Often they disambiguate an ideogram by spelling out the first or last syllable of the word; occasionally (as in Linear B) they may instead abbreviate an adjective that modifies the logogram.
For example, the kanji 生, pronounced shō or sei in borrowed Chinese vocabulary, stands for several native Japanese words as well.
As such, it may be written 生まれる [生mareru], reflecting its derivation, or 生れる [生reru], as with other verbs ending in elidable -ru.
Instead, about 90% of Chinese characters are compounds of a determinative (called a 'radical'), which may not exist independently, and a phonetic complement indicates the approximate pronunciation of the morpheme.
However, the phonetic element is basic, and these might be better thought of as characters used for multiple near homonyms, the identity of which is constrained by the determiner.
A handful of Korean gukja are also constructed as phono-semantic characters, such as 乭 (pronounced as 돌, dol) whose phonetic complement is the bottom 乙.
Some of Japanese Kokuji are phono-semantic characters, like 働, 腺, 鑓, whose phonetic complement is 動, 泉, 遣 respectively.