Character jump is not smooth. ( Fixed )

Unity Engine
1

Hello, i’ve just started scripting in c#, i’ve managed to create a Character movement script, but my jump isn’t really smooth, it just seems like it transports in the air, and falls down with gravity.
How can i make him go smooth? so it goes smoothly up and down?
This is my script:

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {

    public float playerSpeed = 5.0f;
    public float jumpheight = 5.0f;
    public bool isFalling = false;

    // Use this for initialization
    void Start () {
   
    }
   
    // Update is called once per frame
    void Update () {

    if (Input.GetKey (KeyCode.D)) {
            transform.Translate (Vector3.right * playerSpeed * Time.deltaTime);
        }
    if (Input.GetKey (KeyCode.A)) {
            transform.Translate (Vector3.left * playerSpeed * Time.deltaTime);
        }
    if (Input.GetKeyDown (KeyCode.Space) && isFalling == false) {
            isFalling = true;
            transform.Translate (Vector3.up * jumpheight * Time.deltaTime);
        }
   
    }

    void OnCollisionEnter (Collision col)
    {
        isFalling = false; }

}

Thanks in advance!

2

It is fixed, the new working code for the people who need it:

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {

    public float playerSpeed = 5.0f;
    public float jumpheight = 5.0f;
    public bool isFalling = false;

    // Use this for initialization
    void Start () {
   
    }
   
    // Update is called once per frame
    void Update () {

    if (Input.GetKey (KeyCode.D)) {
            transform.Translate (Vector3.right * playerSpeed * Time.deltaTime);
        }
    if (Input.GetKey (KeyCode.A)) {
            transform.Translate (Vector3.left * playerSpeed * Time.deltaTime);
        }
    if (Input.GetKeyDown (KeyCode.Space) && isFalling == false) {
            isFalling = true;
            //transform.Translate (Vector3.up * jumpheight * Time.deltaTime);
            GetComponent<Rigidbody>().AddForce(Vector3.up * jumpheight, ForceMode.Impulse);
        }
   
    }

    void OnCollisionEnter (Collision col)
    {
        isFalling = false; }

}