A beam of infrared light of wavelength 850 nm carries a power of 1.0 mW onto a silicon photodiode…

Step 1. The photon energy. $E = \dfrac{1240\ \text{eV nm}}{850\ \text{nm}} = 1.46$ eV. This is above $1.12$ eV, so every photon can make a pair.

Step 2. The photon energy in joule. $E = 1.46\ \text{eV} \times 1.602 \times 10^{-19}\ \text{J/eV} = 2.337 \times 10^{-19}\ \text{J}$.

Step 3. Photons each second. $\text{rate} = \dfrac{P}{E} = \dfrac{1.0 \times 10^{-3}\ \text{W}}{2.337 \times 10^{-19}\ \text{J}} = 4.28 \times 10^{15}\ \text{s}^{-1}$.

Step 4. The greatest current. One electron for each photon gives $I = 4.28 \times 10^{15}\ \text{s}^{-1} \times 1.602 \times 10^{-19}\ \text{C} = 0.685$ mA.