A method of forming a three-dimensional memory device, comprising: forming a layer stack over a substrate, wherein the layer stack comprises an alternating stack of insulating layers and sacrificial material layers; forming memory stack structures through the alternating stack (32, 42); forming a backside trench through the alternating stack (32, 42); forming backside recesses by removing the sacrificial material layers selective to the insulating layers; forming electrically conductive layers in the backside recesses; and forming a dielectric isolation structure in drain select level of the three-dimensional memory device after formation of the electrically conductive layers the method further comprising forming an isolation trench through the drain select level, wherein the dielectric isolation structure is formed in the isolation trench, characterized in that: the drain select level comprises an additional electrically conductive layer which comprises a drain select gate which is formed after formation of the electrically conductive layers; the layer stack further comprises an additional material layer overlying the alternating stack; the additional electrically conductive layer is formed by modifying the additional material layer by introducing a conductive material through the isolation trench, the additional material layer comprises a material different from materials of the insulating layers and the sacrificial material layers; and the additional material layer is modified after formation of the electrically conductive layers, and wherein: the additional material layer comprises a semiconductor material; the additional electrically conductive layer is a metal-semiconductor alloy layer; the conductive material is a metal; and the additional material layer is modified into the metal-semiconductor alloy layer by reacting with the metal deposited on the additional material layer through the isolation trench. forming a layer stack over a substrate, wherein the layer stack comprises an alternating stack of insulating layers and sacrificial material layers; forming memory stack structures through the alternating stack (32, 42); forming a backside trench through the alternating stack (32, 42); forming backside recesses by removing the sacrificial material layers selective to the insulating layers; forming electrically conductive layers in the backside recesses; and forming a dielectric isolation structure in drain select level of the three-dimensional memory device after formation of the electrically conductive layers the method further comprising forming an isolation trench through the drain select level, wherein the dielectric isolation structure is formed in the isolation trench, characterized in that: the drain select level comprises an additional electrically conductive layer which comprises a drain select gate which is formed after formation of the electrically conductive layers; the layer stack further comprises an additional material layer overlying the alternating stack; the additional electrically conductive layer is formed by modifying the additional material layer by introducing a conductive material through the isolation trench, the additional material layer comprises a material different from materials of the insulating layers and the sacrificial material layers; and the additional material layer is modified after formation of the electrically conductive layers, and wherein: the additional material layer comprises a semiconductor material; the additional electrically conductive layer is a metal-semiconductor alloy layer; the conductive material is a metal; and the additional material layer is modified into the metal-semiconductor alloy layer by reacting with the metal deposited on the additional material layer through the isolation trench.