This invention relates to an integrated semiconductor device. More particularly, the invention relates both to an integrated bipolar power transistor and a class B output stage.
As is known, it is common practice with integrated power transistors to divide the emitter area into plural regions and accommodate the latter in the base areas, as suitably subdivided to obtain high output currents. Of course, the collector regions extend parallel to one another and are separated from one another by an area including the base area and emitter area, thus forming elementary transistors which extend adjacently to one another. In practice, interleaved emitter and collector contact areas are obtained which form, therefore, a typical geometry called "interdigited geometry".
As for the class B output stages, this geometry is repeated for each of the two power transistors making up the stage, so as to obtain two such interdigited structures, generally disposed adjacent to each other.
Structures of this kind afford a definitely improvement in high current gain over other structures, but are affected by the problem of the direct secondary breakdown (I.sub.s/b).
As is known, the direct secondary breakdown is the main cause for failure of power transistors and is due to voltage disuniformity on the junctions and to temperature disuniformity in the different areas of the transistor (see the article "La rottura secondaria nei circuiti integrati di potenza" by F. Villa, Elettronica e telecomunicazioni, No. 3, 1984).
In particular, the main obstacle to the achievement of better direct secondary breakdown strength concerns the electrothermal interaction between the different power dissipating areas of the transistors.
Several solutions have been proposed already for improving the direct secondary breakdown strength of transistors. In particular a solution consists of utilizing resistors, called ballast resistors, in series with the emitter of each elementary transistor to introduce a negative feedback which stabilizes its behavior.
Another known solution (see English Pat. No. 1 467 612) consists of replacing each elementary transistor, e.g. of the NPN type, with a pair of NPN transistors of one of which forms the drive transistor and the other forms the output transistor, cascode connected to each other or in the Darlington configuration and being disposed geometrically such that the output transistor is coupled thermally with the drive transistor of another pair instead of with that of its own pair so as to obtain compensation for the thermal unbalances.
A definitely more substantial improvement is achieved with the solution indicated in Italian patent application No. 21028 A/84 by this same assignee. According to that solution, the bases of the elementary transistors forming the power transistor or every power transistor in the output stage, instead of being shorted together, are independent of one another and each of them is controlled by means of a respective current source, constructed with a PNP type bipolar transistor the connector whereof is connected to the aforesaid bases. This may be accomplished without any wasted area by subdividing the collector of the PNP drive transistor of the near complementary symmetry output stage and thus forming a multi-collector PNP structure. That structure is shown in FIGS. 1 and 2 for an integrated bipolar power transistor, and in FIG. 6 for a class B output stage.
By virtue of the solution known from the cited application, those regenerative phenomena of the electrothermal type which arise in a traditional power structure are decreased in that the collector current variations of each elementary transistor as a function of temperature are now dependent solely on the variation of the individual gains with the temperature itself. That variation is of about 0.5%/.degree.C., and therefore, well below the temperature variation of the I.sub.c (V.sub.BE) which is equal, in the instance of small temperature ranges, to 8%/.degree.C.
However, the known solutions only permit a partial reduction of direct secondary breakdown and are not always devoid of disadvantages. For example, in the instance of ballast resistors being utilized, there occurs an increase in the saturation voltage of the power transistor.