Noise reduction technique for breakdown diodes

Abstract: A device including a first semiconductor PN junction (16) which is reverse biased to operate in the avalanche breakdown region. A second semiconductor PN junction (20) is biased to operate in the forward active region such that carriers are injected through the second semiconductor PN junction and diffuse to the first semiconductor PN junction wherein the noise generated from the first semiconductor PN junction operating in the avalanche breakdown region is substantially reduced.

Description: Background of the Invention This invention relates to diodes such as diodes operating in the avalanche breakdown region. Any reverse biased PN junction has a small reverse current flow due to the presence of holes and electrons in the vicinity of the depletion region. These holes and electrons are swept across the depletion region by the applied electric field and contribute to the leakage current of the PN junction. As the reverse bias on the PN junction is increased, the applied electric field increases and the carriers acquire increasing amounts of energy. At a predetermined critical electric field, many of the carriers traversing the depletion region acquire sufficient energy to create new hole-electron pairs upon colliding with lattice atoms. This is called the avalanche multiplication process and leads to a large and sudden increase in the reverse bias current since the newly created carriers are also capable of producing new hole-electron pairs upon colliding with other lattice atoms. It should be noted that for a large reverse current to flow by avalanche breakdown, two conditions must exist: first, there must be sufficient reverse bias potential across the PN junction to create a sufficient electric field in the depletion region to support avalanche multiplication which enables holes and electrons to accelerate across the PN junction and, thus, create many additional carriers. Second, there must be carriers available in the vicinity of the PN junction to begin and sustain the avalanche breakdown process. PN junctions operating in the avalanche region, as described above, are widely used as voltage references and are typically called zener diodes, but are more correctly called avalanche breakdown diodes. However, one major problem with zener diodes is that they exhibit large levels of noise, or short term variations in the breakdown current, when biased around their breakdown voltage. One simplistic approach for reducing the noise due to avalanche breakdown of a PN junction is to place a capacitor across the PN junction. In general, as the value of the capacitor increases, the level of the noise decreases. However, a large capacitor is difficult to fabricate on an integrated circuit and typically reduces the speed of the circuit. Another approach for reducing the noise due to avalanche breakdown of a PN junction is to bias the PN junction at a higher current. However, this approach consumes a large amount of power while only slightly reducing the noise. Patent Abstracts of Japan, vol. 7, no. 76 (E-167) and Japanese patent application no. JP-A-58004980 provide examples of improving noise characteristics of a transistor. Hence, there exists a need for providing a PN junction device that operates in the avalanche breakdown region while having reduced noise characteristics. Summary of the Invention Briefly, there is provided a diode operating in the avalanche region, comprising: a first semiconductor region having a first conductivity type; a second semiconductor region having a second conductivity type, said first and second semiconductor regions forming a first semiconductor junction, said first semiconductor junction being arranged to be reverse biased to operate in an avalanche breakdown mode; and a third semiconductor region having said first conductivity type, said second and third semiconductor regions forming a second semiconductor junction, said second semiconductor junction being arranged to be forward biased such that said third semiconductor region injects carriers into said second semiconductor region and said carriers subsequently diffuse to said first semiconductor junction wherein noise associated with said first semiconductor junction operating in said avalanche breakdown mode is substantially reduced; a first electrode coupled to the first semiconductor region, the first electrode serving as a cathode of the diode; a second electrode coupled to the second semiconductor region, the second electrode serving as an anode of the diode; and a third electrode coupled to the third semiconductor region, the third electrode being coupled to the second electrode. In another aspect, the invention provides a method for reducing the noise during avalanche breakdown of a diode PN junction, the method comprising the steps of: forming a first PN junction that is reverse biased to operate in avalanche breakdown; forming a second PN junction that is forward biased; and injecting carriers from said second PN junction to said first PN junction such that noise resulting from said avalanche breakdown of said first PN junction is substantially reduced. It is an advantage of the present invention to provide a PN junction device having reduced noise characteristics while operating within the avalanche breakdown region. It is also an advantage of the present invention to provide a voltage reference circuit which includes a diode that operates in the avalanche breakdown mode for providing an output voltage having reduced noise characteristics. The above and other features and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings. Brief Description of the Drawings FIG. 1 is a detailed diagram illustrating a first embodiment of a diode in accordance with the present invention; FIG. 2 is a detailed diagram illustrating a second embodiment of a diode in accordance with the present invention; and FIG. 3 is a detailed diagram illustrating a third embodiment of a diode in accordance with the present invention. Detailed Description of the Present Invention Referring the FIG. 1, a detailed diagram illustrating a first embodiment of diode 10 in accordance with the present invention is shown comprising N-type semiconductor region 12, P-type semiconductor region 14 