Reverse conducting thyristor with specific resistor structures between main cathode and amplifying, reverse conducting portions

A reverse conducting thyristor includes a thyristor section, a diode section and a semiconductor separator section for electrically separating both the sections. The thyristor section includes: a first region of first conductivity type, a second region of a second conductivity type, a third region of the first conductivity type, a main emitter region of the second conductivity type, an auxiliary emitter region formed, with intervention of the exposed portion of said third region, facing at least a part of the periphery of said main emitter region which does not contact the separate section and a cathode electrode, an auxiliary gate electrode contacting the auxiliary emitter region and enclosing, with intervention of the exposed portion of the third region, at least a part of the periphery of said main emitter region which does not contact the separate section, and a main gate electrode formed on the exposure surface of the third region contacting the side wall of the auxiliary emitter region which does not face the main emitter region. The respective components are successively layered on a first electrode acting as an anode electrode. The diode section includes a fourth region of the second conductivity type formed on the first or anode electrode, a fifth region of the first conductivity type formed on the fourth region, and a second electrode formed on the fifth region and connected to the cathode electrode. The separate section includes a sixth region formed on the first electrode and of the first conductivity type, a seventh region formed on the sixth region and of the second conductivity type and an eighth region formed on the seventh region and of the first conductivity type. The resistance values of the semiconductor layers between the cathode electrode and the second electrode and between the cathode electrode and the auxiliary gate electrode are each 2 to 18 Ohms.

BACKGROUND OF THE INVENTION 
The present invention relates to a reverse conducting thyristor and, more 
particularly, to one for reducing the possibility of failure of the 
turn-off and destruction of the thyristor even if the turn-off should 
fail. 
The reverse conducting thyristor includes in integral form a thyristor 
section and a diode section. This type thyristor generally is used for the 
chopper or inverter. This requires good turn-off performance for the 
thyristor. The improvement of the turn-off performance of the thyristor 
alone fails to prevent the failure of the turn-off when it is incorporated 
into a circuit. It is advisable, therefore, that the thyristor per se is 
hard to be destroyed even if the turn-off of the thyristor should fail. 
For a better understanding of the reverse conducting thyristor, explanation 
will be made of a conventional chopper circuit using the reverse 
conducting type thyristors, referring to FIGS. 1 and 2A to 2D. As shown in 
FIG. 1, a main reverse conducting thyristor I is connected through a load 
M between the positive side and negative side of a DC power source. An 
auxiliary reverse conducting thyristor II is connected across the main 
thyristor I through a series circuit consisting of a capacitor C and an 
inductor L. In operation, a gate signal is applied to the gate of the 
thyristor I to make current I.sub.DC flow into the load M. Then, another 
gate signal is applied to the gate of the thyristor II to be conductive 
and to turn-off the main thyristor I. When the current I.sub.DC flows 
through the load M, the capacitor C is charged with the polarity as shown 
in the figure. When the auxiliary thyristor II is turned on, the current 
I.sub.DC and the discharge current from the capacitor C flow in 
superposition into the thyristor section of the main thyristor I during 
the period of time t.sub.o, as shown in FIG. 2A. Then, in the period 
t.sub.p1, the polarity of the capacitor C is reversed so that the 
discharge current from the capacitor C flows into the diode section of the 
thyristor I, thereby turning the thyristor I on. On the other hand, in the 
auxiliary thyristor II, discharge currrent with a pulse width t.sub.p2 
which is dependent on the circuit constant, flows into the thyristor 
section and diode section, as shown in FIG. 2B. The current flowing into 
the diode section of the main thyristor I equals the value that the 
current I.sub.DC is subtracted from the current flowing into the diode 
section of the auxiliary thyristor II. The conduction period t.sub.p1 
thereof is shorter than that t.sub.p2 of the auxiliary thyristor II. The 
main thyristor I has no serious problem, since the current I.sub.DC flows 
to sufficiently spread the conduction area of the thyristor section and at 
this time the discharge current is superposed thereon during the period 
t.sub.0. On the other hand, in the case of the auxiliary thyristor II, the 
current flowing into the thyristor section is of pulse and hence it flows 
concentrated in the vicinity of the gate of the thyristor section. The 
current concentration locally raises the temperature in the vicinity of 
the gate. For this reason, the turn-off time of the auxiliary thyristor II 
must be shorter than that of the main thyristor I. In the main thyristor 
I, when the turn-off time t.sub.p1 of the circuit is shorter than that of 
the element, its turn-off operation fails. In the auxiliary thyristor II, 
when the turn-off time t.sub.p2 of the circuit is shorter than that of the 
element due to the temperature rise in the vicinity of the gate of the 
thyristor section, the turn-off thereof fails. As mentioned referring to 
FIG. 2A, the conduction width t.sub.p1 of the diode section of the main 
thyristor I is narrow. The peak value of the current flowing through the 
diode section of the auxiliary thyristor II is large. Therefore, the 
current reduction rate -di/dt in the diode sections of the main and 
auxiliary thyristors I and II is made small. This means that a large 
amount of charges resides in the diode sections. When the amount of the 
residual charges exceeds a predetermined value, the residual charges 
trigger the thyristor section to turn it on again, resulting in failure of 
the turn-off. FIG. 2C shows a wave form of the current in the main 
thyristor I at the turn-off failure, and FIG. 2D a wave form of the 
auxiliary thyristor II current at the turn-off failure. Particularly, the 
shaded wave forms in these figures appear at the turn-off failure. 
