An optical semiconductor device comprising: a light-emitting semiconductor element having an electrode surface, a light-emitting surface opposite to the electrode surface, a peripheral side surface located between the electrode surface and the light-emitting surface, and a plurality of electrodes each comprising an upper surface and a lower surface opposite to the upper surface, the plurality of electrodes being arranged on the light-emitting semiconductor element such the upper surface of the electrodes contacts the electrode surface of light-emitting semiconductor element; a phosphor layer covering the light emitting surface and the peripheral side surface of the light-emitting semiconductor element, the phosphor layer comprising an upper surface, a lower surface opposite to the upper surface and a peripheral side surface located between the upper surface and the lower surface; a light reflecting layer covering the peripheral side surface of the phosphor layer, the light reflecting layer comprising an upper surface, a lower surface opposite to the upper surface, an inner side surface located between the upper surface and the lower surface, and an outer side surface opposite to the inner side surface, wherein the outer side surface is perpendicular to the upper surface and the lower surface, and the outer side surface of the light reflecting layer is exposed; wherein the upper surface of the phosphor layer and the upper surface of the light reflecting layer are exposed and flush with each other; wherein the lower surface of the light reflecting layer, the lower surface of the phosphor layer, and the lower surface of the plurality of electrodes are exposed and flush with each other; wherein the upper surface of the light reflecting layer has a width W6, the lower surface of the light reflecting layer has a width W5, and the light reflecting layer has a thickness T5; wherein the width of the light reflecting layer is tapered from the lower surface towards the upper surface, such that there is an angle α between the peripheral side surface of the phosphor layer and the outer side surface of the light reflecting layer; wherein W5, T5, α, and W6 comply with the formula: W5 = (T5 × tan α) + W6. a light-emitting semiconductor element having an electrode surface, a light-emitting surface opposite to the electrode surface, a peripheral side surface located between the electrode surface and the light-emitting surface, and an electrode surface, a light-emitting surface opposite to the electrode surface, a peripheral side surface located between the electrode surface and the light-emitting surface, and a plurality of electrodes each comprising an upper surface and a lower surface opposite to the upper surface, the plurality of electrodes being arranged on the light-emitting semiconductor element such the upper surface of the electrodes contacts the electrode surface of light-emitting semiconductor element; a phosphor layer covering the light emitting surface and the peripheral side surface of the light-emitting semiconductor element, the phosphor layer comprising an upper surface, a lower surface opposite to the upper surface and a peripheral side surface located between the upper surface and the lower surface; a light reflecting layer covering the peripheral side surface of the phosphor layer, the light reflecting layer comprising an upper surface, a lower surface opposite to the upper surface, an inner side surface located between the upper surface and the lower surface, and an outer side surface opposite to the inner side surface, wherein the outer side surface is perpendicular to the upper surface and the lower surface, and the outer side surface of the light reflecting layer is exposed; wherein the upper surface of the phosphor layer and the upper surface of the light reflecting layer are exposed and flush with each other; wherein the lower surface of the light reflecting layer, the lower surface of the phosphor layer, and the lower surface of the plurality of electrodes are exposed and flush with each other; wherein the upper surface of the light reflecting layer has a width W6, the lower surface of the light reflecting layer has a width W5, and the light reflecting layer has a thickness T5; wherein the width of the light reflecting layer is tapered from the lower surface towards the upper surface, such that there is an angle α between the peripheral side surface of the phosphor layer and the outer side surface of the light reflecting layer; wherein W5, T5, α, and W6 comply with the formula: W5 = (T5 × tan α) + W6.