Semiconductor controlled rectifier with configured cathode to eliminate hot-spots

A semiconductor controlled rectifier comprises a semiconductor substrate having four layers of alternate n- and p-type conductivities and includes two main surfaces one of which is formed of the exposed surface of first and second layers and the other of which is formed of the exposed surface of a fourth layer. A gate electrode of a rectangular shape is disposed on the second layer on the one main surface and a cathode electrode is disposed on the first layer so as to extend along at least two sides of the rectangular gate electrode. The cathode electrode portion extending along the short side of the rectangular gate extends slightly beyond a p-n junction defined between the first and second layers, so as to be in ohmic contact with the second layer.

FIELD OF THE INVENTION 
The present invention relates to a semiconductor controlled rectifier and 
more particularly pertains to a structure of a semiconductor controlled 
rectifier in which initial firing due to a gate trigger current can be 
established uniformly along a relatively wide region of the p-n junction 
between two semiconductor layers connected to the gate and cathode 
electrodes. 
BACKGROUND OF THE INVENTION 
As is well known, a typical semiconductor controlled rectifier comprises a 
semiconductor substrate having first, second, third and fourth layers of 
alternate n- and p-types of conductivities. One main surface of the 
semiconductor substrate is formed of the exposed surfaces of the first and 
the second layers and the other main surface is formed of the exposed 
surface of the fourth layer. A cathode electrode is disposed in ohmic 
contact with the first layer on one main surface and a gate electrode is 
disposed in ohmic contact with the second layer on the same surface as 
mentioned above. On the other hand, an anode electrode is formed in ohmic 
contact with the fourth layer on the other main surface. An example of a 
semiconductor controlled rectifier device that employs amplifying gate 
structure may be found in the U.S. Pat. No. to Kimura et al. 4,063,270. 
If a voltage in the forward direction is applied across the anode and the 
cathode of such a semiconductor controlled rectifier and, further, if a 
pulse-like gate signal is applied across the gate and cathode, electric 
current flows between the anode and the cathode rendering the 
semiconductor controlled rectifier conductive. The conversion of the 
semiconductor controlled rectifier from the cut-off state to the 
conductive state is referred to as "turn on". 
If the initial firing, which causes the semiconductor controlled rectifier 
to be turned on, takes place in a narrow region, a heavy firing current 
flows through the firing region resulting in an increase in temperature 
and producing a so-called hot spot. Such a phenomenon develops 
conspicuously particularly when the rate of current increase di/dt (where 
i denotes a current flowing between the anode and the cathode) flowing 
into the semiconductor controlled rectifier device is high. The reason is 
because when di/dt is great, large switching power is dissipated causing 
the temperature of the narrow firing region to be increased. The 
development of the hot spot presents the probability of damage to the 
semiconductor controlled rectifier device, thereby greatly decreasing the 
reliability of the device. 
SUMMARY OF THE INVENTION 
The principal object of the present invention is to provide a semiconductor 
controlled rectifier which is so constructed that the initial firing when 
the rectifier is turned on is uniformly established over a relatively wide 
region. 
In more detail, the object of the present invention is to provide a 
semiconductor controlled rectifier having increased reliability by 
permitting the initial firing to take place uniformly over a relatively 
wide region so that a hot spot will not be developed when di/dt is great, 
i.e., even when a large amount of switching power is consumed. 
One of the features of the present invention for attaining the abovesaid 
object consists of disposing a cathode electrode a predetermined distance 
from both sides of a gate electrode having relatively long sides and 
relatively short sides in a manner such that the cathode electrode faces 
the gate electrode, and such that the firing is established in a p-n 
junction region immediately beneath the cathode electrode facing to the 
long side of the gate electrode. 
Another feature in the present invention consists of disposing a cathode 
electrode a predetermined distance from both sides of a gate electrode 
having relatively long sides and relatively short sides in such a manner 
that the cathode electrode faces the gate electrode, and extending the 
cathode electrode facing the short sides of the gate electrode slightly 
beyond the end of junction between a first semiconductor layer and a 
second semiconductor layer, such that the cathode electrode comes into 
ohmic contact with the second semiconductor layer. 
