Jumping higher the more you hold space!

Hi, im making a fast paced jumping game and thought when I was asleep how I could make it better! (my game dosent need to move left right and forward and backward, just jump) I want someone kind to please help me make a script where when you tap space it makes you jump a small amount but when you hold it you jump higher.

Thanks

Matt

public float jumpHeight;

void Start()
{
    rb = GetComponent<Rigidbody>();
}

void FixedUpdate()
{
if(Input.GetKeyDown(KeyCode.Space)
{
rb.AddForce(Vector3.up * jumpHeight);
}
}