Abstract:
In one preferred embodiment, a semiconductor diode includes a first layer formed with a p-type semiconductor, a second layer formed with an n-type semiconductor, and a third active depletion layer contained between the first and second layers. The third layer is formed with a radioisotope of the p-type and n-type semiconductors (preferably Si 32) such that initial emission of beta particles begins in the active depletion region and substantially all of the emitted beta particles are contained within the first, second and third layers during operation. The p-type and n-type layers each have sufficient depth to contain substantially all of beta particles emitted from the depletion layer. The depth of each of the p-type and n-type layers is substantially equal to or greater than the maximum beta emission depth of the radioisotope.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention (Navy Case No. 102,478) is assigned to the United States Government and is available for licensing for commercial purposes. Licensing and technical inquiries may be directed to the Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; voice (619) 553-2778; email T2@spawar.navy.mil. 
    
    
     BACKGROUND 
     The present invention relates to a semiconductor diode, and more particularly to a beta voltaic semiconductor diode fabricated from a radioisotope. Beta voltaics convert the energy of radioactive decay products directly into electrical power. They operate much the same way as a solar cell except that the beta particles (high energy electrons) are used, rather than photons. The beta particles can produce many electron-hole pairs in the diode per incident particle. The accepted method of construction is to coat a diode with a beta emitter (i.e. a radioisotope that undergoes beta decay) such as Nickel 63, tritium (usually as a metal hydride), or promethium 147. Radiation damage is often an issue, therefore silicon carbide, (being more radiation hard than silicon) is primarily used. The high energy electrons (beta particles) do not penetrate very far into silicon. This presents issues for fabrication of the diodes and favors high surface to volume geometries (i.e., pillar or comb structures are employed). 
     SUMMARY 
     In one preferred embodiment, a semiconductor diode includes a first layer formed with a p-type semiconductor, a second layer formed with an n-type semiconductor, and a third active depletion layer contained between the first and second layers. The third layer is formed with a radioisotope of the p-type and n-type semiconductors (preferably Si 32) such that initial emission of beta particles begins in the active depletion region and substantially all of the emitted beta particles are contained within the first, second and third layers during operation. The p-type and n-type layers each have sufficient depth to contain substantially all of beta particles emitted from the depletion layer. The depth of each of the p-type and n-type layers is substantially equal to or greater than the maximum beta emission depth of the radioisotope. Incorporation of the isotope as the diode material overcomes the short range of the beta particles and simplifies the device fabrication geometry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Throughout the several views, like elements are referenced using like reference numerals, wherein: 
         FIG. 1  is a graph showing the range of high energy electrons in silicon carbide for three isotopes. 
         FIG. 2  is a graph showing a diode with various isotopes coated on the surface. 
         FIG. 3  shows a view of a diode of the present invention with the depletion region made from Si32 where the average and maximum ranges are shown as cross hatching. 
         FIG. 4  shows another view of a diode embodiment based on  FIG. 3 . 
         FIG. 5  shows another embodiment of a semiconductor diode of the present invention. 
         FIG. 6  shows a variation of the embodiment shown in  FIG. 5 . 
         FIG. 7  shows views of a Schottky diode. 
         FIG. 8  shows a view of a beta voltaic Schottky diode. 
         FIG. 9  a top view of a beta voltaic Schottky diode. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention relates to a semiconductor diode, and more particularly to a beta voltaic semiconductor diode fabricated from a radioisotope. Beta voltaics are generators of electrical current, in effect a form of a battery, which use energy from a radioactive source emitting beta particles (high energy electrons). Beta voltaics are particularly well-suited to low-power electrical applications where long life of the energy source is needed, such as implantable medical devices or military or space applications. Beta voltaics convert the energy of radioactive decay products directly into electrical power. 
