Semiconductor device having symmetrical current distribution

A semicondutor device has an electrode with a configuration having a trunk portion and at least one comb-like portion. The comb-like portion has a back with ends and a plurality of conductive fingers extending therefrom. The trunk portion has a segment which is substantially parallel to the back of the comb-like portion. The trunk portion contacts the comb-like portion at a point intermediate the ends of the back.

The present invention relates generally to semiconductor devices, and, in 
particular, relates to those devices having at least one surface region to 
be contacted by an electrode. 
A major problem of conventional semiconductor devices, particularly 
transistors having interdigitated base-emitter structures, is that each 
finger portion of the interdigitated region operates at a different 
voltage potential. That is, where an electrode extends from a terminal pad 
and weaves along the periphery of the finger portions there is a 
considerable voltage drop between the finger portions near the terminal 
pad and the finger portions further away, the relative distance of the 
finger portions to the terminal pad being measured along the electrode. 
The same condition exists in an electrode configuration wherein a trunk 
portion extends from a terminal pad and has a plurality of conductive 
fingers protruding directly therefrom into the finger portions of the 
interdigitated region. Such a voltage imbalance between the finger 
portions can cause a semiconductor device to be unstable. 
In additon to the unbalanced voltage distribution itself, the electrode 
fingers nearer the terminal pad, in order to assure that the electrode 
fingers further away are maintained at some minimum operating voltage, are 
usually overbiased, i.e., maintained at a voltage which is greater than 
necessary for proper operation. Consequently, more current flows into the 
finger portions of the interdigitated region nearer the terminal pad than 
into the finger portions further away. This results in excessive current 
flow in those finger portions which can create hot spots which can damage 
or destroy a device. 
Conventional attempts to solve the above problems have generally been 
directed toward reducing the resistance of the trunk and thereby reducing 
the voltage drop differences between the fingers. One such solution is to 
form the trunk from a material having an increased conductivity. Another 
solution is to increase either the thickness or width, or both, of the 
electrode. 
Although these solutions succeed in reducing the resistance of the trunk, 
and hence the voltage drop differences between the electrode fingers, the 
basic problem remains. It is thus desirable to provide an electrode 
configuration which directly substantially eliminates the voltage drop 
differences between electrode fingers.

A semiconductor device, indicated generally at 10 in FIG. 1, comprises a 
body 12 of semiconductor material having a surface 14. 
A first region 16 having a one type conductivity, for example, P type, is 
within the body 12 and adjacent the surface 14. While the first region 16 
is described herein as having a P type conductivity, it can also be of N 
type conductivity so long as all other conductivity types specified herein 
are likewise changed. 
At least one second region 18 having another type conductivity, N type in 
this embodiment, is within the first region 16 and adjacent the surface 
14. The second region 18 forms a PN junction 20 with the first region 16 
at the interface therewith. Preferably the first region 16 and the second 
region 18 are interdigitated although other shapes may also be used, for 
example the second region 18 can have a surface intercept in the shape of 
a grid, not shown in the drawings. By interdigitated it is meant that 
finger portions of the first region 16 are interlaced with similar finger 
portions of the second region 18. 
A layer 22 of conductive material electrically contacts the first region 16 
and constitutes an electrode. Preferably, the layer 22 contacts the first 
region 16 only at the finger portions thereof. That is, the layer 22 on 
the body 12 may be spaced apart from the first region 16 by a layer of 
insulating material, not shown in the drawings, which has openings therein 
which expose the finger portions and allow the layer 22 to contact the 
first region 16 therethrough. The layer 22 has a configuration comprising 
a comb-like portion 24 having a back 26 with ends and a plurality of 
fingers 28 extending over the finger portions of the first region 16. The 
configuration also comprises a trunk portion 30 which has a segment 32 
thereof which is substantially parallel to the back 26 of the comb-like 
portion 24. The trunk portion 30 extends from a terminal pad 34 and one 
end thereof contacts the back 26 of the comb-like portion 24 at a point 
intermediate the ends thereof. 
