Curable organopolysiloxane composition, encapsulant and semiconductor device

A curable organopolysiloxane composition, comprising:

• (A) a branched organopolysiloxane having at least one silicon-bonded alkenyl group and at least one silicon-bonded aryl group per molecule, and having siloxane units of the formula:
• RSiO3/2 where R is a substituted or unsubstituted monovalent hydrocarbon group;
• (B) a linear organopolysiloxane with both terminal ends of the linear organopolysiloane bearing silicon-bonded hydrogen atoms and having at least one silicon-bonded aryl group per molecule;
• (C) a hydrosilylation catalyst;
• and,
• (D) at least 1 parts by weight, relative to 100 parts by weight of the total of components (A) and (B), of a low molecular weight siloxane having at least one silicon-bonded alkenyl group per molecule, represented by the average formula
• (R53SiO1/2)f(R52SiO2/2)g(R5SiO3/2)h(SiO4/2)i
• where each R5 can be the same or different and is independently selected from a substituted or unsubstituted monovalent hydrocarbon group, wherein at least one of R5 per molecule is an alkenyl group, with the proviso that the ratio between alkenyl groups and silicon atoms is from 0.3 to 1,
• f, g, h, and i are independently 0 or positive, and such that
• the weight average molecular weight Mw of the siloxane, as determined by Size Exclusion Chromatography, is less than 1,000 g/mol, and
• wherein component (D) is selected from the group consisting of DVi4, MVi4Q, MVi6Q2, MVi3T, and mixtures thereof, and
• wherein the curable organopolysiloxane composition does not comprise a linear organopolysiloxane having at least two silicon-bonded alkenyl groups and at least one silicon-bonded aryl group per molecule.