  This paper is devoted to the existence, uniqueness and comparison theorem on
unbounded solutions of a scalar backward stochastic differential equation
(BSDE) whose generator grows (with respect to both unknown variables $y$ and
$z$) in a super-linear way like $|y||\ln |y||^{(\lambda+1/2)\wedge 1}+|z||\ln
|z||^{\lambda}$ for some $\lambda\geq 0$. For the following four different
ranges of the growth power parameter $\lambda$: $\lambda=0$, $\lambda\in
(0,1/2)$, $\lambda=1/2$ and $\lambda>1/2$, we give reasonably weakest possible
different integrability conditions of the terminal value for the existence of
an unbounded solution to the BSDE. In the first two cases, they are stronger
than the $L\ln L$-integrability and weaker than any $L^p$-integrability with
$p>1$; in the third case, the integrability condition is just some
$L^p$-integrability for $p>1$; and in the last case, the integrability
condition is stronger than any $L^p$-integrability with $p>1$ and weaker than
any $\exp(L^\epsilon)$-integrability with $\epsilon\in (0,1)$. We also
establish the comparison theorem, which yields naturally the uniqueness, when
either generator of both BSDEs is convex (concave) in both unknown variables
$(y,z)$, or satisfies a one-sided Osgood condition in the first unknown
variable $y$ and a uniform continuity condition in the second unknown variable
$z$.
