By the nature of its construction, a MOSFET has a built-in anti-parallel diode – this is refered to as the intrinsic “body diode”. It is generally very slow, unless the MOSFET is optimized to have a fast body diode, which unfortunately often has the side effect of increasing the Rds(on) of the MOSFET.
It is usually easier to find fast body diodes in lower voltage MOSFETs than in higher voltage ones, so the body diode tends to be less of an issue with lower power amplifiers (i.e. lower voltage) than with higher power amplifiers (i.e. higher voltage). IXYS has a notable exception with their “PolarHV™ HiPerFETs” – a series of high voltage MOSFETs with fast intrinsic body diodes.
This article explores the issue of the slow body diode in greater detail and also gives some example means for dealing with it.