semiannual vs half-yearly

semiannual and half-yearly both are adjectives.

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

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

Other synonyms of semiannual include biannual, biyearly.

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

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