How to smoothly add force to a rigidbody?

Okay, so I've made a bouncy platform in my 2D platformer that bounces the player up when they jump on it. The problem is when the player jumps on the platform, it adds the force instantly and it looks like the player just teleports upwards very quickly. Is there any way to make it add force smoothly so it doesn't look so unnatural?

Here's my code:

 var player : GameObject;
 player = GameObject.Find("Player");
 
 function OnTriggerEnter(Trigger : Collider)
 {
  if (Trigger.gameObject.tag == "Player") // if player collides
  {
  Debug.Log ("Trampoline collision detected"); // print that it's detected 
  player.rigidbody.AddForce(0, 20000, 0); // add force up on player
  }
 }
 
 function OnTriggerExit(Trigger : Collider) 
 {
  if (Trigger.gameObject.tag == "Player") // if player leaves collider
  {
  player.GetComponent(Rigidbody).isKinematic = true; // remove player's rigidbody
  yield WaitForSeconds (0.3); // wait 0.3 seconds
  player.GetComponent(Rigidbody).isKinematic = false; // re-add player's rigibody
  }
 
 }