SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

362 CHAPTER 8. FIELD EFFECT TRANSISTORS

This gives

Js=8×(300)^2 × 4. 34 × 10 −^14 =3. 125 × 10 −^8 A/cm^2

For thep+-gate we have fromp-ndiode theory (see chapter 4)

J 0 =

eDppn

Lp

This gives

J 0 =

1. 6 × 10 −^19 × 20 × 3. 38 × 10 −^5

10 −^4

=1. 08 × 10 −^18 A/cm^2

We see that the gate current is much smaller for the JFET case. However for the GaAs case

considered here, the MESFET gate current is small enough for most applications.

8.3 CURRENT-VOLTAGE CHARACTERISTICS .................

The MESFET is one of the simplest three terminal devices to fabricate and to conceptually
understand. The most common material used in MESFETs is GaAs. Other compound semicon-
ductors can also be used although is common to use a HFET approach for most materials. It is
important that one have a high resistivity substrate to avoid current flow through the substrate.
This is usually done by impurity doping. These impurities create levels at midgap, pinning the
Fermi level..
We will first present a very simple model for the current-voltage relation in the MESFET.
Then we will describe a more accurate model. However, to obtain realistic results one needs to
use computer simulation tools.
In figure 8.6 we show the device structure along with the band profile under the gate. In
figure 8.7 we show the MESFET cross-section along with the depletion width under the gate
region. In the absence of any bias, there is a uniform depletion region under the gate region, as
shown in figure 8.7a. If the gate bias is made more negative, the depletion width spreads further
into the active region until eventually the channel is completely depleted. Thus, as the gate bias
is increased (to negative values), the total charge available for conduction decreases until the
channel is pinched off.
If the gate bias is fixed and the drain voltage is increased toward positive values, current
starts to flow in the channel. The depletion region now becomes larger near the drain side, as
shown in figure 8.7b. As the drain voltage is increased, the depletion width toward the drain end
starts to increase, since the potential difference between the gate and the drain end of the channel
increases. The channel then starts to pinch off at the drain end. As this happens, the current starts
to saturate. Once the drain voltage reaches a valueVDS(sat)such that the channel pinches off
at the drain end, the current remains essentially constant even as the drain voltage is increased.

8.3.1 TheOhmicRegime.............................

As noted earlier we will start with a very simple model which, though not accurate for mod-
ern short gate devices, provides insight into the device operation. We will use the following