A further feature of the present invention is that the distance between the 
gate electrode and a p-n junction defined between the first and second 
layers, which is formed along the long side of the rectangular electrode 
separated therefrom, is shorter than the distance between the gate 
electrode and the p-n junction formed along the short sides of the gate 
electrode. 
Other objects and features of the invention will become more apparent from 
the following detailed description in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For the purpose of better understanding of the present invention, the 
structure of a representative semiconductor controlled rectifier and the 
mechanism of firing will be described below with reference to FIGS. 1a, 1b 
and 3. 
Referring to FIGS. 1a and 1b, reference numerals 1 to 4 represent first, 
second, third and fourth semiconductor layers, respectively, the p-type 
semiconductor layers and the n-type semiconductor layers being 
alternatingly laminated on a semiconductor substrate. Although the 
drawings show the semiconductor composed of four layers, it is permissible 
to employ more than four layers. The first semiconductor layer 1 is formed 
by masking part of the p-type second semiconductor layer 2, and diffusing 
n-type impurities into other portions. In these drawings, for the purpose 
of illustration, the first semiconductor layer is formed in a partially 
cut-away circular form. On the exposed surface of the first semiconductor 
layer there is formed a cathode electrode 5 in ohmic contact therewith, 
and on the exposed surface of the second semiconductor layer there is 
formed a rectangular gate electrode 7 in ohmic contact therewith. In this 
specification, the rectangular shape is so defined as to include a 
perfectly rectangular shape, as well as slender arcuate shapes, oval 
shapes, and other slightly modified shapes. What is important in the 
present invention is that the shape of the gate electrode consists of 
relatively long sides and relatively short sides. Usually, the gate 
electrode is surrounded by the cathode electrode in such a manner that the 
two short sides and one long side of the rectangular gate electrode are 
spaced apart by a predetermined distance from the cathode electrode, so 
that the exposed surfaces of the first and the second semiconductor layers 
may be utilized as effectively as possible. A portion J.sub.1 of the p-n 
junction between the first semiconductor layer and the second 
semiconductor layer facing the gate electrode 7 is slightly extended 
toward the gate electrode as compared to the cathode electrode 5. A number 
of holes 8 which are usually referred to as short-circuiting holes are 
formed in the first semiconductor layer in order to improve the 
characteristic dv/dt (the rate of rise in the forward voltage across the 
anode and the cathode). The second semiconductor layer and the cathode 
electrode 5 are in ohmic contact to each other through the 
short-circuiting holes 8. On the exposed surface of the fourth 
semiconductor layer 4 there is formed an anode electrode 6 in ohmic 
contact therewith. 
The turn-on mechanism of the semiconductor controlled rectifier having the 
abovementioned construction will be described below with reference to the 
equivalent circuit of FIG. 3. In FIG. 3, the same portions as those of 
FIGS. 1a and 1b are represented by the same reference numerals. The 
resistance of the current path from the gate electrode 7 to the p-n 
junction J.sub.1 via the second semiconductor layer is denoted by R.sub.1, 
and the resistance of the current path from said p-n junction J.sub.1 to 
the cathode electrode 5 via a short-circuiting hole closest to said 
junction J.sub.1 is denoted by R.sub.2. If a voltage is applied to the 
gate while a voltage in the forward direction is being applied across the 
cathode electrode 5 and the anode electrode 6, a small current flows from 
the gate electrode 7 toward the cathode electrode 5 through resistors 
R.sub.1 and R.sub.2. If a voltage drop produced by the above small current 
through the resistor R.sub.2 exceeds the build-in voltage level of the p-n 
junction J.sub.1, the gate current is permitted to flow through the p-n 
