Field effect transistors (FETs) are semiconductor devices used in a wide variety of electronics applications. A FET has three terminals: a source, a drain and a gate. During operation of the FET, current flows between source and drain terminals through a channel region. The gate electrode, positioned between the source and the drain, enables the current through the FET to be controlled based on the strength of the signal applied to the gate. The signal and bias present at the gate, source and drain determines the electric field profile in the channel region between the source and the drain. The performance of the FET, e.g., factors such as current gain, carrier mobility, and transconductance (gm), are determined by the profile of the electric field in the channel region.
In conventional FETs, the strength of the electric field varies over the length of the channel, being typically weaker near the source and stronger near the drain (in depletion mode). A non-uniform field can lead to decreased performance of the FET, because electrons near the source are accelerated slowly due to the relatively weak field in this region. Electrons near the drain may acquire too much energy due to the relatively strong field in this region, possibly causing damage to a gate insulator. An excessively strong electric field in one region can cause mobility degradation, hot electrons and impact ionization, and can generate gate leakage. FIG. 1A illustrates an example of a conventional metal semiconductor field effect transistor (MESFET) having a source 104, drain 106 and gate 108 formed on a substrate 102. FIG. 1B shows a curve 110 that illustrates an example of the magnitude of the electric field E in the channel region while the MESFET is in depletion mode. In this example, the magnitude of the electric field is relatively weak near the source and relatively strong near the drain. A non-uniform electric field, such as that illustrated by curve 110, can lead to decreased carrier mobility, non-linearity and non-constant transconductance.
Various techniques have been used to mitigate the effect of the non-uniform electric field, such as using a lightly-doped drain, delta doping of the channel, or using one or more field plates behind the gate. However, these methods lack flexibility to tailor the field in response to a range of operational voltages on the gate of the FET. Furthermore, no known field effect transistor provides constant transconductance.