Integrated Power Amplifier

My present invention relates to an integrated power amplifier composed of a multiplicity of elemental transistors which are epitaxially grown on a semiconductor chip.

Conventional techniques of selective doping, metal deposition and photoengraving enable the mass production of such modular semiconductor units with active areas of different types and degrees of conductivity located between inactive regions along the chip surface. Emitters, bases and collectors of elemental transistors can thus be formed on the intrinsically conductive substrate and can be interlinked in part by conductor zones of predetermined resistivity (substantially less than that of the intrinsic regions) and in part by superposed metallic strips. Such strips may be used to connect respective amplifier terminals, common to all the elemental transistors, in parallel to a set of emitter, base and collector electrodes overlying the corresponding active areas of each of these transistors.

In a power amplifier of this type there are several desiderata to be satisfied, including particularly:

A. HIGH GAIN, ALSO WITH LARGE OPERATING CURRENTS;

B. LOW SATURATION VOLTAGE;

C. ABSENCE OF SECONDARY BREAKDOWNS; AND

D. COMPACT STRUCTURE.

Requirement (a) is satisfied by making the emitter area of each elemental transistor as large as possible and uniformly distributing the current density in that area at all current levels. The satisfaction of requirement (b) depends essentially on the series resistance of the collector and may be achieved by various measures such as the interleaving of the collector and emitter areas to minimize the current path therebetween. Requirement (c) can be fulfilled by a balancing of the individual emitter currents with the aid of suitably dimensioned emitter resistors formed by certain zones of the chip. Requirement (d), finally, demands a well-planned layout and optimum activation of the electrode-supporting areas of the transistor array.

In order to obtain a large emitter area with uniform current distribution, it is known to divide the transistor array into several parallel rows and to dispose a metallic emitter lead along the centerline of each row between symmetrical sets of emitter areas. A pair of such areas form part of each elemental transistor and are linked with that emitter lead by narrow conductive zones perpendicular to that lead, these zones constituting the aforementioned emitter resistors. Such a layout is satisfactory from a viewpoint of high gain at elevated current levels and, particularly with several rows of this description connected in parallel between the base and emitter terminals, also affords good utilization of the available chip surface. Within a planar array of this type, however, imbalances still exist among the elemental transistors of a row as well as among the several rows; as a result, there is no optimum utilization of the available surface area. Moreover, especially at high current levels, these imbalances may lead to local overheating entailing the risk of secondary breakdowns.

The object of my present invention, therefore, is to provide an improved integrated power amplifier of the type referred to which satisfies all the aforestated desiderata and obviates the drawbacks just mentioned.

In accordance with my present invention, the resistive zones linking the emitters of the elemental transistors of any row with a common metallic strip, serving as the emitter lead for that row, have resistances which vary progressively along the row for substantially equalizing the current distribution among the elemental transistors of each row. More particularly, these resistances should progressively increase from the first transistor of the row, i.e. the one whose base is electrically closest to the base terminal of the amplifier, to the last transistor whose emitter is electrically closest to the common emitter terminal; the term "electrically closest", of course, means the least amount of resistance between the electrode and the terminal involved.

The above and other features of my invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a diagram showing the layout of part of a conventional power amplifier of the general type here considered;

FIG. 2 shows the equivalent circuit for the amplifier section illustrated in FIG. 1;

FIG. 3 shows the layout of a representative portion of a power amplifier embodying my present improvement;

FIG. 4 is a diagram comparable to that of FIG. 1, showing a section of the layout of FIG. 3 drawn to a larger scale; and

FIG. 5 shows the equivalent circuit for the amplifier portion included in the layout of FIG. 3.

FIG. 1 shows part of a row of elemental transistors in a planar array of such transistors constituting a conventional power amplifier of the integrated-circuit type discussed above. Each elemental transistor of this amplifier comprises two lateral active emitter areas 1 constituted by a serrated layer of one conductivity type epitaxially grown on a base layer 6 of the opposite conductivity type, other layers 7 of the first conductivity type forming associated collectors. (Usually, in such transistor structures, the collector layers extend underneath the base layers; see, for example, U.S. Pat. No. 3,619,741.)A central zone 2 of emitter material is linked with the active zones 1 via narrow rectangular zones 3 of limited conductivity flanked by highly resistive regions 8 of intrinsic material. The central zone 2 is overlain by a metallic strip 4 extending to a common emitter terminal indicated at E in FIG. 2. Electrodes 5 on base layer 6 are similarly connected to a common base terminal B (FIG. 2) via conductor strips that have not been illustrated. There is also a collector terminal C, likewise shown in FIG. 2, connected to zones 7 in an analogous manner.

