High current Darlington amplifier

The invention teaches the placement of a plurality of conducting paths in the vicinity of the base of the final stage of a Darlington configured transistor group. When an abrupt change in carrier density occurs, such as during switching intervals, the added conduction paths facilitate the removal of carriers from the base region, thereby facilitating the turn-off process. The extra conductive paths allow a more even current density within the volume of the base during changes in carrier density.

This invention is also described in Application No. 62-310505 dated Dec. 8, 
1987, filed in the Patent Office of the Japanese Government. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improvement in the internal wiring of 
semiconductors in a Darlington amplifier configuration and more 
particularly, to a Darlington amplifier configuration in which speed-up 
diodes are connected between the base of the front-stage transistor(s) and 
the base of the rear-stage transistor(s) to allow for better current 
distribution in the base region of the rear-stage transistor(s) during 
periods of abrupt changes in device current density. 
2. Description of the Related Art 
To make a large capacity transistor, a plurality of individual transistors 
are connected to form a Darlington configuration. This configuration 
typically consists of rear stage transistor(s) and front stage 
transistor(s). One advantage of the Darlington configuration is that the 
gain of the combination of the two stages is the product of the gain of 
each of the multiple transistors comprised within the stages. Furthermore, 
each of the transistors within the stages can be optimized for power 
handling capability, gain, frequency response and/or other parameters of 
interest. 
In typical prior art examples, in a multiple transistor configuration, the 
base of the rear-stage transistor of the Darlington circuit is connected 
with the base of the front-stage transistor via a speed-up diode to 
shorten the turn-off time. Equivalent circuits of such Darlington 
configurations are shown in FIGS. 2(a) and 2(b). 
A Darlington amplifier consisting of two transistors is shown in FIG. 2(a). 
Here a speed-up diode 21 has its cathode connected to the base of NPN 
transistor 11. This diode 21 is connected in a forward direction between 
the base of the rear-stage, or output stage, NPN transistor 12 and the 
base of the front-stage, or input stage, NPN transistor 11. 
A Darlington amplifier comprising three transistors is shown in FIG. 2(b), 
where a first speed-up diode 21 is connected in a forward direction 
between the base of the second-stage NPN transistor 12 and the base of the 
first-stage NPN transistor 11. A second speed-up diode 22 is connected in 
a forward direction between the base of the final-stage, or output-stage, 
NPN transistor 13 and the base of the front-stage transistor 11. 
FIGS. 3(a) and 3(b) show chip level electrical interconnections of the 
Darlington configurations which are shown schematically in FIG. 2. The 
interconnections shown in FIG. 3 are bonded to the semiconductor material 
constituting the Darlington structure. 
Referring to FIG. 3(a), in the Darlington amplifier configuration 
comprising the two transistors, a base terminal B is connected to the base 
32 of the rear-stage transistor via speed-up diode 21 and aluminum leads 
4. Base 32 also acts as the emitter of the front-stage transistor and is 
connected to base 31 of the front-stage transistor and the speed-up diode 
21 by aluminum leads 4. Bases 31 and 32 are both positioned on 
semiconductor chip 1. An emitter terminal E is connected to the plural 
emitters 33 of the rear-stage transistor by aluminum leads 4. 
Referring to FIG. 3(b), in the Darlington amplifier comprising three 
transistors, a base terminal B is connected to the base 32 of the 
second-stage transistor serving also as the emitter of the front-stage 
transistor, via the base 31 of the front-stage transistor and the speed-up 
diode 21 with aluminum leads 4. Also, the base terminal B is connected to 
the base 34 of the final-stage transistor via the speed-up diode 22 and 
aluminum lead 4, acting also as the emitter of the second-stage 
transistor. An emitter terminal E is connected to the plural emitters 35 
of the final-stage, or output stage, transistor by aluminum leads 4. 
In the prior art circuit configurations, when a reverse bias is applied 
between the base and the emitter of each Darlington amplifier to turn it 
off, carriers are not drained uniformly from the vicinities of a bonding 
pad 51 for the base 32 of the rear-stage transistor shown in FIG. 3(a) and 
from a portion away from the pad, for example the portion 52. This portion 
52, in which carriers concentrate, may not be rapidly turned off but 
rather keeps conducting, forcing a transitional high carrier current 
density in the portion 52 of the device during transition periods in 
operating states. The high current density associated with the on to off 
transition creates heat, which in turn may destroy the device. 
