Two-terminal Monolithic Voltage Regulator And Reach-through Transistor

Abstract

A double diffused transistor structure having a base region which is sufficiently thin so that the reverse breakdown voltage between the emitter and the collector (BV.sub.eco) is less than the reverse breakdown voltage between the emitter and the base (BV.sub.ebo). The transistor structure is connected in an upside-down fashion as a two-terminal network, the terminals being formed by the emitter and the collector in a manner similar to that heretofore employed with Zener diodes to provide a "reach-through" at a predetermined voltage thereacross. A circuit utilizing such a "reach-through" transistor structure connected in the upside-down fashion in conjunction with other components to operate the transistor structure at a constant current to maintain the reverse breakdown voltage constant.

Description

STATEMENT OF THE INVENTION

This invention relates to voltage regulators, and, more particularly, to a two-terminal monolithic voltage regulator utilizing a reach-through transistor structure either alone or in combination with other circuitry for maintaining the current therethrough constant.

DESCRIPTION OF THE PRIOR ART

Low voltage regulators are required for numerous applications, such as simple voltage regulation, level shifting, and the generation of abrupt transitions for use in reproducing nonlinear transfer functions. One device which is inexpensive and reliable and has found very extensive use for low voltage regulation application is the Zener diode. Zener diodes, however, have a rather soft characteristic at low voltage, and do not have the speed of response found desirable particularly in high-speed computer and similar applications.

To overcome the limitations of Zener diodes, a narrow base, planar junction, punch-through diode has been developed which is described in U.S. Pat. No. 2,975,342 which issued on Mar. 14, 1961. In accordance with this Pat., a single junction diode is provided which has an ohmic contact located parallel to and spaced a very small distance apart from the PN junction. When the applied voltage across the junction is increased, the depth of the space charge region or depletion layer produced by the voltage likewise increases. When the voltage across a junction is increased to a value at which the depletion layer depth punches through to the ohmic contact, the diode develops a low dynamic impedance. This punch-through diode has been found to be considerably faster than the Zener diode, and has a somewhat sharper breakdown characteristic.

However, the sharpness of the breakdown characteristic of the punch-through diode still leaves something to be desired, particularly where regulation is needed over a wide range of currents, say from 10 microamps to 10 milliamps.

OBJECTS OF THE INVENTION

It is therefore a primary object of the present invention to provide an improved low voltage regulator.

It is a further object of this present invention to provide a low voltage regulator of the two-terminal monolithic type which is the electrical equivalent of a breakdown diode.

It is also an object of the present invention to operate a transistor structure, having a base region too thin for normal operation, in a new mode to provide a two-terminal voltage regulator which has a hard characteristic.

It is a further object of the present invention to provide a two-terminal monolithic voltage regulator which has a sharp breakdown characteristic over a current range extending from below 10 microamps to above 10 milliamps.

It is an additional object of the present invention to provide a regulator device which includes a transistor structure which is operated in a different than the normal mode to exhibit reach-through breakdown at a predetermined voltage, the transistor structure being combined in an integrated circuit chip with other circuit elements to form a new and novel regulator device.

SUMMARY OF THE INVENTION

In accordance with the preferred embodiment of the present invention, a double diffused planar transistor is constructed in such a manner that the second diffusion is deeply diffused into the first diffusion region so that the PN junction defining the base-emitter boundary and the PN junction defining the base-collector boundary have at least a portion in close proximity to one another. The close proximity portion of these two junctions is selected in such a manner that the application of a reverse voltage (emitter more positive than collector for an NPN device) across the emitter and the collector electrodes produces a "reach-through" or breakdown of the transistor at a voltage which is lower than the reverse voltage which will produce an avalanche breakdown when applied between the emitter and the base terminals.

In other words, the transistor structure of this invention is operated in the upside-down mode in which the emitter of a NPN structure, or the collector of a PNP structure, is maintained positive with respect to the base. To avoid an avalanche breakdown between the emitter and base when connected in this upside-down configuration, the base region is made sufficiently thin so that the depletion layer crosses the base-collector junction before an avalanche is reached.

In accordance with one embodiment of the invention, this reach-through transistor structure may be utilized in the upside-down configuration as a two-terminal regulator, the two-terminal networks being formed respectively by the emitter and the collector. In accordance with another embodiment of the present invention, the reach-through transistor is operated at a constant reverse current and amplifier circuitry is used to regulate the terminal voltage against variations in current.