     In electronics, a diode is a two-terminal electronic component that conducts electric current in only one direction. A semiconductor diode is fabricated from a crystal of semiconductor such as silicon that has impurities added to it to create a region on one side of a junction that contains negative charge carriers (electrons), called n-type semiconductor, and a region on the other side of that junction that contains positive charge carriers (holes), called p-type semiconductor. The diode&#39;s terminals are attached to each of these regions, and the boundary within the diode between these two regions is called a PN junction, in which the action or operation of the diode takes place. 
     There are many types of junction diodes, which either emphasize a different physical aspect of a diode often by geometric scaling, doping level, choosing the right electrodes, or just in the application of a diode in a special circuit. For example, a Schottky diode is typically fabricated from the contact between a metal and a semiconductor, rather than by a PN junction. A Schottky diode has a potential barrier formed at the metal-semiconductor junction which has rectifying characteristics, suitable for use as a semiconductor diode. 
     Accordingly, the term “semiconductor diode” as used and claimed herein is intended to cover many types of semiconductor diodes, as will become apparent from the following description, when taken in conjunction with the accompanying drawings. 
     In one preferred embodiment, the present invention relates to a beta voltaic or ‘nuclear battery” using an isotope of silicon (Si32) as the source (beta emitter), where the diode itself is made from the isotope. The present invention provides a long term power source for remote power generation of high efficiency and long term operation. 
     In one embodiment, the present invention would make the diode out of an isotope of silicon (silicon-32 or Si32). The diode could be either silicon or silicon carbide. Silicon-32 is a pure beta emitter with no gamma radiation. It has a long half life of about 150 years and decays to phosphorus 32 (another strong beta emitter). Since the silicon-32 is internal to the diode structure, the short range of the beta particles is overcome and a simple planer geometry can be used. The use of silicon-32 vs. the naturally occurring (stable) isotopes of silicon should cause no material difference in the operation of the diode beyond the effects of radioactive decay. 
     In a preferred embodiment, one aspect is to use a radioisotope (beta emitter) within the diode itself rather than applying it to the surface. The energy can be more efficiently harvested since the beta particles are emitted in the active region of the diode. Silicon-32 is one preferred candidate. Silicon-32 is a pure beta emitter, with no gamma rays. Silicon and silicon carbide diodes are made with silicon, therefore no “impurities” need to be added to the diode. Silicon has a 150 year half-life, ensuring commensurate long power output. 
     The simple planer geometry with silicon-32 inside the device would be relatively straightforward to make, by using silicon-32 during manufacturing. The intended uses of such devices are for long missions, using low average power, where it would be difficult to change a traditional battery (such as deep sea, space probes, medical implant, remote location data collection etc.). 
       FIG. 1  is a graph showing the range of high energy electrons in silicon carbide. Note the vertical axis (Y-axis) is shown on a log scale in  FIG. 1  for clarity. 
     For each isotope, the electrons are emitted at different energies. In  FIG. 1 , the average range is for the average-energy electron, the maximum range is the distance traveled for the maximum energy electron. The isotopes Pm 147  and Si 32  have equivalent emitted electron energies and therefore equivalent ranges. These ranges were calculated using data from NIST (National Institute of Standards and Technology), using the continuously slowing down approximation, which includes collisions and bremsstrahlung radiation (which can defined as electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus). 
     Note that beta particle from tritium does not penetrate far into the silicon carbide. Also note the average and maximum ranges in depth shown in silicon carbide shown in  FIG. 1  for Tritium, Nickel 63, and Pm 147/Si 32. In particular, the average-energy electron travel depths for Pm 147 and Si 32 shown in  FIG. 1  are more than 20 microns (on the log scale) and the maximum-energy electron travel depth for Pm 147 and Si 32 shown in  FIG. 1  (again on the log scale) are less than 200 microns in depth. 
       FIG. 2  is a graph illustrating the general problems described above.  FIG. 2  shows a diode  20  with various isotopes coated on the surface  22  (the top layer  22  in the graph of  FIG. 2 ). The diode  20  in  FIG. 2  includes a layer of a p-type region  24 , a layer of an n-type region  25 , and a depletion region  26 . The depth (or width) of the p-type layer  24  and n-type layer  25  are each approximately 200 microns, or more than the maximum energy electron levels for Pm 147 and Si 32 shown in  FIG. 1 . 
