Is a ball carrier down by contact...

[Kajun Ref N Texas]

Here is the rule from NCAA.

RULE 4
Ball in Play, Dead Ball, Out of Bounds
SECTION 1. Ball in Play—Dead Ball
Ball Declared Dead
ARTICLE 3. A live ball becomes dead and an official shall sound his whistle
or declare it dead:
a. When it goes out of bounds other than a kick that scores a field goal after
touching the uprights or crossbar, when a ball carrier is out of bounds,
or when a ball carrier is so held that his forward progress is stopped.
When in question, the ball is dead (A.R. 4-2-1-II).
b. When any part of the ball carrier’s body, except his hand or foot, touches
the ground or when the ball carrier is tackled or otherwise falls and loses
possession of the ball as he contacts the ground with any part of his
body, except his hand or foot [Exception: The ball remains alive when
an offensive player has simulated a kick or is in position to kick the ball
held for a place kick by a teammate. The ball may be kicked, passed or
advanced by rule] (A.R. 4-1-3-I).