Objects and advantages other than those set forth above will be apparent from the following description, when read in connection with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of a reach-through transistor structure constructed in accordance with the present invention;

FIG. 2 is a schematic wiring diagram of an NPN reach-through transistor structure of the type illustrated in FIG. 1, connected for operation in the voltage regulator mode, in accordance with the present invention;

FIG. 3 is a schematic circuit diagram of a regulator device employing the reach-through structure of the present invention;

FIG. 4 is a graph comparing the performance characteristics of the regulator device of FIG. 3 with that of a conventional alloy-junction diode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a double diffused transistor device 10 which includes an emitter (E) region 12, a base (B) region 13 and a collector (C) region 14. The emitter region is deeply diffused during the manufacture of device 10 into the base region to produce the desired characteristics for reach-through operation in accordance with this invention. Because of this deep penetration during the second diffusion, the thickness of base 13, defined as the distance between the NP junctions formed by the emitter-base junction and the base-collector junction, is less than found in ordinary transistors, a condition necessary for the phenomena of reach-through to occur.

The phenomena of reach-through occurs when the depletion region of the reverse biased emitter-base junction penetrates or "reaches through" narrow base region 13 to collector region 14. This results in an abrupt increase in current flow between the emitter and collector. In accordance with this invention, the reach-through phenomenon is utilized to produce a regulating action in the circuit in which the device is connected.

It is important in the present invention that the device be fabricated in such a manner that the voltage at which reach-through occurs (between emitter and collector) is less than the voltage at which avalanche breakdown occurs (between emitter and base). That is, the reach-through voltage between the emitter and collector, commonly designated as BV.sub.eco, must be less than the avalanche voltage between the emitter and the base, commonly designated as BV.sub.ebo. The condition that BV.sub.ebo must be greater than BV.sub.eco usually makes the device of this invention unsuitable as an ordinary transistor device, as the narrow base region causes a low breakdown voltage in the normal operating mode of the transistor.

To obtain a transistor device in which the BV.sub.ebo is greater than the BV.sub.eco, the second diffusion must be carefully controlled. In practice, a test sample is utilized and frequently tested during the second diffusion and the diffusion is stopped when the sample shows the desired breakdown voltage between the emitter and collector.

The reach-through transistor of the present invention may be utilized in a number of ways to provide voltage regulation. In perhaps the simplest application, which is illustrated in FIG. 2 of the drawing, a reach-through transistor device 15 has its emitter 15E connected through a resistance R to terminal 16. The collector 15C is connected to ground and the base 15B is preferably shorted to the collector 15C, although it need not necessarily be so connected. When an unregulated voltage is applied to the circuit at terminal 16 the operating characteristics of the transistor 15, as explained above, are such that a regulated output voltage can be obtained at the emitter terminal 15E. Instead of using an NPN configured reach-through transistor structure, the same effect can be obtained using a PNP configured structure by simply reversing the circuit polarities.

A somewhat more efficient voltage regulator is illustrated in FIG. 3. A reach-through transistor structure 20 is shown connected to elements including a field effect transistor 17 which uses the base diffusion of the NPN transistor as a channel and the NPN emitter as a gate. Transistor 17 serves to establish the operating current of device 20 at a level greater than the base current of a transistor 18, which may be an ordinary transistor, and provides most of the gain for the regulator. Transistor 18 has an emitter 18a, a base 18 b which is connected to the emitter of device 20, and a collector 18c. The regulator also employs another transistor having an emitter 19a, a base 19b and a collector 19c. From a practical standpoint, all of the the transistor elements of FIG. 3 are preferably formed on the same chip using integrated circuit techniques. For example, the regulator may be fabricated on a silicon die with the six-mask planar-epitaxial process used for the great majority of linear and digital integrated circuits. Transistor 19 may be a vertical PNP transistor which is made using the NPN base and an emitter, the NPN collector as a base and the P-type substrate of the integrated circuit as a collector. It will be apparent that this PNP transistor, which is required to handle large currents up to 10 mA, must be made quite large because of the low emitter efficiency of the NPN base diffusion.

The regulator also includes a pinch resistor 21 which sets the operating collector current of transistor 18. The resistance value of resistor 21 may be chosen so that transistor 18 sees a minimum variation of collector current in the midrange of operating currents for the completed device, giving best regulation in this region.

In operation of the regulator, as voltage is applied across the device, the emitter-base junction of diode device 20 is reverse biased. When a voltage is reached where the emitter depletion region penetrates the base to the collector, reverse reach-through occurs and device 20 beings to conduct, turning on transistor 18 which regulates the voltage across the terminals of the regulator.

The characteristics of a regulator such as shown in FIG. 3, relative to the characteristic of a typical alloy-junction diode, are shown in the graphs of FIG. 4. Curve 23 represents the characteristic of the alloy-junction device, while curve 24 shows the characteristics of a regulator as shown in FIG. 2. These graphs clearly illustrate the relatively soft characteristic of the alloy-junction device compared with the hard and sharply breaking characteristics of the regulator of the present invention.

