Integrated circuit with high output current I.sup.2 L transistor

The invention relates to an integrated circuit having a transistor suitable for integrated injection logic (I.sup.2 L) with a single collector output region and having at least one base contact disposed between the collector output region (C.sub.60, C'.sub.60) and the injector (IN.sub.6), the surface of the collector output region being several times larger than that of a logic gate of the I.sup.2 L multi-collector type. The base (B.sub.60) has at least two rows of interconnected contacts: a first row (CB.sub.60, CB.sub.61, CB.sub.62) constituting the base contact disposed between the collector and the injector, and at least a second row (CB.sub.63, CB.sub.64, CB.sub.65) situated at the perimeter of the collector (C.sub.60,C'.sub.60), which can consist of one or several parts. The injector (IN.sub.6) may also have a row of interconnected contacts (CIN.sub.1, CIN.sub.2, CIN.sub.3).

BACKGROUND OF THE INVENTION 
The invention relates to an integrated circuit comprising a multi-collector 
I.sup.2 L gate and a transistor for internal injection logic (I.sup.2 L) 
with a single collector output region and at least one base contact 
arranged between the collector output region and an injector, the surface 
area of the collector output region being several times larger than that 
of a collector of a logic multi-collector I.sup.2 L gate. 
Transistors of the above type capable of operating with a comparatively 
high output current are frequently used, especially for being mounted in 
cascade arrangement at the output of a logic I.sup.2 L gate so that it 
constitutes an interface circuit with other elements. 
In such transistors of high current level, the voltage drop in the base is 
larger than in a logic I.sup.2 L gate, which results in that the output 
current is considerably smaller than could be expected from the ratio 
between the surface area of the single collector output of the transistor 
and that of a collector of the logic gate. 
Prior art U.S. Pat. Nos. 4,512,075 and 4,322,882 disclose I.sup.2 L 
transistors, in which the base resistance is reduced due to the presence 
of a base contact extending throughout the surface of the base. However it 
has been found that such a provision has the disadvantage of increasing 
the phenomena of recombination in the base and of resulting in a reduction 
of the gain, which is particularly unfavourable in the case of transistors 
operating at a high current and hence having larger dimensions. Also see 
U.S. Pat. Nos. 4,075,039 and 4,546,539. 
SUMMARY OF THE INVENTION 
The invention has for its object to provide an integrated circuit which 
comprises an I.sup.2 L transistor with high output current in which the 
voltage drop in the base is reduced, while maintaining a high gain. Such a 
transistor is especially suitable for arrangement in cascade with the 
I.sup.2 L-gate to form a stage. The basic idea of the invention resides in 
that, surprisingly, only local base contacts are obtained. According to 
the invention, the integrated circuit is characterized in that the base of 
the I.sup.2 L transistor has at least two rows of interconnected contacts, 
a first row constituting the said base contact arranged between the 
collector output region and the injector and at least a second row being 
situated at the perimeter of the collector output region. 
According to a first embodiment corresponding to a technology according to 
which an interconnection can be formed above the collector output region 
and can be separated therefrom by an insulator, the collector output 
region consists of one part and has at least one row of inclusions, 
through which the base adjoins the surface, while the contacts of the 
second row of contacts are arranged on the said row of inclusions. 
According to a second embodiment, the collector output region is divided 
into at least two interconnected parts and advantageously into two parts 
and at least a said second row of base contacts is arranged between the 
said parts. 
According to a preferred variation of this embodiment, the collector output 
region consists of two parts and the surface area of the part closest to 
the injector is twice that of the other part. 
According to a preferred embodiment, the injector has a row of injector 
contacts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, an I.sup.2 L stage of an integrated circuit according 
to the prior art comprises two I.sup.2 L transistors T.sub.1 and T.sub.2 
connected in cascade arrangement at the output of one of the collectors 
C.sub.1 of an I.sup.2 L gate P which constitutes an interface circuit with 
one or several other elements. A gate or a transistor of the I.sup.2 L 
type comprises an injection transistor, generally a PNP transistor, and a 
control transistor of the opposite type. The I.sup.2 L gate P comprises, 
for example, four collectors C.sub.1 . . . C.sub.4, on which are formed 
collector contacts (not shown), a base B and an injector I of great 
length, which is common to the I.sup.2 L transistors T.sub.1 and T.sub.2 
and a base contact CB between the collector C.sub.1 and the injector I. 