As described above, the chopper circuit design is inherently accompanied by 
variation of the conduction period t.sub.p1 of the current flowing through 
the diode section of the main thyristor and of t.sub.p2 in the auxiliary 
thyristor II, and increment of the current reduction rate -di/dt in the 
diode sections of the main and auxiliary thyristors. 
It is theoretically and practically possible to reduce the possibility of 
failure of the turn-off and destruction of the thyristor per se even if 
the turn-off should fail. 
For avoiding the refiring of the thyristor due to the residual charges in 
the diode section, the countermeasure thus far taken is to provide an 
electric separation or isolation region or to accelerate the spreading 
rate of the thyristor current by using an auxiliary gate, i.e. a pilot 
gate. Such the contermeasures, however, fail to attain desired or 
satisfactory results. 
Accordingly, an object of the present invention is to provide a reverse 
conducting thyristor reducing possibility of the turn-off failure and 
complete destruction of the thryistor even if failure of the turn-off 
should occur. 
SUMMARY OF THE INVENTION 
A reverse conducting thyristor of the invention has in integral form a 
thyristor section, a diode section, and a semiconductor separator section 
for preventing an electrical interference between the thyristor and diode 
sections. The thyristor section includes a first region of a first 
conductive type formed on a first electrode, a second region of a second 
conductivity type formed on the first region, a third region of the first 
conductivity type formed on the second region, a main emitter region of 
the second conductivity type formed in the third region, a cathode 
electrode formed on the main emitter region, an auxiliary emitter region 
formed, with intervention of the exposure layer of the third region, 
facing at least a part of the periphery of the main emitter region which 
does not contact the separator section, an auxiliary gate electrode 
contacting the auxiliary emitter region and enclosing, with intervention 
of the exposure portion of the third region, at least a part of the 
periphery of the main emitter region which does not contact the separator 
section, and a main gate electrode formed on the exposure surface of the 
third region contacting the side wall of the auxiliary emitter region 
which does not face the main emitter region. The diode section includes a 
fourth region of the second conductivity type formed on the first 
electrode, a fifth region of the first conductivity type formed on the 
fourth region, and a second electrode formed on the fifth region and 
connnected to the cathode electrode. The separator section includes a 
sixth region formed on the first electrode and of the first conductivity 
type, a seventh region formed on the sixth region and of the second 
conductivity type and an eighth region formed on the seventh region and of 
the first conductivity type. The resistance values of the semicoductor 
layers between the cathode electrode and the second electrode and between 
the cathode electrode and the auxiliary gate electrode are each 2 to 18 
ohms. 
