Vector3.back not working

Hi, I’m doing a 2D arcade game for Android devices and I’m having an issue with Vector3.back.
What I’m trying to achive is that the player avatar moves to the part of the screen that the player had touched. Vector3.left, .right and .forward are working nice but when I use Vector3.back or -Vector3.forward the object doesn’t moves to the touched position.
The script I’m using is the following one (if someone knows a better and cheaper way to do this I’d be pleased to know :D):

	if (mobileControls == true) {

		if(Input.touchCount > 0 && Input.GetTouch(0).phase == TouchPhase.Stationary) {

			double halfXScreen = Screen.width / 2.0;
			double halfZScreen = Screen.height / 2.0;
			Vector3 touchPosition = Input.GetTouch(0).position;

			if(touchPosition.x < halfXScreen) { //works nice
				gameObject.transform.Translate(Vector3.left * speed * Time.deltaTime);
			else if (touchPosition.x > halfXScreen) { //works nice
				gameObject.transform.Translate(Vector3.right * speed* Time.deltaTime);

			if(touchPosition.z < halfZScreen) { //works nice
				gameObject.transform.Translate(Vector3.forward * speed * Time.deltaTime);
			else if (touchPosition.z > halfZScreen) { //doesn't works :(
				gameObject.transform.Translate(Vector3.back * speed * Time.deltaTime);

I’m using Unity 5.4.1f1 Personal Edition (64 bits)

Input.GetTouch(0).position gives you a Vector2. With Vector3 touchPosition = Input.GetTouch(0).position; you turn it into Vector3, where the z-component of the vector will always be 0. So touchPosition.z < halfZScreen is always true and touchPosition.z > halfZScreen always false and therefor the translation in Vector3.back direction is never called. Use touchPosition.y instead of touchPosition.z and see if it works for you.

It works!!! Thank you very much saschandroid! Do you know a cheaper way to doo the same or how can I use the default movement inputs (horizontal and vertical) to do the same?