Patent Number: 
Section: claims

1. A betavoltaic device, comprising:an N+ doped semiconductor substrate having a top surface and a bottom surface and a thickness tN+ between the top and bottom surfaces, where tN+ is equal to or less than 100 micrometers (μm);an electrically conductive layer disposed immediately adjacent the bottom surface of the substrate;an N− doped epitaxial layer having a top surface, disposed immediately adjacent the top surface of the substrate;a P+ doped epitaxial layer having a top surface, disposed immediately adjacent the top surface of the N− doped epitaxial layer;an ohmic conductive layer having a top surface, disposed immediately adjacent the top surface of the P+ doped epitaxial layer; anda radioisotope layer disposed immediately adjacent the top surface of the ohmic conductive layer,wherein coincident regions of at least a portion of the radioisotope layer, the second electrically conductive layer, the P+ doped epitaxial layer, and the N− doped epitaxial layer, and the N+ doped substrate are etched so as to provide a plurality of devices including a common N+ doped substrate and first electrically conductive layer. 2. The betavoltaic device of claim 1, wherein the N+ doped semiconductor substrate is silicon carbide (SiC). 3. The betavoltaic device of claim 1, wherein the radioisotope layer is 63Ni. 4. The betavoltaic device of claim 1, wherein the radioisotope layer is 147Pm. 5. The betavoltaic device of claim 1, wherein the radioisotope layer is 3H. 6. The betavoltaic device of claim 1, wherein the radioisotope layer has a thickness, tRad, where tRad is equal to or less than the self-absorption thickness of the radioisotope. 7. The betavoltaic device of claim 3, wherein the radioisotope layer has a thickness, tRad, where tRad is equal to or less than about two micrometers. 8. The betavoltaic device of claim 2, wherein the ohmic conductive layer is an aluminum/titanium layer having a thickness tohm, where tohm is equal to about 250 nanometers (nm). 9. The betavoltaic device of claim 8, wherein the aluminum/titanium layer is 90 wt. % Al and 10 wt. % Ti. 10. The betavoltaic device of claim 2, wherein the P+ doped epitaxial layer has a doping concentration equal to or greater than 1019/cm3. 11. The betavoltaic device of claim 2, wherein the P+ doped epitaxial layer has a thickness tP+, where tP+ is equal to or less than 250 nm. 12. The betavoltaic device of claim 2, wherein the N− doped epitaxial layer has a doping concentration equal to or less than 4.6E14/cm3. 13. The betavoltaic device of claim 1, wherein the N− doped epitaxial layer has a thickness tN−, where tN− is equal to or less than the lesser of the diffusion length of the electron-hole pairs and the penetration depth of incident electrons. 14. The betavoltaic device of claim 13, wherein the radioisotope layer is 63Ni and further wherein tN− is less than 3 μm. 15. The betavoltaic device of claim 13, wherein the radioisotope layer is 147Pm and further wherein tN− is equal to or less than 20 μm. 16. The betavoltaic device of claim 1, wherein 2≦tN+≦50 μm. 17. The betavoltaic device of claim 16, wherein 30≦tN+<50 μm. 18. The betavoltaic device of claim 1, wherein the electrically conductive layer has a thickness tec, where tec is equal to or less than 1 μm. 19. The betavoltaic device of claim 2, wherein the electrically conductive layer is nickel. 20. A betavoltaic device, comprising:at least a first and a second of the betavoltaic devices according to claim 1, wherein the at least a first and a second of the betavoltaic devices are disposed in a series stack; anda positive electrode connected to one of a top and a bottom of the stack and a negative electrode connected to one of the bottom and the top of the stack. 21. The betavoltaic device of claim 20, wherein the N+ doped semiconductor substrate is silicon carbide (SiC). 22. The betavoltaic device of claim 20, further comprising an adhesion layer disposed intermediate and contacting the electrically conductive layer of the first betavoltaic device and the radioisotope layer of the second betavoltaic device. 23. The betavoltaic device of claim 22, wherein the adhesion layer is metal. 24. The betavoltaic device of claim 23, wherein the adhesion layer is aluminum. 25. The betavoltaic device of claim 24, wherein the aluminum adhesion layer has a pre-annealing thickness of about 50 nm. 26. A betavoltaic device, comprising:at least a first and a second of the betavoltaic devices according to claim 1, wherein the at least the first and the second of the betavoltaic devices are disposed in an opposing, facing relationship in a parallel stack; anda positive electrode disposed on a side of the stack and connected to the electrically conductive layers in the stack and a negative electrode disposed on the other side of the stack and connected to the ohmic conductive layers of the stack. 27. The betavoltaic device of claim 26, wherein the N+ doped semiconductor substrate is silicon carbide (SiC). 28. The betavoltaic device of claim 26, further comprising an adhesion layer disposed intermediate and contacting the electrically conductive layer of the first betavoltaic device and the radioisotope layer of the second betavoltaic device. 