junction J.sub.1 into the cathode electrode so that the semiconductor 
controlled rectifier is rendered turned on. Therefore, when the 
semiconducrtor controlled rectifier is turned on, on which portion of the 
p-n junction J.sub.1 the firing will take place is determined by the 
potential distribution of the p-n junction produced by the current flowing 
between the gate electrode 7 and the cathode electrode 8. This potential 
distribution is proportional to the resistance ratio R.sub.2 /(R.sub.1 
+R.sub.2). The graph of FIG. 2 is obtained by measuring the potential 
distribution at the positions X.sub.1 to X.sub.8 along the p-n junction 
J.sub.1 formed adjacent to the three sides of the gate electrode 7. The 
solid line shows the dispersion of resistance of the second semiconductor 
layer 2, and the potential distribution when there is no deviation in 
position of the patterns of the electrodes 7 and 5. In this case, the 
junction portions are spaced apart by an equal distance from the gate 
electrode 7 in the regions X.sub.1 -X.sub.2, X.sub.4 -X.sub.5, and X.sub.7 
-X.sub.8, and the potential is nearly constant. In the regions X.sub.2 
-X.sub.4 and X.sub.5 -X.sub.7, however, the junction portions are 
separated more apart from the gate electrode, whereby the resistance 
R.sub.1 is increased and the potential is decreased. If the resistance of 
the second semiconductor layer is not uniform, or if the positions of the 
electrodes 7 and 5 are deviated from each other, the potential at the 
junction of the regions X.sub.1 -X.sub.2 and X.sub.7 -X.sub.8 may often 
become greater than the potential at other regions of the junction. In the 
regions having a potential higher than that of the other regions, the 
carriers are concentrated in large amounts from the first semiconductor 
layer 1 to the second semiconductor layer 2, so that the regions of the 
initial firing are limited to these narrow regions X.sub.1 -X.sub.2 and 
X.sub.7 -X.sub.8. Even when the junction has a potential distribution as 
represented by the solid line, the firing which took place at some regions 
such as X.sub.1 -X.sub.2 or X.sub.7 -X.sub.8 is not often allowed to 
instantaneously and uniformly spread to the region X.sub.4 -X.sub.5 
interrupted by a low potential at the positions X.sub.3 and X.sub.6. 
If the region of the initial firing is limited to narrow regions as 
mentioned above, a heavy current flows in a concentrated manner through 
the narrow regions thereby developing hot spots. If di/dt (i represents a 
current flowing between the anode and the cathode) of the semiconductor 
controlled rectifier element increases, dv/dt (v denotes a voltage across 
the anode and the cathode) decreases. However, since the gradient of di/dt 
is usually more steep than that of dv/dt, an increased amount of electric 
power is consumed by the element particularly when di/dt is great, whereby 
the temperature is greatly raised giving rise to the development of hot 
spots. The hot spots often result in damage to of the element, greatly 
decreasing the reliability, as mentioned above. 
As will be obvious from the foregoing, the firing regions are restricted to 
narrow regions due to the fact that the junction J.sub.1 includes the 
regions X.sub.1 -X.sub.2 and X.sub.7 -X.sub.8 having high potentials, and 
regions X.sub.2 -X.sub.4 and X.sub.5 -X.sub.7 having low potentials. To 
solve this problem, the initial firing should be forcibly established in 
the region X.sub.4 -X.sub.5. The region X.sub.4 -X.sub.5 corresponds to 
the long side of the gate electrode 7, and exhibits a nearly uniform 
potential distribution over a relatively wide region. Therefore, if the 
firing takes place at any point in the region X.sub.4 -X.sub.5, the firing 
region instantaneously spreads throughout the whole region X.sub.4 
-X.sub.5, preventing the development of hot spots. 
The present invention is constructed based on the abovementioned 
consideration. An embodiment of the invention is shown in FIG. 4a and FIG. 
4b. FIG. 4a is a plan view showing a portion of the semiconductor 
controlled rectifier, and FIG. 4b is a cross-sectional view along the 
section IV--IV of FIG. 4a, in which the same portions as those of FIGS. 1a 
and 1b are represented by the same reference numerals. 