Other objects and features of the present invention will be apparent from 
the following description taken in connection with the accompanying 
drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made to FIGS. 3A to 3C illustrating a first 
embodiment of a reverse conducting thyristor of the invention. As shown, 
the reverse conducting thyristor comprises a thyristor section 10, a diode 
section 12, and a semiconductor separate section 11 for electrically 
separating the thyristor and diode sections. The thyristor section 10 
comprises a first electrode 12, a first region (P1) 14 of, for example, 
p-conductivity type, a second region (N) 15 of, for example, 
n-conductivity type, and a third region (P2) 16 of, for example, 
p-conductivity type. These regions are successively multilayered on the 
first electrode 13. In the surface region of the third region a main 
emitter region (N.sup.+) 17 is formed with its side walls partly 
contacting the side wall of the separator section 11. An auxiliary emitter 
region (N.sup.+) 18 is embedded in the third region 16 at the opposite 
side to the one contacting the separator section 11. A cathode electrode 
19 is layered over the main emitter region 17. An auxiliary gate electrode 
20 is formed on the parts of the top surface of the auxiliary emitter 
region 18 and the exposure surface of the third region 16 sandwiched 
between the regions 17 and 18. As viewed from top in the drawing, the 
auxiliary gate electrode 20 is placed around a greater part of the 
periphery of the cathode electrode 19, with the exposure portion of the 
third region therebetween, as shown in FIG. 3A. The remaining part of the 
electrode 19 periphery is surrounded by a second electrode 24, with a 
groove 28 intervened therebetween. A main gate electrode 21 is connected 
to a part of the exposure surface of the third region 16 at the outer side 
of the auxiliary emitter region 18. 
The diode section 12 comprises a 4th region (N) 22 which is an extension of 
the second region 15, a 5th region (P2) 23 which is an extension of the 
third region 16, and a second electrode 24 separated from the cathode 
electrode 19 by groove 28. 
The separator region 11 comprises a 6th region (P1) 25 which is an 
extension of the first region 14, a 7th region (N) 26 which is an 
extension of the second region 15, and an 8th region (P2) 27 of an 
extension the third region 16. The separator section 11 contacts at one 
side the thyristor region 10 while at the other side the diode region 12. 
A part of the top exposure surface of the 8th region 27 contacts the 
second electrode 24. 
The groove 28 is formed in the semiconductor layer lying between the 
cathode electrode 19 and the second electrode 24. The semiconductor layer 
has a surface layer resistance R1 of 2 to 18 ohms. Between the cathode 
electrode 19 and the auxiliary gate electrode 20, another groove 29 is 
formed in the semiconductor layer having a surface resistance R2 of 2 to 
18 ohms. The main emitter region 17 has a lot of gaps 30 which are filled 
with the third region 16 of which the tops are exposed contacting the 
cathode electrode 19. That is, the so called shorted emitter is 
constructed in such a way. A part of the third region 16 serves to guide 
displacement current which is produced in the thyristor section and is 
proportional to the differentiation of a forward impression voltage V with 
respect to time t, dV/dt, to the cathode electrode 19. 
The explanation to follow is the operation of the reverse conducting 
thyristor shown in FIGS. 3A to 3C. A forward voltage is first applied 
between the first electrode 13 serving as an anode for the thyristor 
section 10 and the cathode electrode 19, and then a gate voltage is 
applied between the main gate 21 and the cathode 19. Upon the application 
of the gate voltage, the auxiliary thyristor section corresponding to the 
auxiliary emitter region 18 is turned on, with the result that the 
potential of the auxiliary gate electrode 20 rises. With the potential 
rise, the main thyristor corresponding to the main emitter region 17 is 
turned on. In this case, the turn-on area gradually spreads along the 
entire periphery of the main emitter region 17 facing the auxiliary gate 
electrode 20. This, therefore, improves the di/dt endurance of the 
thyristor, where i is thyristor current. From the fact that the turn-on 
area of the thyristor starts to spread from most of the entire periphery 
of the emitter region 17, it will be apparent that the possibility of 
failure of turn-on due to temperature rise in the vicinity of the gate is 
reduced as previously mentioned, and that, even if a thyristor current 
with large di/dt flows after failure of the turn-on, possibility of 
complete destruction of the thyristor device is also reduced. However, the 
resistance value of the semiconductor layer between the cathode electrode 
19 and the auxiliary gate electrode 20 must be properly selected. When the 
resistance value is too small, current flowing from the auxiliary gate 
electrode 20 to the cathode electrode 19 is large to possibly melt the 
terminal of the cathode electrode. Conversely, when the resistance value 
is too large, the current is small, resulting in deterioration of current 
injection efficiency, and thus the di/dt endurance. For example, in the 
case of the thyristor with 2,500 V forward voltage, if the resistance 
value becomes below 1 ohm, the di/dt endurance suddenly reduces to be 
below 100 A/.mu.s; if it is above 18 ohms, the di/dt endurance becomes 
below 200 A/.mu.s. It is for this reason that, in the reverse conducting 
thyristor of the invention, the resistance value is restricted to be 2 to 
9 ohms. 
