Physical and electrical bandwidths of integrated photodiodes in standard CMOS technology

The influence of different geometries (layouts) and structures of high-speed photodiodes in fully standard 0.18 μm CMOS technology on their intrinsic (physical) and electrical bandwidths is analyzed. Three photodiode structures are studied: nwell/p-substrate, p+/nwell/p-substrate and p+/nwell. The photodiode bandwidths are compared for λ=650 nm wavelength which is used in today's DVD optical pick-ups. Slow substrate current component limits the intrinsic bandwidth of nwell/p-substrate and p+/nwell/p-substrate photodiodes to 70 MHz and 100 MHz, respectively. The electrical bandwidth of these diodes in combination with typical transimpedance amplifiers, will be larger than the calculated intrinsic bandwidths. Thus, the parasitic diode capacitance has almost no influence on the total bandwidth of both photodiodes. By using only a p+/nwell photodiode (not connecting a substrate), the intrinsic diode bandwidth is 1 GHz. However, the electrical bandwidth limitation of this diode due to its parasitic capacitance is important and can limit the total diode bandwidth which is by approximation the lower of the physical and the electrical bandwidth. The calculated responsivity of p+/nwell photodiode is 10 dB lower than in other two defined diodes structures, requiring higher sensitivity of the subsequent electronic circuitry