The rectangular linking zones 3 throughout the array of elemental transistors are all of the same length A and width D so as to represent identical emitterresistances RE; FIG. 2 shows two elemental transistors T, T' each with a pair of such emitter resistances RE, RE' constituted by the two transverse zones 3 flanking the central zone 2. The bases of transisitors T and T' are separated by a resistor RB representing the resistance of the sections of metallic strip (not shown) which interconnect the base electrodes 5 of successive elemental transistors. Base resistance RB is small in comparison with the mutually identical emitter resistances RE and RE'. The central emiter zone 2, which is in direct contact with strip 4 and is therefore connected to terminal E without intervening resistance RE, has not been illustrated in the diagram of FIG. 2 and with its underlying base layer 6 may be considered part of a separate elemental transistor.

The metallic strip 4 interconnecting the emitters of the several transistors T, T' of a row also forms a resistance RM between consecutive transistors, i.e. between the junctions of their transverse zones 3 with the central zone 2. In order to minimize the effect of this series resistance RM upon the current distribution within the transistor array, it has heretofore been the practice to make the length A of the connecting zones 3 much larger than their width D, thereby greatly increasing the resistance ratio RE/RM. Such a dimensioning, however, results in a wide separation of the two active emitter zones 1 on opposite sides of central zone 2 and in incomplete utilization of the available surface area, in view of the presence of relatively large regions 8 or intrinsic conductivity.

Reference will now be made to FIGS. 3 - 5 representing an improved power amplifier according to my invention, it being understood that the equivalent circuit of FIG. 2 applies also to this improvement.

FIG. 3 shows as substantial part of a semiconductor chip with an array of elemental transistors 13 divided into three parallel rows 10, 11 and 12 of six, six and four transistors, respectively. FIG. 3 also shows three resistive zones R.sub.1, R.sub.2, R.sub.3, formed in the usual manner by the diffusion of impurities into the substrate, through which the bases of the uppermost transistors of these rows are connected to a common terminal B not shown in FIG. 3 but illustrated in FIG. 5.

As will be more clearly apparent from the detailed showing of FIG. 4, each elemental transistor 13 comprises two active emitter zones 17 deposited on base layers 16 between collector zones 15. The active lateral emitter zones 17 of each elemental transistor communicate with a central zone 18, carrying a metallic emitter electrode 20, via resistive zones 19 (also formed by the controlled diffusion of impurities) which are generally similar to the zones 3 of FIG. 1 but are of considerably reduced length. The emitter electrodes 20 of a row are interconnected by an overlying metallic strip 22 extending along the centerline of the row. It will be noted that adjacent transistors 13, 13' are separated from each other by generally H-shaped intrinsic regions 14 occupying substantially less space than the two intrinsic areas 8 present between adjoining transistors in the conventional layout of FIG. 1.

In accordance with an important feature of my invention, the width D of the transverse zones 19, 19' of adjoining transistors 13, 13' decreases progressively from one end of the row to the other, i.e. from left to right in FIGS. 4 and 5. This narrowing of the transverse connecting zones represents a progressive increase of the emitter resistances represented thereby. The same progressive decrease in the width D can be observed, from the top down, in each of the rows 10- 12 of the array shown in FIG. 3. Thus, the lowest emitter resistances are provided at the transistors proximal to respective base of input resistors R.sub.1 - R.sub.3 of the three rows. Conversely, the highest emitter resistances are tied to the transistors proximal to the common terminal E (FIG. 5) which lie at the bottom of each row in FIG. 3.

