Can anybody help me fix this wave spawning script please?

I have this script here for spawning enemies in waves but when you play it, it only lays the number of enemies you enter and then stops, it doesn't start a new wave after that, so if anybody could please fix that and add anything necessary of completing the script so it can project waves of enemies, that would be GREAT. Thankyou.

Here is the script ... #pragma strict

 var spawnPoints : Transform[];
 var enemyPrefabs : GameObject[];
  
 var yieldTimeMin = 2;
 var yieldTimeMax = 5;
  
 static var enemyCounter = 0;
  
 var spawnXOffsetMin = 0;
 var spawnXOffsetMax = 0;
  
 var spawnZOffsetMin = 0;
 var spawnZOffsetMax = 0;
  
 var defaultSpawnNumber = 5;
  
 var waveNumber = 1;
  
 var isSpawning = false;
  
 function SpawnEnemies(wave : int)
 {
         var spawnNum = (defaultSpawnNumber + 5 * (wave - 1));
  
         isSpawning = true;
  
         for(var i = 0; i < spawnNum;i++)
         {
                 yield WaitForSeconds(Random.Range(yieldTimeMin, yieldTimeMax));
  
                 var object : GameObject = enemyPrefabs[Random.Range(0, enemyPrefabs.Length)];
                 var position : Transform = spawnPoints[Random.Range(0, spawnPoints.Length)];
  
                 Instantiate(object, position.position + 
                         Vector3(Random.Range(spawnXOffsetMin, spawnXOffsetMax), 0, 
                                 Random.Range(spawnZOffsetMin, spawnZOffsetMax)), position.rotation);
  
                 enemyCounter++;
         }
  
         isSpawning = false;
 }
  
 function UpdateWave()
 {
         waveNumber++;
         SpawnEnemies(waveNumber);
 }
  
  
 function Start () 
 {
         SpawnEnemies(waveNumber);
 }
  
 function Update () 
 {
         if(enemyCounter == 0 && !isSpawning)
         {
                 UpdateWave();
         }
 }