[Overview] [Problem to be Solved] To provide a switching transistor and a semiconductor module having lower distortion generated in a signal. [Solution] A switching transistor including: a channel layer including a compound semiconductor and having sheet electron density equal to or higher than 1.7×1013 cm−2; a barrier layer formed on the channel layer by using a compound semiconductor that is of a different type from the channel layer; a gate electrode provided on the barrier layer; and a source electrode and a drain electrode provided on the barrier layer with the gate electrode interposed between the source electrode and the drain electrode.