The field effect transistor

Previous work on field-effect transistors considered the performance of the device when operated with electric fields in the channel below the critical field, E
c
, where the mobility of carriers becomes dependent on field. This work is reviewed and it is shown that, in this range of operation, the frequency cut-off, f, and transconductance, g
m
, of the device increase with increasing values of electric field. New theory is derived for the performance with electric fields greater than E
c
, where the mobility is proportional to E
^−1/2
. It is shown that, although both f and g
m
continue to increase with electric field in this range, the corresponding increase in the power dissipated is so rapid that such designs are unattractive. It is concluded that a good compromise is to operate with the average channel field equal to E
c
. The performance in this particular case is considered in detail and the results summarized in a design nomograph. It is found that f is inversely proportional to the “pinch-off” voltage. The pinch-off voltage cannot, however, be made indefinitely small because the gate junction must be in the saturated condition. A reasonable estimate of the minimum voltage is ½ volt and this leads to a maximum value of f of 1,000 mc/s. A description is given of the fabrication and performance of several field-effect transistors operating in both the constant and non-constant mobility ranges. It is shown that the performance of these units is in agreement with theory. One of these units had a frequency cut-off of 50 mc/s with a transconductance of 1.6 ma/v when operated at 40 volts and 40 ma.