helvet loads Adobe's version of Helvetica or, more likely, URW's, if Adobe's isn't available. The family name is
Another similar option is TeX Gyre Heros,
However, none of these offer semi-bold. The series available are: medium condensed, medium, bold condensed and bold.
Medium is just regular - what you get without switching to bold.
Here's a demonstration of the series. Obviously, you would not usually select fonts this way in a document, but it works for demonstration purposes.
Here's a comparison with HelveticaNeue:
Querulous squirrels waved merrily\textbullet
Sphinx of black quartz\textbullet
Flabbergasted eagles juxtaposed eyesores readily\par