A similar situation occurs between a bonding pad 51 for the base 33 of the 
final-stage, or output stage, transistor of the three-stage Darlington 
amplifier shown in FIG. 3(b), and a relatively remote portion 52 opposite 
pad 51. 
It is an object of the invention to provide a Darlington amplifier in which 
electric current flowing through the base of the output-stage, or 
final-stage, transistor is less likely to be distributed non-uniformly 
during transition (turn off) times. It is desired to optimize the current 
distribution in the semiconductor material of the Darlington configuration 
so that when the amplifier is turned off, undesirable excess current 
concentration is reduced in portions where carriers linger, and turn-off 
conditions are not reached quickly. The goal is to remove carriers 
uniformly from the base of the output-stage transistor and thereby extend 
the safe operating region during the application of a reverse bias on said 
Darlington configuration. 
SUMMARY OF THE INVENTION 
The above object is achieved in accordance with the teachings of this 
invention by a Darlington amplifier comprising a plurality of transistors, 
where the base of the rear-stage transistor is connected to the base of 
the front-stage transistor via speed-up diodes. The Darlington amplifier 
according to the present invention is characterized in that the speed-up 
diodes are connected by leads to the base of the final-stage transistor at 
plural locations spaced from each other on the base region. 
Since the base of the output-stage transistor is connected via the speed-up 
diodes to the front-stage transistor by the leads at plural locations 
distributed over the base, carriers are uniformly taken from the base 
region of the output-stage transistor when the amplifier is turned off. 
Consequently, high current concentrations during turn-off or other state 
transitions where the current density in the device changes abruptly, are 
less likely to occur.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Darlington amplifiers constructed in accordance with the invention are 
shown in FIGS. 1(a) and 1(b). These amplifiers comprise two transistors 
and three transistors, respectively. It is to be noted that like 
components are denoted by like reference numerals in FIGS. 1, 2, 3. 
The Darlington amplifier shown in FIG. 1(a) comprises two transistors. The 
base 32 of the rear-stage transistor acts also as the emitter of the 
front-stage transistor. Six portions of the base 32 are connected to the 
base terminal B via three speed-up diodes 21 by aluminum leads 4. 
Similarly, in the Darlington amplifier shown in FIG. 1(b) and consisting of 
three transistors, the base electrode 34 serves also as the emitter of the 
second-stage transistors, and six portions of the base 34 are connected to 
the base terminal B via three speed-up diodes 22 by aluminum leads 4. The 
base 32 of the second-stage transistor is connected at one location to the 
base terminal B via a speed-up diode 21. The front-stage transistors have 
two bases 31 which act also as emitters of the second-stage transistor. 
These two bases 31 are connected to the base terminal B. 
The leads 4, which are remotely located from the speed-up diodes connected 
to the bases 32, 34 of the transistors, form extensions to the leads 4 
close to the diodes to reduce the number of leads and to avoid 
short-circuit between the leads. 
In FIG. 4 , the curve 41 shows the safe operating region of the novel 
Darlington amplifier depicted in FIG. 1(a) when a reverse bias is applied. 
The safe operating region of a conventional Darlington amplifier shown in 
FIG. 3(a) when a reverse bias is applied is indicated by the curve 42 in 
FIG. 4. It can be seen from this graph that the invention provides an 
extended safe operating region for the Darlington amplifier. 
In accordance with the invention, the base of the output-stage transistor 
of a Darlington amplifier is connected by leads to the base of the 
front-stage transistor via speed-up diodes spaced from each other on the 
base of the output-stage transistor. Thus, carriers are taken uniformly 
from the base of the output-stage transistor. As a result, destruction 
which would have been heretofore caused by uneven current distribution 
within the semiconductor material is prevented. Hence, the safe operating 
region applicable during reverse bias condition is extended. 
While the invention has been described with reference to preferred 
embodiments, it will be understood by those skilled in the art that 
various changes in form and detail may be made without departing from the 
spirit and scope of the invention as defined in the following claims.