amsmath 말고 다른 패키지를 쓰시면 어떨까요?

\documentclass{article}
\usepackage{mathtools}
\begin{document}
$\xrightarrow{\text{hello}}$
$\xrightarrow[\text{world}]{\text{hello}}$
$\xRightarrow[\text{world}]{\text{hello}}$
$\xrightarrow[g(x)]{f(x)}$
$\xRightarrow[g(x)]{f(x)}$  
\end{document}
% https://tex.stackexchange. com/questions/103988/rightarrow-with-text-above-it

결과