Bulk CMOS photodiodes for λ = 400 nm

Abstract

The photodiode bandwidth is a strong function of the wavelength. The previous two chapters assumed λ=850 nm. This chapter presents both time domain and frequency domain analyses of monosilicon photodiodes in a standard 0.18 µm CMOS technology, for λ = 400 nm.

For monosilicon diodes, the maximum calculated intrinsic −3 dB bandwidth is up to 6 GHz at λ = 400 nm; this corresponds to a cut-off frequency of about 4 GHz. The photodiodes designed in twin-well technology have smaller bandwidth because of the limited size of the vertical depletion region. Measurements on p+/nwell/p-substrate photodiode designed in 0.18 µm CMOS, showed that the total diode bandwidth is 1.7 GHz, which was limited by the electrical diode bandwidth in our measurements.