Hall effect is one of the most important effect in physics and especially in semiconductor research. It tells us the most fundamental information of the majority charge carriers in semiconductor: their type (P or N), density (n) and mobility (m). Consider a semiconductor under light illumination such as in solar cell that has both majority and minority carriers. Until recently, we did not know how to extract both majority and minority carrier information simultaneously. We present an exciting discovery of a mathematical identity  in a photo Hall experiment  – hidden for 140 years ever since the original discovery of Hall effect in 1879: 

where Dm is the hole and electron mobility difference, H is the Hall coefficient and s is the conductivity. This equation allows us to extract both majority and minority carrier information simultaneously and unlocks an astonishing array  of semiconductor parameters such as mobility, photo-generated carrier density, recombination lifetime, diffusion lengths and recombination coefficients. The experiment is enabled by another new discovery in electromagnetism: a natural magnetic trap system for ac field generation called parallel dipole line  (PDL) system. We report a collaboration between IBM, KAIST and KRICT that has applied this technique in world champion perovskite and  kesterite solar cell films, demonstrating a new exciting capability of studying semiconductor materials with unprecedented details.