In the equivalent-circuit diagram of FIG. 5, the four transistors of row 12 have been designated T.sub.1 - T.sub.4, those of row 11 are labeled T.sub.5 - T.sub.10 and those of row 10 have been marked T.sub.11 - T.sub.16. Resistors R.sub.1, R.sub.2 and R.sub.3 lie between the common base terminal B and the bases of transistors T.sub.1, T.sub.5 and T.sub.11, respectively; they are traversed by input currents IB.sub.1, IB.sub.2 and IB.sub.3. Consecutive transistors of rows 12, 11 and 10 have their bases separated by resistors RB.sub.2 - RB.sub.4, RB.sub.6 - RB.sub.10 and RB.sub.12 - RB.sub.16 whose significance has been explained in connection with resistor RB of FIG. 2. Each elemental transistor has a collector tied to common terminal C, in a manner not relevant to my present improvement, and has two emitters with series resistors RE.sub.1 - RE.sub.16 formed by the aforementioned zones 19. The intervening sections of strips 22 (FIG. 4) are represented in FIG. 5 by series resistors RM.sub.1 - RM.sub.16 ; emitter terminal E is connected to resistor RM.sub.10 through another resistor R'.sub.2 and to resistor RM.sub.16 through the series combination of this latter resistor with a further resistor R'.sub.3 (which could be combined with resistor RM.sub.16).

Transistor rows 12, 11 and 10 have output currents I.sub.1, I.sub.2 and I.sub.3 respectively traversing resistors RM.sub.4, RM.sub.10 + R'.sub.2 and RM.sub.16 + R'.sub.3 + R'.sub.2 downstream of the last transistors T.sub.4, T.sub.10 and T.sub.16 of these rows. Each of these output currents is composed of the individual emitter currents passing through the resistance pairs RE.sub.1 - RE.sub.4 , RE.sub.5 - RE.sub.10 and RE.sub.11 - RE.sub.16 of the corresponding groups of elemental transistors.

Uniformization of the current distribution requires that the input or base currents IB.sub.1 - IB.sub.3 have relative magnitudes proportional to the numbers of parallel transistors in their respective rows. In the illustrated array, therefore

furthermore, the sums of the voltage drops in the input and the output of each row should be the same; thus:

given a gain G for a chosen emitter current, the values of base currents IB.sub.1 - IB.sub.3 can be determined; with due consideration of thermal and electrical stability, one of the three input resistances R.sub.1 - R.sub.3 can be freely selected. With resistances RM.sub.4, RM.sub.10 and RM.sub.16 fixed by the spacing of the emitter electrodes 20 and the dimensions of conductor strip 22 (FIG. 4), the two other input resistances can be derived from equations (1) and (2).

Let Vbe.sub.n and Vbe.sub.n+1 be the base/emitter voltages of an upstream and a downstream elemental transistor immediately adjoining each other in a row, with RE.sub.n and RE.sub.n+1 denoting the corresponding emitter resistances, RM.sub.n being the intervening strip resistance, IE.sub.n and IE.sub.n+1 being the emitter currents and RB.sub.n+1 being the corresponding interbase resistance traversed by an interval base current IB.sub.n+1. Kirchhoff's law then requires that

With RB.sub.n+1 negligible in comparison with RE.sub.n and Re.sub.n+1 , and with identical transistors so that Vbe.sub.n = Vbe.sub.n+1 , equation (3) simplifies to

Equation (4) means that the different between the voltage drops across consecutive emitter resistances RE.sub.n, RE.sub.n+1 (the latter corresponding to RE' in FIG. 2) equals twice the voltage drop across the intervening strip resistance RM.sub.n, the factor of 2 being of course due to the presence of two emitter resistances on opposite sides of strip 22 in each elemental transistor 13.

Finally, in order to establish the desired current-distribution pattern one should observe the relationship ##EQU1## where Z.sub.n and Z.sub.n+1 are the areas of the active emitter zones of two consecutive elemental transistors.

From the gain G and a given value for the overall emitter current on terminal E, in combination with the known emitter areas Z.sub.n and Z.sub.n+1, it is possible to obtain particular values for all the variables of equations (4) and (5) except emitter resistances RE.sub.n, RE.sub.n+1. Selection of one such emitter resistance, again with due consideration of thermal and electrical stability as well as the physical characteristics of the elemental transistors, then provides the remaining values; care should be taken in that selection to avoid overloading of the central zones 18 in comparison with the other emitter areas and to prevent crowding, with maintenance of a suitable current gradient in the active emitter zones.

Naturally, the geometrical configuration of the transistor array may be varied and other modifications may be introduced in the disclosed modular unit without departing from the spirit and scope of my invention. Thus, for example, the elemental transistors have been indicated as being of the NPN type, yet analogous principles apply to PNP transistors.