Problem with transparence in quad

Hi.

I have a scene in Unity 4.3 with a Sprite and a camera and i want make a fade effect, to do this, im creating by script a new Quad gameobject and modifying the position, the material and the alpha color of this material.

The Script is:

Code (csharp):

1. void Start ()
2.     {
3.         pixel = GameObject.CreatePrimitive (PrimitiveType.Quad);
4.         pixel.name = "pixel";
5.  
6.         float height = Camera.main.orthographicSize * 2.0f;
7.         float width = height / Screen.height * Screen.width;
8.         pixel.transform.localScale = new Vector3 (width, height, 1);
9.         pixel.transform.position= new Vector3(pixel.transform.position.x,pixel.transform.position.y,-0.1f);
10.  
11.         color.a = 0.5f;
12.         pixel.renderer.material = new Material(Shader.Find("Transparent/Diffuse"));
13.         pixel.renderer.material.color = color;

The result is:


The red background is the Quad and the transparence works but the Sprite is near to camera independently that the Quad position.

somebody can help me?

Thanks!