A Compound semiconductor is composed of elements from two or more different groups of the chemical periodic table, e.g. (1) Group III (B, Al, Ga, In) and Group V (N, P, As, Sb, Bi) for the compounds AlN, AlP, AlAs, GaN, GaP, GaAs, InP, InAs, InSb, AlInGaP, AlGaAs etc, or (2) the compounds of Group II (Zn, Cd, Hg) and Group VI (O, S, Se, Te) such as ZnS, ZnSe, ZnTe, CdTe, HgTe, CdHgTe etc. Although Si and Ge are elemental (or element) semiconductors, it's worthwhile to note that some Si-based semiconductors that are formed by two elements from the same group (e.g. SiC and SiGe) are also termed as "Compound Semiconductor" in the literature. ([1]).