Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Having heterojunction
Reexamination Certificate
1998-04-21
2002-03-26
Crane, Sara (Department: 2811)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Having heterojunction
C257S487000, C257S496000, C257S565000
Reexamination Certificate
active
06362064
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the elimination of premature avalanche breakdown and subsequent breakdown voltage walkout in high voltage semiconductor devices, in particular, to eliminating premature initial breakdown and subsequent breakdown voltage walkout in trench isolated high voltage semiconductor devices by applying voltage directly to silicon adjacent to the trench isolating the device.
2. Description of the Related Art
The phenomenon of breakdown voltage “walkout” can present a serious problem to semiconductor designs that must exhibit a fixed initial breakdown voltage. Defined simply, “walkout” is the alteration in the breakdown voltage of a semiconducting device that results from at least one prior occurrence of avalanche breakdown.
FIGS. 1A-1F
illustrate the phenomenon of walkout at the base-collector junction of a conventional LOCOS isolated NPN bipolar transistor.
FIG. 1A
illustrates a cross-sectional view of the base/collector PN junction
100
of a conventional NPN bipolar transistor. The NPN bipolar transistor includes field oxide
102
, a heavily doped p type base region
104
, having base contact
106
, and a lightly doped n type collector region
108
having a collector contact
110
. For ease of illustration, collector contact
110
is shown as being on the side of collector
108
, rather than at its actual location at the silicon surface.
As shown in
FIG. 1B
, when the base/collector PN junction is forward bias, a positive potential difference is applied between the base contact
106
and the collector contact
110
. When the base/collector junction is biased in this manner, with the base contact at a more positive potential than the collector contact, holes
112
flow from base
104
to collector
108
, and electrons
113
flow from collector
108
to base
104
.
As shown in
FIG. 1C
, when the base/collector PN junction is reverse biased, a negative potential difference is applied between the base contact
106
and collector contact
110
. When the base/collector junction is biased in this manner, with the collector contact at a more positive potential than the base contact, depletion region
114
in collector
108
and depletion region
115
in base
104
are formed. The potential difference between collector contact
110
and base contact
106
is dropped across the adjacent depletion regions
114
-
115
.
The thickness of depletion regions
114
and
115
depends upon 1) the magnitude of the potential difference between collector contact
110
and base contact
106
; and 2) the dopant concentrations of collector
108
and base
104
. Because the dopant concentration of the collector (typically 10
14
-10
17
atoms/cm
3
), is generally lower than the dopant concentration of the base (typically 5×10
17
-10
19
atoms/cm
3
) depletion region
114
formed in collector
108
is generally wider than depletion region
115
formed in base
104
.
Under the reverse biased conditions described above, only a very small reverse current flows through the NPN transistor. However, if a large enough positive voltage difference is applied between collector contact
110
and base contact
106
, avalanche breakdown of the PN junction will occur and substantial current will flow across the base/collector PN junction
100
.
FIG. 1D
illustrates avalanche breakdown of base/collector PN junction
100
.
Avalanche breakdown as shown in
FIG. 1D
is triggered when the electric field at any point within the depletion regions
114
-
115
exceeds a certain critical value. The dopant concentrations of collector
108
and base
104
, and the junction depth of base
104
are among the factors which determine how large the potential difference between collector contact
110
and base contact
106
can be before the critical electric field is reached in any part of depletion regions
114
-
115
.
During avalanche breakdown, electrical charge carriers can acquire high energy and be injected into the field oxide. These injected charge carriers can remain embedded and affect the conductivity of surrounding silicon regions.
FIG. 1E
shows a cross-sectional view of the base/collector PN junction
100
following an avalanche breakdown event in which electrons
116
have been injected into the region
118
of field oxide
102
immediately adjacent to base/collector PN junction
100
.
FIG. 1F
illustrates that upon reverse-biasing base/collector PN junction
100
of
FIG. 1E
, embedded electrons
116
distort the shape of depletion region
114
. Their effect is to widen depletion region
114
, and therefore, for a given potential difference between collector contact
110
and base contact
106
, to reduce the peak electric field in the portion of depletion regions
114
-
115
adjacent to field oxide
102
. Since the highest electric field in depletion regions
114
-
115
usually is located close to where PN junction
100
intersects the interface between silicon and field oxide
102
, the embedded charge
116
has the effect of increasing the breakdown voltage of base/collector PN junction
100
.
To summarize, following an initial avalanche breakdown that injects high energy carriers into an oxide, the breakdown voltage of a device can increase as the embedded carriers reduce the peak electric field near the field oxide.
The aforementioned increase in breakdown voltage is commonly known as “walkout”. Walkout is illustrated graphically in FIG.
2
.
FIG. 2
plots voltage versus current in a semiconductor device that is experiencing walkout.
FIG. 2
illustrates that following an initial avalanche breakdown event triggered by application of a first initial breakdown voltage (BV
init
), the voltage required to recreate avalanche breakdown is increased, or “walked out”, to a higher voltage (BV
1
).
FIG. 2
shows that subsequent breakdown events can result in a cumulative increase in breakdown voltage (i.e. BV
init
<BV
2
<BV
3
).
Field plates have conventionally been used to increase PN junction breakdown voltages by reducing electric field where a PN junction intersects an oxide/silicon interface. The use of a field plate for this purpose is shown in
FIGS. 3A-3B
.
FIG. 3A
shows a cross-sectional view of the base/collector PN junction
300
of an NPN bipolar transistor equipped with a conductive field plate
301
having a field plate contact
301
a.
Conductive field plate
301
is in contact with surface portion
319
of field oxide
302
proximate to the field oxide/silicon interface
318
.
FIG. 3B
shows that if field plate contact
301
A is held at the same potential as base contact
306
, while base/collector PN junction
300
is reverse biased, the width of depletion region
314
proximate to field oxide/silicon interface
318
is increased. This reduces the peak electric field in the portion of depletion region
314
proximate to field oxide/silicon interface
318
, and therefore increases the breakdown voltage of base/collector PN junction
300
.
The effect of the field plate is very similar to the effect of embedded negative charge shown in FIG.
1
F. Though the field plate is usually electrically connected to base contact
306
, it may be independently biased. As long as the bias on the field plate is less positive than the bias on the collector contact
310
, the effect will be to increase the breakdown voltage of base/collector PN junction
300
.
FIGS
1
A-
1
F and
FIGS. 3A-3B
depict only the base/collector junction of an NPN bipolar transistor. However, if trench isolation is used to isolate individual transistors in an integrated circuit, the avalanche breakdown determining the highest collector voltage may occur initially not at the base/collector PN junction, but proximate to the trenches which isolate the individual transistors.
FIG. 4
illustrates a plan view of a high voltage trench isolated NPN bipolar transistor
400
. High voltage trench isolated transistor
400
includes a heavily doped n type emitter region
402
having an emitter contact
404
, formed within p type base region
406
having base con
Bashir Rashid
McGregor Joel M.
Yindeepol Wipawan
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