voltage of the zener diode, and successively, by applying a current not less than a predetermined value to the respective diodes, the respective pad electrodes are fused, thereby causing short-circuiting between the adjacent pad electrodes. Consequently, it becomes possible to protect the gate insulator by using the diode characteristics of the zener diode.
Therefore, in the present invention, since the screening can be carried out by using a comparatively high voltage, it is possible to provide a voltage-controlled power semiconductor device with high reliability, which allows elimination of initially defective products with ease and can sufficiently protect the gate oxide in use.
Moreover, in the power semiconductor device of the present invention, in order to carry out screening in the off state of the voltage-controlled transistor, it is preferable to set the polarity of the respective diodes be the same as the polarity of the zener diode.
In the power semiconductor device of the present invention, by setting the respective diodes to have the same polarity as that of the zener diodes, screening can be carried out in the OFF state of the voltage control transistor, thereby making it possible to reduce power consumption at the time of screening.
Moreover, in the power semiconductor device of the present invention in which a plurality of diodes are provided, it is preferable to set the adjacent diodes to have polarities revered to each other. With this arrangement, screening can be carried out by applying either of the positive and negative voltages and consequently to carry out screening with ease.
Furthermore, in the power semiconductor device of the present invention, the gate can be made from po
Fenty Jesse A.
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Saadat Mahshid
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