Patent Number: 
Section: claims

1. A device for producing electricity, comprising:an indium gallium phosphide semiconductor material comprising a plurality of indium gallium phosphide material layers each layer having different dopant concentrations and doped with either n-type dopants or p-type dopants;a first terminal on a first surface of the semiconductor material;a beta particle source proximate the first surface for emitting beta particles that penetrate into the semiconductor material;a second terminal on a second surface of the semiconductor material;the semiconductor material for producing current between the first and second terminals responsive to the beta particles penetrating into the semiconductor material; anda back surface field reflector layer comprising one of InAIP, InAIGaP and ZnSe. 2. The device of claim 1 wherein the beta particle source comprises one of a tritium metal hydride source or a polymer containing tritium. 3. The device of claim 1 wherein the semiconductor material comprises In0.49 Ga0.51 P. 4. The device of claim 1 wherein a material of the semiconductor layer has a band gap of about 1.9 eV. 5. The device of claim 1 wherein the thickness of the betavoltaic source layer is between about 100 nanometers and about one micron. 6. The device of claim 1 further comprising a physical barrier for shielding outer periphery edges of the semiconductor material from the beta particles, the physical barrier disposed on outer periphery edges of the first surface. 7. The device of claim 6 wherein the barrier comprises perimeter grid lines disposed on the first surface. 8. The device of claim 1 wherein the beta particle source is positioned such that the beta particles from the beta particle source do not impinge cannot outer periphery edges of the semiconductor material. 9. The device of claim 1 further comprising a housing, wherein the device is hermetically sealed within the housing. 10. The device of claim 1 the first terminal comprising a contact disposed at outer peripheral edges of the first surface. 11. The device of claim 1 the first and second surfaces comprising opposing surfaces of the semiconductor material. 12. The device of claim 1 wherein the plurality of indium gallium phosphide material layers comprises a base layer and wherein a bandgap of the back surface field reflector layer is greater than the bandgap of the base layer. 13. The device of claim 1 wherein the plurality of indium gallium phosphide material layers comprises a window layer and an emitter layer. 14. The device of claim 13 wherein the window layer comprises one of a pseudomorphic material, InAIGaP, ZnSe, AlAs and AlAsP. 15. The device of claim 1 further comprising a cap layer atop an uppermost one of the plurality of indium gallium phosphide material layers and a conductor atop regions of the cap layer, the cap layer having a first thickness in the regions below the conductors and a second thickness elsewhere. 16. The device of claim 15 wherein the beta particle source is atop the conductor. 17. The device of claim 15 wherein the beta particle source is atop the cap layer and bounded by the conductor. 18. The device of claim 15 wherein the second thickness is less than about 100 Angstroms. 19. The device of claim 15 wherein a material of the cap layer comprises gallium arsenide.