In the operation of the regulator shown in FIG. 3 for reverse voltages up to about further 1 volt of the nominal breakdown voltage, the reverse current is primarily junction leakage current in the low nanoampere range. Above this voltage, device 20 breaks down, conducting into the On resistance of transistor 17. Any further increase in voltage causes device 20 to conduct more, but at the same time it pinches OFF transistor 17, giving rise to an initial hump in the reverse characteristic. This continues until transistor 18 turns ON, and its added gain initiates breakdown and hardens the diode characteristic. The diode voltage will then vary by about 60 mv./decade until the current rises to 50 .mu.A. At this point, transistor 19 turns ON, providing even more gain and reducing the voltage variation to approximately 15 mv./decade. At currents much above 1 mA, the base current of the output PNP transistor 19 becomes appreciable by comparison to the bleed current through resistor 21, so that the reverse characteristic softens somewhat.

The change of breakdown voltage with temperature is a nearly linear 3.3 mv./.degree. C., independent of the nominal breakdown voltage. Because of this predictable temperature drift, the device can be used in building relatively simple temperature compensated regulators for operation with input voltages down to 3v. and output voltages down to 1V. This is a distinct advantage, since it is difficult to make any other type of temperature compensated reference in the voltage range from 2V to 6v., and tunnel diodes and most integrated circuits operate at voltages below that of temperature-compensated references.

From the foregoing, it will be seen that there is provided a two-terminal monolithic voltage regulator which is electrically equivalent to a breakdown diode. At low voltages, the regulator provides reverse characteristics which are more than 10 times sharper than those obtainable with single-junction Zener diodes. The regulator of this invention has been manufactured with breakdown voltages from 1.8v. to 5.6v. As indicated above, the regulator is well suited to fabrication using integrated circuit techniques.

While the above detailed description has shown, described and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

Claims

I claim:

1. A two-terminal monolithic voltage regulator comprising:

a first terminal and a second terminal across which a voltage to be regulated may be applied;

a first transistor means including a first region and a second region of a first conductivity type separated by a third region of a second conductivity type, at least a portion of said third region being so thin that a depletion layer caused by reverse biasing the junction formed between said first region and said third region extends through said portion at a potential less than the avalanche breakdown potential of said junction, said first region being coupled to said first terminal;

resistance means coupling said second region to said second terminal; and

a second transistor means having a first base, a first emitter and a first collector, said first emitter being coupled to one of said first and second terminals, said first collector being coupled to the other of said first and second terminals and said first base being coupled to said second region of said first transistor means, said second transistor means being responsive to the current flow through said first transistor means to regulate said voltage.

2. A two-terminal monolithic voltage regulator as recited in claim 1 wherein said resistance means includes a field effect transistor having a gate, a source and a drain, said gate being coupled to one of said first and second terminals, said source being coupled to said second terminal and said drain being coupled to said second region.

3. A two-terminal monolithic voltage regulator as recited in claim 2 and further including a third transistor means having a second base, a second emitter and a second collector, said second emitter being coupled to one of said first and second terminals, said second collector being coupled to the other of said first and second terminals and said second base being responsive to the output of said second transistor means.

4. A two-terminal monolithic voltage regulator as recited in claim 3 and further including means coupling said third region to said second terminal.

5. A two-terminal voltage regulator comprising:

a first terminal and a second terminal across which a voltage to be regulated may be applied;

a first transistor means including a first emitter and a first collector separated by a first base, and defining a first PN junction between said first emitter and said first base, and a second PN junction between said first base and said first collector, the thickness of said base separating said first and second junctions being so small that a depletion layer caused by reverse biasing said first junction extends to said second junction at a biasing potential less than that required to cause avalanche breakdown across said first junction, said first emitter being coupled to said first terminal;

resistance means coupling said first collector to said second terminal; and

a second transistor means including a second base, a second emitter and a second collector, said second base being coupled to said first collector, said second collector being coupled to said first terminal and said second emitter being coupled to said second terminal.

6. A two-terminal voltage regulator as recited in claim 5 wherein said resistance means includes a field effect transistor having a gate, source and drain, said gate being coupled to said first terminal, said drain being coupled to said first collector and said source being coupled to said second terminal.

7. A two-terminal voltage regulator as recited in claim 6 and further including a third transistor means having a third base, a third emitter and a third collector, said third base being coupled to said second collector, said third emitter being coupled to said first terminal and said third collector being coupled to said second terminal.

8. A two-terminal voltage regulator as recited in claim 7 wherein said first transistor means and said second transistor means are of a first conductivity type and said third transistor means is of the opposite conductivity type.

9. A two-terminal voltage regulator as recited in claim 6 and further including means coupling said first base to said second terminal.