and PN junction 16 formed between regions 12 and 14. It is understood that N-type semiconductor region 12 can be formed within P-type semiconductor region 14. Also, N-type semiconductor region 18 is formed within P-type semiconductor region 14 whereby PN junction 20 is formed. Region 12 has an electrode coupled to output terminal 22 and to a first terminal of resistor 24. The second terminal of resistor 24 is returned to ground reference through source of potential (or battery) 26. Also, region 14 has an electrode returned to ground reference. Region 18 has an electrode coupled to the first terminal of resistor 28 through source of potential (or battery) 30. Finally, the second terminal of resistor 28 is returned to ground reference. The present invention provides a method for increasing the number of minority carriers in the vicinity of PN junction 16 prior to avalanche breakdown thereby helping to trigger the avalanche current-multiplying events. In other words, the present invention provides many additional carriers at PN junction 16 to be accelerated thereacross such that the noise associated with the avalanche breakdown of PN junction 16 is substantially reduced. Further, the present invention provides a circuit including a diode which operates in the avalanche breakdown mode for providing a reference voltage at an output. In operation, PN junction 16 is biased by battery 26 and resistor 24 such that battery 26 supplies a predetermined reverse bias potential to PN junction 16 that is greater than the predetermined avalanche breakdown voltage of PN junction 16. Further, PN junction 20 is forward biased by battery 30 and resistor 28 such that N-type semiconductor region 18 injects electrons (minority carriers) into P-type semiconductor region 14 wherein at least some of the electrons subsequently diffuse to PN junction 16 thereby creating an increased number of electrons at PN junction 16. Thus, the two conditions required for avalanche breakdown exist: 1) there is a sufficient electric field as provided by reverse biasing PN junction 16 through battery 26, and 2) there are a sufficient number of the minority carriers present at PN junction 16 to initiate the avalanche breakdown process. Further, it must be understood that N-type semiconductor region 18 continuously injects electrons into P-type semiconductor region 14 thereby providing a sufficient number of electrons at PN junction 16 in order to sustain avalanche breakdown. Due to N-type semiconductor region 18 injecting electrons into P-type semiconductor region 14, it must be realized that the noise associated with the avalanche breakdown of PN junction 16 is substantially reduced since there is always a large amount of minority carriers (electrons) available in P-type semiconductor region 14 to initiate and sustain avalanche breakdown. Without N-type semiconductor region 18 injecting electrons into P-type semiconductor region 14, the only electrons available at PN junction 16 are those few which are thermally generated. Therefore, the avalanche breakdown process occurs at various time intervals when the thermally generated electrons create enough carriers, by colliding with lattice atoms, to provide a sufficient number of minority carriers to initiate avalanche breakdown. Further, at each time interval when the avalanche process occurs, it is accompanied by large noise spikes of current through the PN junction. In contrast, the present invention substantially reduces the noise across PN junction 16 by continuously providing a sufficient number of minority carriers at PN junction 16 to initiate and sustain avalanche breakdown. In other words, the present invention does not rely on the thermally generated carriers to initiate breakdown. Instead, the present invention provides additional carriers at PN junction 16 to insure that avalanche breakdown is initiated and sustained thereby reducing the noise associated with PN junction 16 operating in its avalanche breakdown mode. It should be realized that voltage V OUT , which appears between output terminal 22 and ground reference, is a predetermined voltage due to the diode, formed from semiconductor regions 12 and 14, operating within its avalanche breakdown region. Further, since the noise associated with PN junction 16 is substantially reduced, it follows that the noise associated with voltage V OUT is also substantially reduced. Thus, a low noise reference voltage is provided at output terminal 22. As an example, the output voltage of a SmartMOS 13 volt avalanche diode device was observed to have 6 volts peak-to-peak noise with a 50 microamp bias current flowing therethrough. However, when this same SmartMOS device included carrier injector means for increasing the number of carriers at its PN junction as described in the present invention, an astonishing result was observed. There was less than 34 millivolts of noise on the output voltage. Further, the SmartMOS device was biased at only 0.4 microamps with the carrier injector means as opposed to the aforementioned 50 microamps without the carrier injector means. Thus, by continuously providing a sufficient number of carriers at a PN junction which is operating in the avalanche breakdown region, a substantial reduction in noise is achieved. It is worth noting that the present invention can easily be incorporated into a three-terminal integrated circuit device whereby the first terminal is coupled to the electrode of N-type semiconductor region 12, the second terminal is coupled to the electrode of P-type semiconductor region 14, and the third terminal is coupled to the electrode of N-type semiconductor region 18. Referring to FIG. 2, a detailed diagram illustrating a second embodiment of a diode in accordance with the present invention is shown. It is understood that components similar to those of FIG. 1 are designated by similar reference numerals. The embodiment of FIG. 2 further comprises P-type semiconductor region 34 being formed within N-type semiconductor region 12 thereby forming PN junction 36. Further, region 34 has an electrode coupled to the first terminal of resistor 38 through battery 40. Also, the second terminal of resistor 38 is returned to ground reference. The operation of the embodiment in FIG. 2 is similar to the operation