NOVELTY - The monolithic, quadruple two-junction tandem solar cell (10) comprises a first cell comprising germanium (Ge) substrate (15) with a bandgap of 0.67 eV, a second cell comprising gallium arsenide (GaAs) and gallium phosphide (GaP), where the GaAs and GaP are isoelectronically co-doped with an acceptor element and a donor element to have an effective bandgap of 1.05 eV and 1.75 eV on the first cell respectively, a third cell comprising GaAs with a bandgap of 1.42 eV on the second cell, and a fourth cell comprising indium gallium phosphide (InGaP) with a bandgap of 1.90 eV on the third cell.    USE - The monolithic, quadruple two-junction tandem solar cell is useful in a heterojunction bipolar transistor device (claimed), spintronics and near-infrared devices, where the solar cell is thermophotovoltaic cells.    ADVANTAGE - The monolithic, quadruple two-junction tandem solar cell can be efficiently manufactured with increased carrier mobility, long lifetime and high quality.    DETAILED DESCRIPTION - The monolithic, quadruple two-junction tandem solar cell (10) comprises a first cell comprising germanium (Ge) substrate (15) with a bandgap of 0.67 eV, a second cell comprising gallium arsenide (GaAs) and gallium phosphide (GaP), where the GaAs and GaP are isoelectronically co-doped with an acceptor element and a donor element to have an effective bandgap of 1.05 eV and 1.75 eV on the first cell respectively, a third cell comprising GaAs with a bandgap of 1.42 eV on the second cell, a fourth cell comprising indium gallium phosphide (InGaP) with a bandgap of 1.90 eV on the third cell, a p++-n++ charge-doped Ge tunnel junction between the first and second cells, a p++-n++ charge-doped tunnel junction of isoelectronically co-doped GaAs and GaP between the second and third cells respectively, a p++-n++ charge-doped GaAs tunnel junction between the third and fourth cells, and a first conductive contact on the substrate and a second conductive contact on the third and fourth cells. The acceptor element is boron (B) and the donor element is bismuth (Bi) to form GaAs:Bi:B crystal lattice and GaP:Bi:B crystal lattice that is lattice matched to the substrate. The Ge first-, GaAs third- and InGaP fourth-cells have a charge-doped p-n junction (21). The isoelectronically co-doped GaAs and GaP second cell have the charge-doped p-n junction. The p-n junctions comprising the second and third cells are sandwiched between back surface reflectors (BSR) layers (26, 27, 28, 29, 30), where each BSR layers has a higher bandgap than the respective p-n junction located between the second and third cells. The p-n junction of the fourth cell is sandwiched between an n-type aluminum indium phosphide (AlInP) window layer (25) and the BSR layer. A content of the: B in the GaAs:B:Bi crystal lattices is 2 at.% B; Bi in the GaAs:B:Bi crystal lattice is 3.8 at.% Bi; GaP:B:Bi crystal lattice is 5 at.% B and 2.2 at.% Bi; and other GaP:B:Bi crystal lattice of the third cell is 7 at.% B and 4.5 at.% Bi. The first cell further comprises a first charge-doped p-n junction between the GaP window layer and the BSR layer of GaP:B:Bi. The second cell is adjacent to the first cell, and further comprises a second charge-doped p-n junction. The BSR layer of GaP;B:Bi has a higher bandgap than the first p-n junction. The third cell is adjacent to the second cell, and further comprises GaP that is isoelectronically co-doped with an acceptor element and a deep donor element to have an effective bandgap of about 2.05 eV. The second p-n junction of the second cell is sandwiched between BSR layers. The third p-n junction of the third cell is sandwiched between the window layer and a particular BSR layer. An INDEPENDENT CLAIM is included for a heterojunction bipolar transistor device.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-section view of a monolithic, quadruple junction solar cell.    Solar cell (10)    Germanium cell (11)    Germanium substrate (15)    P-N junction (21)    Window layer (25)    Back surface reflectors layers. (26, 27, 28, 29, 30)