29. The betavoltaic device of claim 28, wherein the adhesion layer is metal. 30. The betavoltaic device of claim 29, wherein the adhesion layer is aluminum. 31. The betavoltaic device of claim 30, wherein the aluminum adhesion layer has a pre-annealing thickness of about 50 nm. 32. A method of making a series-type betavoltaic device, comprising:providing at least a first and a second of the betavoltaic devices according to claim 1;providing a connecting layer intermediate to and contacting the electrically conductive layer of the first betavoltaic device and the radioisotope layer of the second betavoltaic device;stacking the at least first and second betavoltaic devices and the intermediate connecting layer in series;annealing the device at or above the melting temperature of the connecting layer; andproviding a positive and a negative electrode for the device at opposite surfaces, respectively. 33. The method of claim 32, wherein the step of providing an N+ doped semiconductor substrate further comprises providing an N+ doped silicon carbide (SiC) substrate. 34. A method of making a series-type betavoltaic device, comprising:providing at least a first and a second of the betavoltaic devices according to claim 1 in an opposing, facing relationship;providing a connecting layer intermediate to and contacting the electrically conductive layer of the first betavoltaic device and the radioisotope layer of the second betavoltaic device;stacking the at least first and second betavoltaic devices and the intermediate connecting layer in parallel;annealing the device at or above the melting temperature of the connecting layer; andproviding a positive electrode on a side of the stack connected to the electrically conductive layers in the stack and a negative electrode on the other side of the stack connected to the ohmic conductive layers of the stack. 35. The method of claim 34, wherein the step of providing an N+ doped semiconductor substrate further comprises providing an N+ doped silicon carbide (SiC) substrate. 36. A method for making a betavoltaic device, comprising:providing an N+ doped substrate having a thickness that is greater than about 100 μm;providing an N− doped epitaxial layer on a top surface of the substrate;providing a P+ doped epitaxial layer on a top surface of the N− doped epitaxial layer;providing an ohmic conductive layer on a top surface of the P+ doped epitaxial layer;thinning the substrate from a bottom surface thereof to a thickness tN+ that is equal to or less than 100 μm;providing an electrically conductive layer on the bottom surface of the thinned substrate;suitably annealing the device;providing a radioisotope layer on a top surface of the ohmic conductive layer; andetching coincident regions of at least a portion of the radioisotope layer, the second electrically conductive layer, the P+ doped epitaxial layer, N− doped epitaxial layer, and the N+ doped substrate so as to provide a plurality of devices including a common N+ doped substrate and first electrically conductive layer. 37. The method of claim 36, wherein the step of providing an N+ doped semiconductor substrate further comprises providing an N+ doped silicon carbide (SiC) substrate. 38. The method of claim 36, further comprising providing external electrodes for the device. 39. The method of claim 36, further comprising etching the device to provide individual device isolation. 40. The method of claim 36, wherein the step of providing a radioisotope layer further comprises providing a layer of at least one of 63Ni, and 3H. 41. The method of claim 36, wherein the step of providing a radioisotope layer further comprises providing the layer having a thickness tRad that is equal to or less than the self-absorption thickness of the radioisotope. 42. The method of claim 36, wherein the step of providing an ohmic conductive layer comprises providing a suitable metallization layer. 43. The method of claim 37, wherein the step of providing the P+ doped epitaxial layer further comprises providing the layer with a doping concentration equal to or greater than 1019/cm3. 44. The method of claim 43, further comprising providing the P+ doped epitaxial layer having a thickness equal to or less than about 250 nm. 45. The method of claim 37, wherein the step of providing the N− doped epitaxial layer further comprises providing the layer with a doping concentration equal to or less than 4.6E14/cm3. 46. The method of claim 45, further comprising providing the N− doped epitaxial layer having a thickness tN− that is equal to or less than the lesser of the diffusion length of the electron-hole pairs and the penetration depth of incident electrons. 47. The method of claim 36, wherein the step of thinning the substrate further comprises thinning the substrate to a thickness tN+, that is between about 3 to 50 μm. 48. The method of claim 36, wherein the step of thinning the substrate further comprises thinning the substrate to a thickness tN+ that is between about 3 to 30 μm. 49. The method of claim 36, wherein the step of providing an electrically conductive layer further comprises providing the electrically conductive layer having a thickness equal to or less than about 1 μm. 50. The method of claim 37, further comprising providing a layer of Nickel.