random place generator

public class enemySpawner : MonoBehaviour

{ public GameObject enemyPrefab;

public float spawnTime;

Vector2 screenBounds;

void Start()
{
    screenBounds = Camera.main.ScreenToWorldPoint(new Vector3(Screen.width, Screen.height, Camera.main.transform.position.z));
    StartCoroutine(alienWave());
}

void enemySpawn()
{
    GameObject a = Instantiate(enemyPrefab) as GameObject;
    a.transform.position = new Vector2(screenBounds.y * 2, Random.Range(-screenBounds.x, screenBounds.x));
}

IEnumerator alienWave()
{
    while (true)
    {
        yield return new WaitForSeconds(spawnTime);
        enemySpawn();
    }
}

}

I keep spawning aliens only on this side. any solutions?[137079-στιγμιοτυπο-οθονης-10.png|137079]

@DCordoba its not the same except instead of spawning on the sides it spawns in the middle. Any other idea?