Operationally it is preferred, in this embodiment, that the plurality of 
fingers 28 be substantially symmetrically distributed along the back 26 
with respect to the point of contact of the one end of the trunk portion 
30. This criteria assures that the maximum voltage drop between the point 
of contact of the one end of the trunk portion 32 with the back 26 and the 
furthest finger 28 away therefrom is considerably less than the voltage 
drop would be if the trunk portion 32 contacted the back 26 at an end 
thereof. 
The degree of difference between the voltage drop of the present embodiment 
and a configuration, not shown in the drawings, wherein the trunk portion 
contacts the back at an end thereof is demonstrated by the calculations 
below. 
The average voltage drop (.DELTA.V) of a conductive electrode is given by 
the general formula .DELTA.V=IR/2 wherein: 
I is the current; and 
R is the resistance of the back 
The resistance R may be written as: R = (l/w).rho..sub.s wherein: 
l is the path length, 
w is the path width; and 
.rho..sub.s is the sheet resistivity of the material of the back. 
Therefore, for the configuration wherein the trunk contacts an end of the 
back, not shown in the drawings, the voltage drop along the back would be 
.DELTA.V.sub.1 = Il/2w .rho..sub.s. 
For the present embodiment, shown in FIG. 1, the current and the path 
length are one-half that of the above configuration since the contact 
point is intermediate the ends of the back 26 and the fingers 28 are 
symmetrical therewith. 
Therefore, for the electrode of the present embodiment: 
##EQU1## 
Hence, the voltage drop differential between the point of contact of the 
one end of the trunk to the back and the finger most distant therefrom is 
4 times less in the present embodiment than in the configuration wherein 
the trunk portion contacts an end of the back. 
A number of inherent advantages are derived from the use of the present 
embodiment and the accompanying relatively large reduction the voltage 
drop differential. Initially, since the primary objective of any electrode 
design is to deliver a particular operating voltage to the finger 28 
furthest from the trunk portion contact point, it is clear that the 
voltage potential, in the present embodiment, at the point of contact 
between the trunk portion 30 and the back 26 can be considerably less, due 
to the reduced voltage drop to the fingers 28, than that required were the 
trunk portion 30 to contact an end of the back 26. Since the maximum 
voltage drop along the back portion 26 is reduced by the present 
embodiment the size of the trunk portion 30 compared to the size of the 
trunk portion which contacts an end of the back, can be reduced. While a 
reduction in size increases the resistance of the trunk portion 30 the 
surface area of the overall device, however, can be reduced. This is an 
important aspect since chip area is usually at a premium in semiconductor 
device technology. 
In addition to the voltage drop between the trunk contact point and the 
fingers furthest therefrom, another consideration in the design of an 
electrode configuration is the voltage drop between adjacent fingers. A 
reduction in the voltage drop between adjacent fingers can be obtained 
through the use of a second embodiment of the present configuration. Such 
a configuration is shown in FIG. 2 of the drawings. 
The device 40 shown in FIG. 2 is similar to the device 10 in that it 
comprises a body 42 of semiconductor material having a surface 44. 
A first region 46 having a one type conductivity is within the body 42 and 
adjacent the surface 44. A second region 48 having another type 
conductivity is within the first region 46 and adjacent the surface 44. 
Preferably the first region 46 and the second region 48 are 
interdigitated. A PN junction 50 is formed at the interdigitated interface 
between the first region 46 and the second region 48. 
A layer 52 of conductive material electrically contacts the first region 46 
and constitutes an electrode. The layer 52 has a configuration comprising 
a comb-like portion 54 having a back 56 with ends and a plurality of 
fingers 58 extending over portions of the first region 46. In this 
embodiment the back has a comparatively wider portion 59 near one end and 
a comparatively narrower portion 60 near the other end. The layer 52 also 
comprises a trunk portion 62 which has a segment 64 thereof substantially 
parallel to the back 56. The trunk portion 62 extends from a terminal pad 
66 and contacts the back 56 of the comb-like portion 54 at a point `A` 
intermediate the ends thereof. Preferably, the point `A` is asymmetrically 
located with respect to the fingers 58. That is, it contacts the back 56 
nearer to the comparatively narrower end thereof. 
