8.2. JFET AND MESFET:CHARGE CONTROL 357
Figure 8.1: MESFETs and JFETs are important devices for high-speed, low-noise amplifiers,
D/A and A/D converters, and much “front-end” processing where high speed is critical. These
devices exploit materials, like GaAs, InP, and InGaAs, that have transport properties that are
superior to Si. (Top) A cutoff cross-section of a 0.1μm MESFET. (Bottom) Top view of the
MESFET.
Let us examine a typical JFET or MESFET structure as shown in figure 8.3a. The device is
based on a low-conductivity substrate on which ann-type region is grown to form an “active”
conducting channel of thicknessh. The gate is formed by ap+region (n+region for ap−type
FET) or a Schottky barrier. The source and drain are ohmic contacts. In figure 8.3b we show
a case where the active channel is partially depleted (say at zero gate bias). A negative bias on
the gate reverse biases the gate-ohmic conductor junction (for ann-type device) and alters the
width of the depletion region. This allows the gate to modulate the conductance of the device.
In figure 8.3c we show a case where the channel is completely depleted.
Consider the cases shown in figure 8.4. In figure 8.4a we show the device with a small source-
drain biasVDSand no gate bias. As the gate bias is increased and the gate semiconductor
junction is reverse biased, the current through the channel decreases until eventually the channel
is “pinched off” and there are no free carriers in it. If the gate bias is fixed and the drain bias
is increased, as shown in figure 8.5, the gate semiconductor junction near the drain becomes
more reverse biased. Eventually, the channel is pinched off near the drain side. At this point the
current cannot increase even if the drain voltage is increased. This is called the saturation region.
Once the device reaches saturation, the current in the channel remains more or less unchanged.