As will be understood from the drawings, according to this embodiment, 
portions of the cathode electrode 5 facing the short sides of the gate 
electrode 7 are defined by the side ends of the first semiconductor layer 
and the second semiconductor layer. The above-mentioned portions of the 
cathode electrode are extended toward the gate electrode beyond the p-n 
junction to come into ohmic contact with the second semiconductor layer 2. 
The portions of the cathode electrode that are in ohmic contact with the 
second semiconductor layer 2 are hereinafter referred to as 
short-circuiting portions. The portion of the cathode electrode facing the 
long side of the gate electrode 7, on the other hand, merely extends up to 
the inner side of the p-n junction between the side end of the first 
semiconductor layer and the second semiconductor layer. As a result, when 
the initial firing has taken place, the electric current flows through a 
path including resistances R.sub.1 and R.sub.2 of FIG. 1b over the region 
in which the cathode electrode 5 faces to the long side of the gate 
electrode 7. In the regions in which the cathode electrode 5 faces the 
short sides of the gate electrode 7, the electric current flows through 
paths indicated by dotted lines of FIG. 4b. That is, the short-circuiting 
portions 10 electrically short-circuit the gate electrode 7 to some 
portions of the cathode electrode, such that the potential is maintained 
at zero in the p-n junction between the first semiconductor layer and the 
second semiconductor layer. 
FIG. 5 shows the potential distribution at the points X.sub.1 -O-X.sub.8 on 
the junction portions between the side ends of the first semiconductor 
layer and the second semiconductor layer when a gate current is caused to 
flow in the forward direction from the gate electrode 7 toward the cathode 
electrode 5. In the regions X.sub.1 -X.sub.3 and X.sub.6 --X.sub.8, the 
potential becomes nearly zero, while a uniform potential is produced over 
the region X.sub.4 -O-X.sub.5. In the regions X.sub.4 -X.sub.3 and X.sub.5 
-X.sub.6 remote from the gate electrode 7, the resistance R.sub.1 
increases and the potential decreases drastically. 
Therefore, the initial firing established by the gate trigger takes place 
over the region X.sub.4 -O-X.sub.5 facing the long side of the gate 
electrode 7; i.e., the initial firing is uniformly established over a 
relatively wide region. 
What is important in the abovementioned embodiment is that the 
short-circuiting portions 10 are formed along the entirety of regions 
X.sub.1 -X.sub.3 and X.sub.6 -X.sub.8 facing the short sides of the gate 
electrode 7. When the short-circuiting portions are formed only at limited 
portions of the cathode electrode, for example, at the positions X.sub.1 
and X.sub.8 only, the firing may take place at p-n junction portions 
facing the gate electrodes, presenting the probability of developing hot 
spots. Two advantages can be provided by the provision of short-circuiting 
portions 10. One advantage is that the initial firing is uniformly 
established over a relatively wide region as mentioned above, making it 
possible to prevent the development of hot spots; the element is not 
destroyed even when di/dt becomes great. 
Another advantage is the prevention of the semiconductor controlled 
rectifier from being misfired by noise. The reason is attributed to the 
fact that the electric current flowing from the gate electrode 7 toward 
the short-circuiting portions 10 turns into a wasteful current that does 
not contribute to the initial firing of the semiconductor controlled 
rectifier, whereby the sensitivity of the firing is decreased by a 
corresponding amount. The semiconductor controlled rectifier having 
excessively high sensitivity for gate trigger current tends to be misfired 
even by small noise. 
FIG. 6 is a plan view showing a portion of the semiconductor controlled 
rectifier according to another embodiment of the present invention. A 
cross-sectional view thereof is not pesented here, since it can be easily 
understood from FIG. 1b. 
