This is the superellipse. For the curves you want, this can be written as
$$ y=(1-(1-x)^{1/p})^p,\quad x\in[0,1] $$
I believe my $p$ is your $\alpha$, but check. My curves look as shown below,
^{1/p})^p,\quad x\in[0,1] $$
I believe my $p$ is your $\alpha$, but check. My curves look as shown below,
![Superellipse variation](