of the embodiment in FIG. 1 wherein PN junction 36 is forward biased by battery 40 and resistor 38 such that P-type semiconductor region 34 injects holes (minority carriers) into N-type semiconductor region 12 which subsequently diffuse to PN junction 16 thereby creating a sufficient number of carriers at PN junction 16 to initiate the avalanche breakdown process. Further, it must be understood that P-type semiconductor region 34 continuously injects holes into N-type semiconductor region 12 thereby providing a sufficient number of carriers at PN junction 16 in order to sustain avalanche breakdown. Likewise, the embodiment of FIG. 2 substantially reduces the noise across PN junction 16 by providing a large number of carriers at PN junction 16 to initiate and sustain avalanche breakdown. Referring to FIG. 3, a detailed diagram illustrating a third embodiment of a diode in accordance with the present invention is shown. It is understood that components similar to those of FIG's. 1 and 2 are designated by similar reference numerals. The embodiment of FIG. 3 includes both N-type semiconductor region 18 being formed within P-type semiconductor region 14 thereby forming PN junction 20, and P-type semiconductor region 34 being formed within N-type semiconductor region 12 thereby forming PN junction 36. The operation of the embodiment of FIG. 3 is similar to the operation of the embodiment of FIG. 1 whereby N-type semiconductor region 18 functions to inject electrons into P-type semiconductor region 14 as aforedescribed for the embodiment of FIG. 1. In addition, the operation of the embodiment of FIG. 3 is similar to the operation of the embodiment of FIG. 2 whereby P-type semiconductor region 34 functions to inject holes into N-type semiconductor region 12 as aforedescribed for the embodiment of FIG. 2. Likewise to the embodiments of FIGs. 1 and 2, it must be understood that N-type semiconductor region 20 continuously injects electrons into P-type semiconductor region 14 while P-type N-type semiconductor region 34 continuously injects holes into semiconductor region 12 thereby providing a sufficient number of carriers at PN junction 16 in order to sustain avalanche breakdown. Therefore, the embodiment of FIG. 3 also substantially reduces the noise across PN junction 16 by providing a large number of carriers at PN junction 16 to initiate and sustain avalanche breakdown. By now it should be apparent from the foregoing discussion that a novel method has been provided for reducing the noise associated with a PN junction operating within its avalanche breakdown region. Further, a novel circuit has been provided for providing a low noise reference voltage by utilizing a diode operating within its avalanche breakdown region.

Claims: A diode operating in the avalanche region, comprising: a first semiconductor region (12) having a first conductivity type; a second semiconductor region (14) having a second conductivity type, said first and second semiconductor regions forming a first semiconductor junction (16), said first semiconductor junction (16) being arranged to be reverse biased to operate in an avalanche breakdown mode; and a third semiconductor region (18) having said first conductivity type, said second (14) and third (18) semiconductor regions forming a second semiconductor junction (20), said second semiconductor junction (20) being arranged to be forward biased such that said third semiconductor region (18) injects carriers into said second semiconductor region (14) and said carriers subsequently diffuse to said first semiconductor junction (16) wherein noise associated with said first semiconductor junction (16) operating in said avalanche breakdown mode is reduced; a first electrode coupled to the first semiconductor region (12), the first electrode serving as a cathode of the diode; a second electrode coupled to the second semiconductor region (14), the second electrode serving as an anode of the diode; and a third electrode coupled to the third semiconductor region (18), the third electrode being coupled to the second electrode. The diode according to claim 1 wherein said first conductivity type is N-type and said second conductivity type is P-type. The diode according to claim 1 wherein said first conductivity type is P-type and said second conductivity type is N-type. The diode according to any preceding claim wherein the third region (18) is formed within the second region (14). A circuit (10) for providing an output voltage at an output (22), comprising: a diode according to any preceding claim, wherein the first electrode is coupled to the output of the circuit (10); a first voltage source (26) having first and second terminals, said first terminal being coupled to the first electrode and said second terminal being coupled to the second electrode, said first voltage source (26) providing a first predetermined voltage to bias said first semiconductor junction (16) to operate in avalanche breakdown; and a second voltage source (30) having first and second terminals, said first terminal of said second voltage source (30) being coupled to the second electrode and said second terminal of said second voltage source (30) being coupled to the third electrode, said second voltage source (30) providing a second predetermined voltage to forward bias the second semiconductor junction (20). The circuit according to claim 5 further comprising: a first resistor (24) coupled between the first electrode and said first terminal of said first voltage source (26); and a second resistor (28) coupled between the second electrode and said first terminal of the second voltage source (30). A method for reducing the noise during avalanche breakdown of a diode PN junction, the method comprising the steps of: forming a first PN junction (16) that is reverse biased to operate in avalanche breakdown; forming a second PN junction (20) that is forward biased; and injecting carriers from said second PN junction to said first PN junction such that noise resulting from said avalanche breakdown of said first PN junction is reduced.

Inventors: PIGOTT JOHN M, WOLLSCHLAGER RANDAALL T, PIGOTT, JOHN M., WOLLSCHLAGER, RANDAALL T.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1991
Main classification: H01L29
IPC codes: H01L29