The feature of the semiconductor controlled rectifier according to this 
embodiment is that the gate electrode 7 is so disposed that the distance l 
between the p-n junction (junction defined by the side end of the first 
semiconductor layer and the second semiconductor layer) facing the short 
sides of the rectangular electrode 7 and the gate electrode 7 is greater 
than the distance l.sub.0 between the p-n junction facing the long side of 
the rectangular gate electrode 7 and the gate electrode 7. By this 
arrangement, the resistance of the second semiconductor layer between the 
gate electrode 7 and the regions X.sub.1 -X.sub.2 and X.sub.7 -X.sub.8 
becomes greater than the resistance between the gate electrode 7 and the 
region X.sub.4 -X.sub.5. Therefore, the potential developed along X.sub.1 
-X.sub.8 by the gate trigger current is as shown in FIG. 8; the potential 
is nearly constant over the region X.sub.4 -X.sub.5, and the potential 
becomes small over the regions X.sub.1 -X.sub.2 and X.sub.7 -X.sub.8. 
Accordingly, the initial firing is established over the region X.sub.4 
-X.sub.5, providing the same effect as that of the embodiment shown in 
FIGS. 4a and 4b. 
Further, the short-circuiting portions 10 may be provided as shown in FIG. 
4a, and the gate electrode may be so disposed as to satisfy the relation 
l&gt;l.sub.0 as shown in FIG. 6. In this case, the initial firing can be 
established not only over the relatively broad region X.sub.4 -X.sub.5, 
but also it is allowed to adjust the intensity of the loss current flowing 
from the gate 7 toward the short-circuiting portions 10, making it 
possible to adjust the sensitivity of firing. 
Referring to FIGS. 4a, 4b and 6, it can be easily understood that the 
short-circuiting holes 8 are not an essential requirement for the present 
invention and may be eliminated. 
FIG. 7 shows a further embodiment according to the present invention. In 
this embodiment, the short-circuiting holes are so disposed that the 
distance d from the p-n junction facing the short sides of the gate 
electrode 7 to a neighboring short-circuiting hole 8 is smaller than the 
distance d.sub.0 from the p-n junction facing the long side of the gate 
electrode 7 to a neighboring short-circuiting hole 8. Moreoever, the 
short-circuiting holes have been so arranged that the distance D among the 
neighboring short-circuiting holes near the p-n junction facing the short 
sides of the gate electrode 7 is shorter than the distance D.sub.0 among 
the neighboring short-circuiting holes located near the p-n junction 
facing the long side of the gate electrode 7. When either one of the 
abovementioned requirements d&lt;d.sub.0 and D&lt;D.sub.0 L is satisfied, the 
resistance from the p-n junction over the X.sub.1 -X.sub.2 and X.sub.7 
-X.sub.8 regions toward the cathode electrode through the second 
semiconductor layer and short-circuiting holes becomes smaller than the 
resistance from the p-n junction over the region X.sub.4 -O-X.sub.5 toward 
the cathode electrode. After all, the potential distribution along X.sub.1 
-X.sub.8 becomes nearly equal to that of FIG. 8. It will therefore be 
apparent that the effect similar to that of the aforementioned embodiment 
is obtained. 
Although some embodiments of the present invention in the foregoing, it 
should be noted that the present invention is not necessarily restricted 
to those of the aforementioned embodiments only but can be variously 
modified without departing from the spirit and scope of the present 
invention. That is, the essential feature of the present invention is that 
the potential distribution produced on the p-n junction between the side 
ends of the first semiconductor layer and the second semiconductor layer 
by the gate trigger current, is so controlled that the potential on the 
p-n junction facing the short sides of the gate electrode is smaller than 
the potential on the p-n junction facing the long side of the gate 
electrode. The abovesaid potential distribution can also be achieved by 
selectively diffusing impurities into the second semiconductor layer 
around the gate electrode to partially vary the resistance of the layer, 
or by forming grooves in the second semiconductor layer by means of 
etching to vary the electrical resistance. 
Further, according to the present invention, the kinds and types of the 
semiconductor controlled rectifiers are not critical; the present 
invention can also be applied to the semiconductor controlled rectifiers 
of the amplifier type having an auxiliary gate and a main gate. 
Moreover, as mentioned already, the shape of the gate electrode needs not 
be limited to a rectangular shape, and the invention can be extensively 
applied to ordinary semiconductor controlled rectifiers having long sides 
and short sides in which the cathode electrode is formed adjacent to both 
of such sides.