It is key to put this in context, by differentiating between **current gain** in a **"MOSFET device"** and a **"MOSFET amplifier"**.

  1. **You will hardly hear about gain current in a single MOSFET device**, as the MOSFET can be ideally thought as a *Voltage Controlled Current Source (VCCS)*, that is, a huge input impedance and a very low output impedance.

      The fact that **the gate is floating**, electrically isolated from the source and drain makes the input impedance huge for low frequency signals. *Note that this impedance has a relevant capacitive component* (i.e. it is not a purely resistive impedance), which must be taken into account and carefully analyzed *when designing very high speed or RF circuits*.

  2. It is much more common to hear about **gain current in a MOSFET amplifier**, as *the amplifier block will usually have a bias network and/or input impedance adapting stage*. In this context gain current makes a lot of sense and it is usually specified.

      Again, when dealing with RF circuits, it is common to design for input impedances of 50/75 ohms (check http://en.wikipedia.org/wiki/Coaxial_cable#Choice_of_impedance).

Beware of **JFET devices** (http://en.wikipedia.org/wiki/JFET), in which the gate is not a MOS (Metal-Oxide-Semiconductor) structure, but a PN junction, usually reversed biased during normal operation. Hence, JFETs will exhibit lower input impedance than their MOSFET relatives. In short, in JFET devices the input impedance characterization is much more important than in MOSFETs.