In this paper, an ultra-compact single-chip solar energy harvesting IC using on-chip solar cell for biomedical implant applications is presented. By employing an on-chip charge pump with parallel connected photodiodes, a 3.5x efficiency improvement can be achieved when compared with the conventional stacked photodiode approach to boost the harvested voltage while preserving a single-chip solution. A photodiode-assisted dual startup circuit (PDSC) is also proposed to improve the area efficiency and increase the startup speed by 77%. By employing an auxiliary charge pump (AQP) using zero threshold voltage (ZVT) devices in parallel with the main charge pump, a low startup voltage of 0.25 V is obtained while minimizing the reversion loss. A 4 V-in gate drive voltage is utilized to reduce the conduction loss. Systematic charge pump and solar cell area optimization is also introduced to improve the energy harvesting efficiency. The proposed system is implemented in a standard 0.18-mu m CMOS technology and occupies an active area of 1.54 mm(2). Measurement results show that the on-chip charge pump can achieve a maximum efficiency of 67%. With an incident power of 1.22 mW/cm(2) from a halogen light source, the proposed energy harvesting IC can deliver an output power of 1.65 mu W at 64% charge pump efficiency. The chip prototype is also verified using in-vitro experiment.