A field effect transistor has the property that the product of its series resistance and its true transconductance is less than one throughout the entire range of drain voltage in the operative state of the transistor, the series resistance being the sum of the resistance from source to channel and the resistance of this channel. In order to prevent an excessive increase in the active resistance of the channel, the channel is made to have an impurity concentration as low as less than 10.sup.15 atoms/cm.sup.3, preferably less than 10.sup.14 atoms/cm.sup.3, so that the depletion layers extending from the gates grow extensively to become contiguous in response to a small increase in the reverse gate voltage applied. As a result, the field effect transistor of this invention has an unsaturated drain current versus drain voltage characteristic.