Super-junction semiconductor device

Details Super-junction semiconductor device Super-junction semiconductor device

2002-03-15

2004-02-24

Fahmy, Wael (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S339000

Reexamination Certificate

active

06696728

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to vertical power semiconductor devices including active devices such as MOSFETs, IGBTs and bipolar transistors, and passive devices such as diodes. Specifically, the present invention relates to vertical power super-junction semiconductor devices which facilitate realizing a high breakdown voltage and a high current capacity.
BACKGROUND OF THE INVENTION
The semiconductor devices may be classified into a lateral device, which arranges the main electrodes thereof on one major surface, and a vertical device, which distributes the main electrodes thereof on two major surfaces facing opposite to each other. In the vertical semiconductor device, a drift current flows in the thickness direction of the semiconductor chip (vertically) in the ON-state of the semiconductor device and depletion layers expand also in the thickness direction of the semiconductor chip (vertically) in the OFF-state of the semiconductor device.
FIG. 9
is a cross sectional view of a conventional planar-type n-channel vertical metal oxide semiconductor field effect transistor (MOSFET). Referring now to
FIG. 9
, the vertical MOSFET includes a drain electrode
18
on the back surface of a semiconductor chip; an n
+
-type drain layer
11
with low electrical resistance in electrical contact with drain electrode
18
; a very resistive n
−
-type drain drift layer
12
on n
+
-type drain layer
11
; p-type base regions
13
formed, as channel diffusion layers, selectively in the surface portion of n
−
-type drain drift layer
12
; a heavily doped n
+
-type source region
14
formed selectively in the surface portion of p-type base region
13
; a heavily doped p
+
-type contact region
19
formed selectively in the surface portion of p-type base region
13
for realizing ohmic contact; a polycrystalline silicon gate electrode layer
16
above the extended portion of p-type base region
13
extended between n
+
-type source region
14
and very resistive n
−
-type drain drift layer
12
with a gate insulation film
15
interposed therebetween; and a source electrode layer
17
in contact with n
+
-type source regions
14
and p
+
-type contact regions
19
. In the vertical semiconductor device as described above, n
−
-type drain drift layer
12
works as a layer, through which a drift current flows vertically in the ON-state of the MOSFET. In the OFF-state of the MOSFET, n
−
-type drain drift layer
12
is depleted by the depletion layers expanding in the depth direction thereof (vertically) from the pn-junctions between n
−
-type drain drift layer
12
and p-type base regions
13
, resulting in a high breakdown voltage.
Thinning very resistive n
−
-type drain drift layer
12
, that is shortening the drift current path, facilitates substantially reducing the on-resistance (the resistance between the drain and the source), since the drift resistance in the ON-state of the semiconductor device is reduced. However, thinning very resistive n
−
-type drain drift layer
12
narrows the width between the drain and the base region, for which depletion layers expand from the pn-junctions between n
−
-type drain drift layer
12
and p-type base regions
13
. Due to the narrow expansion width of the depletion layers, the depletion electric field strength soon reaches the maximum (critical) value for silicon. Therefore, breakdown is caused at a voltage lower than the designed breakdown voltage of the semiconductor device. A high breakdown voltage is obtained by thickening the n
−
-type drain drift layer
12
. However, a thick n
−
-type drain drift layer
12
inevitably causes high on-resistance, which further causes on-loss increase. In other words, there exists a tradeoff relation between the on-resistance (current capacity) and the breakdown voltage. The tradeoff relation between the on-resistance (current capacity) and the breakdown voltage exists in other semiconductor devices, which include a drift layer, such as IGBTs, bipolar transistors and diodes. The tradeoff relation between the on-resistance (current capacity) and the breakdown voltage also exists in lateral semiconductor devices, in which the flow direction of the drift current in the ON-state and the expansion direction of the depletion layers in the OFF-state are different.
European Patent 0 053 854, U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215, Japanese Unexamined Laid Open Patent Application H09-266311 and Japanese Unexamined Laid Open Patent Application H10-223896 disclose semiconductor devices, which facilitate reducing the tradeoff relation between the on-resistance and the breakdown voltage. The drift layers of the disclosed semiconductor devices are formed of an alternating-conductivity-type drain drift layer including heavily doped n-type regions and heavily doped p-type regions arranged alternately. Hereinafter, the alternating-conductivity-type drain drift layer will be referred to sometimes as the “first alternating conductivity type layer” or simply as the “drain drift region”.
FIG. 10
is a cross sectional view of the vertical MOSFET disclosed in U.S. Pat. No. 5,216,275. Referring now to
FIG. 10
, the drift layer of the vertical MOSFET is not a uniform n
−
-type layer (impurity diffusion layer) but a drain drift region
22
formed of thin n-type drift current path regions
22
a
and thin p-type partition regions
22
b
laminated alternately. Hereinafter, the n-type drift current path regions will be referred to as the “n-type drift regions”. The n-type drift regions
22
a
and p-type partition regions
22
b
are shaped with respective thin layers extending vertically. The bottom of p-type base region
13
is connected with p-type partition region
22
b
. The n-type drift region
22
a
is extended between adjacent p-type base regions
13
and
13
. Although alternating conductivity type layer
22
is doped heavily, a high breakdown voltage is obtained, since alternating conductivity type layer
22
is depleted quickly by the depletion layers expanding laterally in the OFF-state of the MOSFET from the pn-junctions extending vertically across alternating conductivity type layer
22
. Hereinafter, the semiconductor device which includes drain drift region
22
formed of an alternating conductivity type layer, which makes a current flow in the ON-state and is depleted in the OFF-state, will be referred to as the “super-junction semiconductor device”.
Although the channel stopper region in the breakdown withstanding region is of the same conductivity type with that of the drift layer usually, the channel stopper region is of the opposite conductivity type opposite to that of the drift layer sometimes depending on the manufacturing process. That is, the n-channel vertical MOSFET, the drift layer thereof is of n-type, includes a channel stopper region of p-type. In this case, the breakdown voltage of the MOSFET is stabilized by extending the channel stopper electrode connected to the outermost p-type region to the side of the active region so that the depletion layer in the peripheral portion of the MOSFET may not reach the outermost p-type region.
However, this structure causes a large leakage current in the n-channel super-junction MOSFET including an alternating conductivity type layer formed of p-type regions and n-type regions arranged alternately in the peripheral portion thereof, since the plural p-type regions of the alternating conductivity type layer connected to the plural p-type base regions in the active region is connected to one of the p-type regions in the channel stopper region. Increase of the leakage current causes not only increase of the losses in the OFF-state of the MOSFET but also breakdown of the MOSFET by thermal runaway.
In view of the foregoing, it would be desirable to provide a super-junction MOSFET reducing the tradeoff relation between the on-resistance and the breakdown voltage greatly and having a peripheral structure, which facilitates reducing the leakage curre

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