     In  FIG. 2 , the range of an average energy electron from each isotope is shown as the square hash pattern (not quite visible for tritium (H3) or nickel 63)—see the previous  FIG. 1  for reference. The range of maximum penetration depth is shown in  FIG. 2  as a diagonal hash mark  28 . Note that for the geometry shown in  FIG. 2 , only promethium 147 penetrates into the depletion region  26  or the active layer of the diode. Also note that roughly half of the emitted electrons are not captured by this geometry shown in  FIG. 2 . 
       FIG. 3  shows a view of a diode  50  of the present invention with the depletion region  52  made from silicon-32 (a radioisotope of silicon). In this geometry shown in  FIG. 3 , the layer  54  of the p-type region and the layer  56  of the n-type region act to slow the emitted electrons down. The square hatch regions  60 ,  62  are the respective stopping ranges for the average energy beta emitted by silicon-32, and the diagonal hatches  64 ,  66  show the range of the maximum energy beta emitted by silicon-32 from depletion region (or layer)  52 . Note how the electrons emitted by silicon-32 are now mostly contained within the diode  50  shown in  FIG. 3 . 
     One advantage of the present invention is that all the emitted electrons start in the active depletion region shown in  FIG. 3 . This means that most of the emitted electrons can be converted to electrical energy. In a standard geometry (isotope coated on a surface), half of the electrons are lost (emitted away from the diode), as seen in  FIG. 2 . Many more electrons do not make it into the active region. This eliminates the need to optimize the surface to volume ratio, as would be required for the structure shown in  FIG. 2 . Note in  FIG. 3 , the emitted electrons are now contained within the diode without extra shielding. 
     For illustrative purposes, the depths (in microns) for the diode device shown in  FIG. 3  are as follows: the p-type region (or layer) is approximately 200 microns in depth (again, more than the maximum energy electron level for Si 32 shown in  FIG. 1 ); the n-type region (or layer) is also approximately 200 microns in depth (also more than the maximum energy electron level for Si 32 shown in  FIG. 1 ); and the depletion region is approximately 100 microns in depth, as shown in  FIG. 3 . 
       FIG. 4  shows another embodiment  70  of the present invention in which an energy converter on the surface of the diode that converts the high energy electrons into photons and with a mirror surface, sends them back into the diode  70  to get converted to electricity as well. A fluorescent coating  74 ,  76  can be added to all the sides of the diode  70 , so that photons are returned into the diode structure  70 , as shown in  FIG. 4 . The Si32 contained inside the diode  70  is a pure beta emitter with a half life of ˜150 years. It is also known that Si 32 =P 32 +e−+v e  and that P 32 =S 32 +e−+v e  (14.2 day half-life). 
       FIG. 5  shows another embodiment of the present invention where the radioisotope is placed outside the depletion region. The semiconductor diode  80  shown in  FIG. 2  includes a depletion region  82 , a p-type layer  83 , and an n-type layer  84 . The depletion region  82  is critical for the functioning of the diode/betavoltaic cell. As shown in  FIG. 5 , one could place the isotope layers  86 ,  87  only outside the critical region  82 , which could increase the operational life of the device. The resulting dimensions would be open to optimization. 
     In the embodiment shown in  FIG. 5 , two isotope layers  86 ,  87  are placed above and below the depletion region  82 . However, one isotope layer could be configured with the present invention (at least one isotope layer would be utilized in such an embodiment). 
       FIG. 6  shows still another embodiment of a semiconductor diode  90  of the present invention, as a further variation of  FIG. 5 . The semiconductor diode  90  of  FIG. 2  further includes scintillator/energy converter layers  91 ,  92 , together with mirror coating layers  94 ,  96 . 
