A trench transistor structure includes a substrate structure, a transistor device, and an electrostatic discharge (ESD) protection device. A first region and a second region are defined in the substrate structure. The substrate structure has a first trench located in the first region and a second trench located in the second region. The transistor device is located in the first region and includes an electrode located in the first trench. The electrode and the substrate structure are isolated from each other. The ESD protection device is located in the second region and includes a main body layer located in the second trench. The main body layer has a planarized top surface. PN junctions are located in the main body layer. The main body layer and the substrate structure are isolated from each other.