NOVELTY - A tunnel diode comprises p-doped semiconductor region (4) and n-doped semiconductor region (5) forming pn-junction (6), where part of pn-junction is formed within nanowire (1).    USE - For oscillators, amplifiers, hetero-junction bipolar transistors, in low power memory cells, multi-junction solar cells, as well as pressure gauges and light emitting diodes.    ADVANTAGE - The tunnel diode provides lower bandgap material that is more susceptible to band bending from doping. The use of material combinations with band alignment supports the creation of tunnel diode by doping, and also helps to choose materials that are highly susceptible to either p- and n-doping in order to fabricate the tunnel diode. The nanowires have narrow lateral size and potentially defect-free epitaxial growth. The bottom-up approach of nanowire growth is easily scalable to smaller diameters and avoids the defects often induced in top-down processes based on etching. The type-II and type-III material combinations have the promise of much better performance due to the reduced tunnel barrier height.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) multi-junction solar cell comprising nanowire that constitutes light absorbing part, where the nanowire comprises first and second semiconductor segment separated by tunnel diode, where first and second semiconductor segment is adapted to absorb light in first and second pre-determined wavelength region of solar spectrum, respectively; and    (2) manufacturing tunnel diode of compound semiconductor material involving providing semiconductor substrate (3) (preferably silicon), and growing nanowire on semiconductor substrate, where pn-junction comprising p-doped and n-doped semiconductor region is formed within the nanowire.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of nanowire tunnel diode in axial configuration.    Nanowire (1)    Semiconductor substrate (3)    P-doped semiconductor region (4)    N-doped semiconductor region (5)    PN-junction (6)