Patent Number: 053961412
Section: summary

TECHNICAL FIELD OF THE INVENTION The present invention generally relates to electrical power sources for electronic circuitry and, more particularly, to a method and apparatus for generating electrical power that employ radioisotope power cells. BACKGROUND OF THE INVENTION Decay of radioactive materials produces electrically charged radioactive particles such as .alpha. particles, .beta. particles, and .gamma. particles. As with other nuclear processes, the charge scale of these types of radiation is millions of times greater than in non-nuclear processes. For example, .alpha. decay of the Am.sub.241 radioisotope has a half-life of 458 years and can introduce 5.5 million electron volts (MeV) into a typical semiconductor material. On the average, however, 3.6 electron volts (eV) are necessary to produce one electron-hole pair the typical semiconductor material. Thus, for every .alpha. particle traveling through the semiconductor material approximately 1.53 million electron-hole pairs may be formed. In contrast, for a typical photo-cell each photon that is absorbed by a photon-responsive semiconductor material generates only one electron-hole pair. If a method and apparatus existed to harness the power that comes from atomic particles, then this energy could be used for a variety of power applications. SUMMARY OF THE INVENTION There is a need, therefore, for a method and apparatus in the form of a radioisotope-based electrical power source. The present invention, accordingly, provides a radioisotope power source in the form of radioisotope power cells using P-N junctions in a semiconductor material that provides a heretofore unavailable source of power to energize electronic circuits. The radioisotope power cell of the present invention provides an electrical power source that includes a semiconductor material and at least one P-N junction within the semiconductor material. A radioisotope or radioactive source associates with the P-N junction and emits electrically-charged radioactive particles into the semiconductor material near the P-N junction. The P-N junction receives the electrically-charged radioactive particles to generate electron-hole pairs therefrom and produce electrical current across the P-N junction. The electrical power source of the present invention may use, for example, a radiation source that emits .alpha. radiation, .beta. radiation, or .gamma. radiation, or even positron radiation. A technical advantage of the present invention is that it recognizes the advantages of a problem that is inherent in packaging integrated circuits. That is, radioactive elements in electronic circuit packaging materials often include traces of uranium and thorium. These trace elements can seriously impair the operation of associated integrated circuits. This is due to the electron-hole pairs that radioactive particles can form in integrated circuits. By providing a method and system for advantageously applying the power from radioactive decay to power electronic circuitry, the present invention provides an attractive alternative power source for electronic circuitry. Another technical advantage of the present invention is that it provides long-lived, inexpensive power for electronic circuitry from relatively minuscule amounts of radioactive material. Because of the magnitude of power per radioactive particle, only a very small amount of radioactive source material is necessary to produce a large number of electron-hole pairs. The large number of electron-hole pairs produces electrical current across the P-N junction to power electronic circuitry. In fact, a sufficient amount of shielding can be applied to the radioactive source to prevent radiation that the radioactive source emits from affecting associated electronic circuitry or from leaving the integrated circuit package. Yet another technical advantage of the present invention is that the power cells may be formed in a variety of configurations or embodiments. For example, one embodiment includes the use of an array of power cells distributed and embedded within a semiconductor chip. This configuration can provide standby power in the event of a primary power source failure. Another embodiment includes growing a P-N junction around a trench within a semiconductor material and embedding a radioactive source in the trench. This aids in preventing the radioactive source from affecting any surrounding electronic circuitry. In still another embodiment, power cells appear on one side of a semiconductor chip, while active integrated circuitry appears on the opposite side. This also prevents the radioactive source from affecting associated electronic or integrated circuitry. The present invention, therefore, possesses this flexibility due in part to the small size requirements of the radioactive source. Still another technical advantage of the present invention is that a wide variety of radioactive materials may be used as the radioactive source for emitting the radioactive particles. Thus, based on engineering design limitations, the present invention may use a long-lived, low-energy system for some applications. On the other hand, some applications may advantageously use relatively short-lived high-energy radioactive sources. Furthermore, based on differing engineering design objectives, it may be more advantageous to use .beta. or .gamma. radiation sources instead of .alpha. radiation sources. The present invention contemplates this degree of flexibility in the radiation source selection.