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.
Oki Gunawan is a Research Staff Member at IBM T. J. Watson Research Center, Yoktown Heights, NY, USA. He received his Ph.D. and M.A. from Princeton University in electrical engineering. His research areas are semiconductor technology and physics such as nanowire transistor, solar cell and novel sensors for internet of things (IoT). At Princeton he conducted the early research in the area of valley-based electronics or “valleytronics”, a new semiconductor device technology after electronics (charge-based) and spintronics (spin-based). The study focused on AlAs two-dimensional electron system, a two-valley electron system. At IBM he manages photovoltaics characterization laboratories and led efforts to develop IBM-PVX system, a portfolio of new systems and patents for advanced solar cell characterizations. Some examples of his recent discoveries and inventions are: the “parallel dipole line” (PDL) trap system, a new natural magnetic trap system that can be used for many novel applications such as earthquake sensor that has been recently tested in Italy. Another invention is the IBM PDL Hall system, a unique high sensitivity ac-field Hall system for photovoltaic and semiconductor research. He has authored or co-authored more than 50 technical publications and holds 36 US and international patents, 9 of which have been licensed to external companies