There is a large degree of freedom in the articulation of open vowels and it is certainly possible to imagine vowels even more open than [a], for example (although such vowels would probably not be phonologically distinct from the "standard" open vowels of the fourth degree of aperture.
On the other hand, the a vowel could not be much more close than the "standard" close vowels (first degree of aperture) and still be vocalic, rather than some kind of spirant or fricative consonant, depending on the degree of muscular tension.
Additionally, the close vowels must have a certain minimum duration in order to be perceived as vocalic rather than consonantal (fricative or spirant).
Semi-vowel is the term for those sounds which, though articulated quite variously, in one way or another fail to meet these above criteria for full vowel-hood.
Examples of Semi-Vowels
You can hear each sound by clicking on his IPA transcription.
Voiceless palatal fricative (semivowel). This sound is produced by articulating a very close unrounded front "vowel" (a close [i]), but without vibration of the vocal cords. The corresponding consonant is the voiceless palatal fricative (hisser).
Voiced palatal fricative (semivowel). This sound is also produced by articulating a very close unrounded front "vowel" (a close [i]), but the realization is extremely brief. The corresponding consonant is the voiced palatal fricative (hisser).
Voiceless labio-velar fricative (semivowel). This sound is produced by articulating a very close rounded back "vowel" (a close [u]), but without vibration of the vocal cords.
Voiced labio-velar fricative (semivowel). This sound is also produced by articulating a very close rounded back "vowel" (a close [u]), but the realization is extremely brief.
Voiced labio-palatal fricative (semivowel). This sound is produced by articulating a very close rounded front "vowel" (a close [y]), but the articulation is extremely brief.
TOP ^