     In  FIG. 6 , the scintillator layers  91 ,  92  could be made from quantum dots, which have a high conversion efficiency. Any scintillator layer that converts the beta particles to light, matched to the band-gap of the diode (such as blue light for silicon carbide) would be suitable. The scintillator layers  91 ,  92  shown in  FIG. 6  converts escaping high energy beta particles (electrons) into light, which is directed back into the depletion region  82 , where it can be converted to an electron-hole pair and give rise to an electric current. A mirror surface layers  94 ,  96  (dielectric mirror tuned to the wavelength of the light emitted by the scintillator) shown in  FIG. 6  reflects the light back into the depletion region  82 . The scintillator, mirror, and p-type region act as radiation shielding as well. 
     The entire device of the present invention could be made using a radioisotope such as silicon-32. Extra shielding for the electrons would be necessary. If a suitable isotope was available, the dopants added to make n or p-type could be radioisotopes. The surface could still be coated with an isotope. If the surface was coated with Pm 147, the device would have high power initially and decay with the 2.62 year half life of Pm 147, then remain powered at a low level for the half-life of silicon-32 (˜150 years). 
     In general, the percent of silicon-32 relative to the stable isotope (silicon-28) could be tailored throughout the diode. The use of silicon-32 in p + n, junctions and Schottky diodes, etc would also be useful. Any diode junction used for generating electric power (photovoltaic) that contains silicon could be made with silicon-32. Note that the main dimensions shown in the figures above are somewhat arbitrary, and are not necessarily shown to scale. 
     It should be understood that silicon-32 could be used as the power source and this would avoid the shallow range of the beta particles in the diode. This eliminates a surface to volume issue during design and manufacturing of such devices. Silicon-32 can be used in just the depletion region and the surrounding layers can then be used to contain the beta particles. 
       FIG. 7  shows several views of a Schottky diode, which is a well known configuration, and with which the features of the present invention can be incorporated.  FIG. 7A  shows a side view of a Schottky diode, with an N-type silicon between a Schottky contact and ohmic contact.  FIG. 7B  shows a perspective view of a Schottky contact and an ohmic contact on a substrate.  FIG. 7C  shows a band diagram of a Schottky diode. 
       FIG. 8  shows a beta voltaic Schottky diode  120  of the present invention, with radioisotope  126  (silicon 32) between Schottky contact  122  and ohmic contact  124 . 
       FIG. 9  shows a top view a metal-semiconductor-metal configuration  130  of radioisotope  136  between Schottky contact  132  and ohmic contact  134 . 
     In  FIGS. 8 and 9 , the energy of the beta particles would excite electrons in the semiconductor (N-type) into the conduction band, where they pass through the electric circuit, generating power. The depth should presumably be contained within the region defined as the depletion region, W, as shown in the energy band diagram of  FIG. 7C . The contact is a Schottky contact if there exists an energy barrier (i.e. Schottky barrier) when the metal is deposited onto the semiconductor. Creating a Schottky barrier is actually much easier than creating an Ohmic contact. With an Ohmic contact the energy level of the metal is chosen to line up precisely with the conduction band or valence band energy levels for n-type and p-type semiconductors respectively. The Schottky contact creates a built-in depletion region just like the diode so they operate in a very similar fashion from that standpoint. 
     From the above description, it is apparent that various techniques may be used for implementing the concepts of the present invention without departing from its scope. The described embodiments are to be considered in all respects as illustrative and not restrictive. The present invention is suitable for use with many types of semiconductor diodes, such as illustrated, for example, in “Diode-Wikipedia, the free encyclopedia”, which is readily accessible via the Internet at http://en.wikipedia.org/wiki/Diode, which shows many types of semiconductor diodes which could be utilized with the present invention. Also see S. M. Sze in “Physics of Semiconductor Devices”, Wiley 2007. It should also be understood that system is not limited to the particular embodiments described herein, but is capable of many embodiments without departing from the scope of the claims.