The transistor T.sub.1 has a base B.sub.10 having an outer contour similar 
to the base of the gate P, a collector C.sub.10 of elongate rectangular 
form occupying a large part of the length of the base and a base contact 
CB.sub.10 arranged between the collector C.sub.10 and the injector. The 
collector C.sub.1 and the base B.sub.10 are interconnected through a 
connection 1 indicated by a dotted line and extending from the contact of 
the collector C.sub.1 to the base contact CB.sub.10. The transistor 
T.sub.2 has a base B.sub.20 having an outer contour of substantially 
square form, of which one side adjoins the base B.sub.10 and of which 
another side adjoins the injector IN. The collector C.sub.20 of the 
transistor T.sub.2 can have a substantially square form and its contour 
occupies the major part of that of the base B.sub.20. A base contact 
CB.sub.20 of elongate rectangular form adjoins one side of the collector 
C.sub.20 and is arranged between the latter and the injector IN. 
Let it be assumed that I is the output current of each of the collectors 
having a surface area S of the gate P. 
The transistor T.sub.1, whose collector C.sub.10 has a surface area 
S.sub.1, supplies an output current I.sub.1 having, in the case in which 
it would be considered as an ideal transistor having a base resistance 
zero and not exhibiting a recombination in the base part not covered by 
the collector, a value: 
##EQU1## 
In the same manner, the transistor T.sub.2, whose collector C.sub.20 has a 
surface area S.sub.2, would have an output current I.sub.2 having a value: 
##EQU2## 
The length L of the injector part IN adjoining the transistor T.sub.2 must 
on the other hand be such that the base current of T.sub.3 can be absorbed 
by the gate P, i.e.: 
##EQU3## 
1 designating the injector part adjoining the control transistor of gate 
P. 
In practice, the base resistance in the transistors T.sub.1 and T.sub.2 is 
higher than in the gate P and this results in that the output current 
I.sub.2 is considerably smaller than the value given above. 
According to a first embodiment of the invention shown in FIG. 2, a 
transistor T.sub.3 has two rows of base contacts. The collector output 
region consists of a collector C.sub.30. A first row is constituted by the 
contacts CB.sub.30 and CB.sub.31 arranged between the collector C.sub.30 
and the injector IN.sub.3 and parallel to the latter and partly included 
in the collector C.sub.30 in the open inclusions 10 and 11 of the base 
B.sub.30. A second row is constituted by the contacts CB.sub.32 and 
CB.sub.33 disposed in closed inclusions 12 and 13 of the base B.sub.30. 
The dimension of the base contacts CB.sub.30 . . . CB.sub.33 is 
advantageously the smallest permitted by the technology. In fact, the 
recombinations are considerably larger under the contact connection 
region, which results in that, when reducing the dimension of the base 
contacts, the gain of the transistor T.sub.3 is improved. 
FIGS. 3a, 3b and 4 show the second embodiment of the invention, in which 
the collector output region of the transistor is divided into two parts, 
between which a row of base contacts is interposed. 
According to FIG. 3a, a transistor T.sub.4 corresponding to the 
configuration of the transistor T.sub.1 of FIG. 1, comprises an injector 
IN.sub.4 and a collector output region composed of two interconnected 
parts, i.e. one on the side of the injector denoted by C.sub.40 and the 
other on the opposite side denoted by C'.sub.40, and both being inscribed 
in the contour of the base B.sub.40. The regions C.sub.40 and C'.sub.40 
have a rectangular elongate contour and preferably the region C.sub.40 has 
a surface area twice that of the region C'.sub.40. 
FIG. 3b shows a transistor T.sub.4 corresponding to the configuration of 
the transistor T.sub.2 of FIG. 1. Its constitutive elements, i.e. the 
injector IN.sub.5, the collector output region of two interconnected parts 
C.sub.50 and C'.sub.50, the base B.sub.50, the first row of base contacts 
CB.sub.50 and CB.sub.51 and the second row of base contacts CB.sub.52 and 
CB.sub.53, are disposed in the same manner as the elements denoted by 
IN.sub.4, C.sub.40, C'.sub.40, B.sub.40, CB.sub.40, CB.sub.41, CB.sub.42 
and CB.sub.43, respectively. On the contrary, it is distinguished from the 
transistor T.sub.3 by the form of its elements corresponding to the 
greatest length of the injector and to the largest surface area of the two 
parts of the collector output region (C.sub.50, C'.sub.50). 