Operationally, by utilizing calculations similar to those presented above, 
the extent of the comparatively wider portion 59 and the point `A` of 
contact can be determined so that the voltage drops between the point `A` 
and each end of the back 56 are equal. Since the resistance of the 
comparatively wider portion 59 is less than that of the comparatively 
narrower portion 60 the point `A` will be nearer the end of the back 56 
having the comparatively narrower portion 60. Because of this location the 
voltage drop between adjacent fingers 58 along the narrower portion 60 is 
reduced, there being a larger voltage potential nearer the end of the 
narrower portion 60 due to the asymmetrical location of `A`. Further, 
because of the lower resistance of the comparatively wider portion 59 the 
voltage drop between adjacent fingers 58 therealong is also reduced. Thus, 
the configuration of the layer 52 of this embodiment not only reduces the 
voltage drop between the trunk portion contact point `A` and the fingers 
58 furthest therefrom but also reduces the voltage drop between adjacent 
fingers. As a result, the fingers 58 are at substantially the same voltage 
potential during the operation of the device 40. 
Referring back to the device 10 as shown in FIG. 1 it is observed that as 
the number of fingers 28 extending from the back 26 increases, the need 
for further voltage drop equalization also increases. This need can be 
substantially fulfilled by utilizing the configuration as shown in FIG. 3 
of the drawings. Therein a device 68 comprises a body 70 of semiconductor 
material having a surface 72. The device 68 further comprises a first 
region 74 having a one type conductivity within the body 70 and adjacent 
the surface 72. 
A second region 76 having another type conductivity is within the first 
region 74 and adjacent the surface 72. Preferably, the first region 74 and 
the second region 76 are interdigitated. An interdigitated PN junction 78 
is formed at the interface between the first region 74 and the second 
region 76. 
A layer 80 of conductive material overlies and electrically contacts the 
first region 74. The layer 80 has a configuration comprising a first 
comb-like portion 82 and a trunk portion 84. The first comb-like portion 
82 has a first back 86 with ends and a first plurality of fingers 88 
extending therefrom. In this embodiment the first plurality of fingers 88 
are preferably substantially equally distributed along the first back 86. 
A second comb-like portion 90, preferably being spaced apart from and 
substantially parallel with the first comb-like portion 82, has a second 
back 92 with ends and a second plurality of fingers 94 extending 
therefrom. The second back 92 is contacted by the first plurality of 
fingers 88. Preferably, in this embodiment, the contacts are substantially 
symmetrical along the second back 92. 
The trunk portion 84 has a segment 96 which is substantially parallel with 
both the first back 86 and the second back 92. The trunk portion 84 
extends from a terminal pad 98 to the first comb-like portion 82 and one 
end of the trunk contacts the first back 86 intermediate the ends thereof. 
Operationally this embodiment can be visualized as being a pair of 
configuraions as described above in relation to the device 10 connected in 
tandem. That is, the first comb-like portion 82 serves as a subfeeder 
wherein each of the first plurality of fingers 88 serves as a trunk-like 
portion contacting the second comb-like portion 90. As discussed above, 
the voltage drops between the trunk portion 84 and each of the first 
plurality of fingers 88 are substantially equal. Hence, since the 
plurality of fingers 88 contact the second back 92 in a substantially 
symmetrical fashion, the voltage drops between each of the first plurality 
of fingers 88 and each of the second plurality of fingers 94 are 
substantially equal. Therefore, the voltage drops between the trunk 
portion 84 and each of the second plurality of fingers 94 are 
substantially the same. This tandem principle can also be used for the 
configuration of the device 40 shown in FIG. 2. 
The layers 22, 52 and 80 of conductive material can be, for example gold, 
nickel-lead or the like. These configurations can be formed using known 
methods in the art such as photolithographic techniques. 
While the above discussion has been directed to devices having 
interdigitated regions the electrode configurations are equally applicable 
to devices having a plurality of discrete areas at the surface which are 
contacted by a single electrode. 
The present novel devices 10, 40 and 68 have substantially reduced voltage 
drops between the fingers 28, 58 and 94, respectively, of the electrodes 
thereof and hence there is little cause to overbias any of those fingers 
28, 58 or 94. Therefore, there is little chance of hot spots, due to 
excessive currents from unbalanced voltage distributions, damaging the 
devices 10, 40 or 68.