An array of deep trenches is formed in a doped portion of the semiconductor substrate, which forms a lower electrode. A dielectric layer is formed on the sidewalls of the array of deep trenches. The array of deep trenches is filled with a doped semiconductor material to form an upper electrode comprising a top plate portion and a plurality of extension portions into the array of trenches. In a depletion mode, the bias condition across the dielectric layer depletes majority carriers within the top electrode, thus providing a low capacitance. In an accumulation mode, the bias condition attracts majority carriers toward the dielectric layer, providing a high capacitance. Thus, the trench metal-oxide-semiconductor (MOS) varactor provides a variable capacitance depending on the polarity of the bias.