The difficulty in constructing high-quality, high-resolution FPAs derives from the materials used. Whereas visible imagers such as CCD and CMOS image sensors are fabricated from silicon, using mature and well-understood processes, IR sensors must be fabricated from other, more exotic materials because silicon is sensitive only in the visible and near-IR spectra. Infrared-sensitive materials commonly used in IR detector arrays include mercury cadmium telluride (HgCdTe, "MerCad", or "MerCadTel"), indium antimonide (InSb, pronounced "Inns-Bee"), indium gallium arsenide (InGaAs, pronounced "Inn-Gas"), and vanadium(V) oxide (VOx, pronounced "Vox"). A variety of lead salts can also be used, but are less common today. None of these materials can be grown into crystals anywhere near the size of modern silicon crystals, nor do the resulting wafers have nearly the uniformity of silicon. Furthermore, the materials used to construct arrays of IR-sensitive pixels cannot be used to construct the electronics needed to transport the resulting charge, voltage, or resistance of each pixel to the measurement circuitry. This set of functions is implemented on a chip called the multiplexer, or readout integrated circuits (ROIC), and is typically fabricated in silicon using standard CMOS processes. The detector array is then hybridized or bonded to the ROIC, typically using indium bump-bonding, and the resulting assembly is called an FPA.