The dV/dt endurance must also be set up within a predetermined range. The 
resistance value R.sub.2 of the semiconductor layer between the auxiliary 
gate electrode 20 and the cathode electrode 19 is given by the equation 
(1) 
EQU V&lt;C.times.S.times.dV/dt.times.R.sub.(RG-K) (1) 
where V is a threshold voltage (about 0.5 V) of the PN junction between the 
second region 15 and the third region 16, C a junction capacitor of the PN 
junction, S an area of the semiconductor layer laid outside the cathode 
electrode 19 in the thyristor section 10, and I the whole displacement 
current produced in the same semiconductor region. The equation (1) may be 
rewritten 
EQU dV/dt&lt;(V/CS).times.1/R.sub.2 (2 ) 
The equation (2) shows that the dV/dt endurance is inversely proportional 
to R.sub.2. Accordingly, in the above-mentioned thyristor, if the R.sub.2 
is above 18 Ohms, the dV/dt endurance is below 100 V/.mu.s. Therefore, it 
is very difficult to design chopper circuits by using such the thyristor. 
FIG. 7 shows the interrelationship of the resistance R.sub.2 versus the 
endurances dV/dt and di/dt. 
Let us now consider the diode section 12. As recalled, the current flowing 
into the diode section is of pulses with narrow width and high peak value. 
See FIGS. 2A and 2B. When such current flows into the diode section 12, a 
comparatively large amount of residual charges remains in the diode 
section 12 at the turn-off of the current. In the case of much of residual 
charges, such charges flow into the thyristor section 10 through the 
separator section 11, as described previously. This causes the thyristor 
section to be turned on again, resulting in failure of the turn-off. In 
this case, the failure of the turn-off may be avoidable by increasing the 
thickness .DELTA.x of the separation section 11, although the residual 
charges tend to flow along the top surface of the separation section into 
the main emitter section 17, thus possibly giving rise to the turn-on. At 
this point, it is to be noted that the thyristor device of the invention 
has the restrictive value 2 to 18 ohms of the resistance R.sub.1 between 
the second electrode 24 and the cathode electrode 19. This resistance 
R.sub.1 suppresses occurrence of the failure of the turn-off. FIG. 7 also 
shows the interrelationship of the resistance R.sub.1 to the endurances 
dv/dt and di/dt. 
From the foregoing description, it will be understood that the thyristor 
devices of the invention successfully prevent the localized temperature 
rise in the thyristor section and refiring of the thyristors due to the 
residual charges in the diode section. Accordingly, possibility of 
occurrence of the failure of the turn-off is reduced and, if the failure 
should occur, the destruction of the devices is minimized, as compared to 
the conventional ones. 
A modification of the embodiment of FIG. 3 of the invention is shown in 
FIGS. 4A and 4B. In this example, like reference numerals are used to 
designate like portions in FIGS. 3A to 3C. As shown, the main emitter 
region 17 is entirely surrounded by the auxiliary gate electrode 20, with 
intervention of a part of the third region 16. 2 to 18 ohms resistance 
value is employed for the resistance values R.sub.1 and R.sub.2 of the 
semiconductor layers laying between the cathode electrode 19 and the 
second electrode 24 and between the cathode electrode 19 and the auxiliary 
gate electrode 20. This example with the same effects as the FIG. 3 one is 
effective in application of it for the case of narrow conduction width of 
the thyristor section. 
Another modification shown in FIGS. 5A and 5B employs a ring diode, and is 
effective when it is applied for the thyristor device of which the 
conduction pulse in the thyristor section has high peak value. The 
operation and effects of this example are equivalent to those of the first 
example. Like portions are designated by like reference numerals in the 
first example. No detailed description of the example will be necessary 
for those skilled in the art. 
FIG. 6 shows still another embodiment of the reverse conducting thyristor 
of the invention. The difference between this embodiment and the FIG. 3 
embodiment is the use of a field initiate (FI) gate electrode 32 separated 
from the cathode electrode 19, by which the main emitter region 17 is 
surrounded, as shown in the figure. 2 to 18 ohms may be used for the 
resistance values R.sub.1 and R.sub.2 of the semiconductor layers between 
the cathode electrode 19 and the auxiliary gate electrode 20 and between 
the cathode electrode 19 and the second electrode 24. The operation and 
effects of this example are equivalent to those of the first example of 
FIG. 3. Details of further description of the example will be omitted.