According to FIG. 4, the injector IN.sub.6 also has a row of interconnected 
contacts CIN.sub.1, CIN.sub.2 and CIN.sub.3. The base B.sub.60 has two 
rows of interconnected contacts, the first row comprising the base 
contacts CB.sub.60, CB.sub.61 and CB.sub.62 and the second row comprising 
the base contacts CB.sub.63, CB.sub.64 and CB.sub.65. The collector output 
region is divided into two interconnected parts C.sub.60 and C'.sub.60, 
the contacts of the second row being disposed between the two parts 
C.sub.60 and C'.sub.60. 
When one continuous injector contact is replaced by several interconnected 
contacts, the gain factor is improved and hence it can be ensured that the 
same performances for a lower current consumption is obtained. 
In connection with FIGS. 4 to 6, it will now be explained how the number 
and the spacing of the base contacts and injector contacts of one row are 
determined. 
As to the base contacts, two conditions are to be fulfilled. 
A first condition relates to the blocking state in which a considerable 
transitory base current exists and for which the behavior is fairly 
complex. 
The current whose current lines are represented in full lines in FIG. 5 is 
very close to the injection current and is a permanent current. The 
current represented by dotted lines is a transitory current whose value 
may be higher than, equal to or lower than that of the preceding current, 
in accordance with the gain G of the control transistor. For a gain G of 
3, it is at most equal to twice that for a structure of two rows of base 
contacts. Let it be assumed that L.sub.b is the pitch of the base contacts 
and l.sub.b is the length of the base contacts. 
For a permitted voltage difference of .DELTA.V.sub.b, there is 
approximately: 
##EQU4## 
expressed in mV. 
This condition applies to all the rows of base contacts. .DELTA.V.sub.b has 
to be chosen to be smaller than the voltage drop across the base-emitter 
path of the transistor during the blocking state and to the voltage drop 
across the collector-emitter path of the controlled transistor. Let it be 
assumed that for G=3, .DELTA.V.sub.b =100 mV and Li-li=10 .mu.m (see 
below), then it follows that (L.sub.b -l.sub.b) is approximately 60 .mu.m. 
The second condition relates to the conductive state and the problem is 
static. The voltage drop in the base must not exceed about 10 mV for a 
conduction current I. This second condition is much more restrictive than 
the first condition. 
The optimization of the structure consists in the highest gain I.sup.2 L is 
obtained, i.e. a maximum output current I.sub.OUT. There is: 
##EQU5## 
S: overall surface area of the base diminished by the surface area of the 
contacts; 
V(ds): polarization voltage of the surface element dS in the emitter-base 
junction, and 
V(ds)=V.sub.CT -.differential.r I(1) dl, 
integral being taken along a current line (see FIG. 4); 
V.sub.CT =base contact voltage. 
The diminution of the surface area of the base contacts is favorable in 
that it leads to an increase of I.sub.OUT on the one hand by reduction of 
the recombination under the metallic contacts and on the other hand 
because this reduction in surface area can be utilized to increase the 
surface area of the collector. 
According to FIG. 6, the length li of an injector contact is chosen so that 
it is the minimum surface area permitted by the technology. The pitch Li 
of the injector contacts must be chosen so that the injection current is 
substantially uniform along the injector. 
Let it be assumed that j is the line density of the current along the 
injector. It is admitted that the current gain of the injection transistor 
(in this case a PNP transistor) is equal to unity. The calculation is 
effected for an injection on either side of the injector. 
Let it be assumed that .DELTA.V.sub.i =V.sub.a =V.sub.b is the maximum 
voltage difference admitted to obtain a density of injection current 
substantially uniform along the injector(b: point situated in the central 
plane between two injectors; a: point situated in the central plane of an 
injector). 
For the sake of simplicity, it is also supposed that the current lines are 
parallel to the injector between the two contacts and perpendicular to 
that at the level of these contacts. 
It results in that Va=Va'. 
With a': point situated in the outer plane of an injector contact. Hence: 
##EQU6## 
current injected between a' and b 
##EQU7## 
resistance of the injector between a' and b 1/2: corresponds to a linear 
decrease in the resistance of the injector. 
W: width of the injector. 
j: density of the injection current 
.rho.: sheet resistance of the injector 
##EQU8## 
With j=1 .mu.A/.mu.m; l.sub.i =4 .mu.m; W=5 .mu.m; .rho.=5000.OMEGA. per 
square and .DELTA.V=9 .mu.mV, this leads to (Li-li) being approximately 10 
.mu.m.