half-yearly vs semiannual

half-yearly and semiannual both are adjectives.

Word	Noun	Adjective	Verb	Adverb
half-yearly	No	Yes	No	Yes
semiannual	No	Yes	No	No

As adjectives, half-yearly and semiannual are synonyms defined as:

Other synonyms of half-yearly include biannual, biyearly.

half-yearly (adjective)	semiannual (adjective)
occurring or payable twice each year	occurring or payable twice each year

half-yearly (adverb)	semiannual (adverb)
every half year, every six months