Metal Oxide Semiconductor Field Effect Transistors  (MOSFETs) and Complementary Metal Oxide Semiconductor  (CMOS) Field Effect Transistors (FET) As with FETs, MOSFETs come with N-channels or  P-channels. They can also be constructed as depletion mode  or enhancement mode devices. This is analogous to a switch  being normally open or normally closed. Depletion mode  MOSFETs have an open channel that is restricted or closed  when voltage is applied to the gate (i.e., normally open).  Enhancement mode MOSFETs allow no current to flow at  zero bias but create a channel for current flow when voltage  is applied to the gate (normally closed). No voltage is used  when the MOSFETs are at zero bias. Millions of enhancement  mode MOSFETs are used in the construction of integrated  circuits. They are installed in complimentary pairs such  that when one is open, the other is closed. This basic design  is known as complementary MOSFET (CMOS), which is  the basis for integrated circuit design in nearly all modern  electronics. Through the use of these transistors, digital logic  gates can be formed, and digital circuitry is constructed. FETs are easier to manufacture than bipolar transistors and  have the advantage of staying on once current flow begins  without continuous gate voltage applied. They have higher  impedance than bipolar transistors and operate cooler. This  makes their use ideal for integrated circuits where millions  of FETs may be in use on the same chip. FETs come in  N-channel and P-channel varieties.  One of the materials in a FET or JFET is called the channel. It  is usually the substrate through which the current needing to  be controlled flows from a source terminal to a drain terminal.  The other type of material intrudes into the channel and acts  as the gate. The polarity and amount of voltage applied to  the gate can widen or narrow the channel due to expansion  or shrinking of the depletion area at the junction of the  semiconductors. This increases or decreases the amount of  current that can flow through the channel. Enough reversed  biased voltage can be applied to the gate to prevent the flow  of current through the channel. This allows the FET to act as a  switch. It can also be used as a voltage-controlled resistance. As shown in the triac and the UJT, creative arrangement  of semiconductor material types can yield devices with a  variety of characteristics. The field effect transistor (FET) is  another such device which is commonly used in electronic  circuits. Its N- and P-type material configuration is shown  in  Figure 11-36 . A FET contains only one junction of the  two types of semiconductor material. It is located at the gate  where it contacts the main current carrying portion of the  device. Because of this, when an FET has a PN junction, it is  known as a junction field effect transistor (JFET). All FETs  operate by expanding and contracting the depletion area at  the junction of the semiconductor materials. The basic FET has been modified in numerous ways and  continues to be at the center of faster and smaller electronic  component development. A version of the FET widely used  is the metal oxide semiconductor field effect transistor  (MOSFET). The MOSFET uses a metal gate with a thin  insulating material between the gate and the semiconductor  material. This essentially creates a capacitor at the gate and  eliminates current leakage in this area. Modern versions of  the MOSFET have a silicon dioxide insulating layer and  many have poly-crystalline silicon gates rather than metal,  but the MOSFET name remains and the basic behavioral  characteristic are the same.  [Figure 11-37] 11-17 Figure 11-37.  A MOSFET has a metal gate and an oxide layer between  it and the semiconductor material to prevent current leakage.  Figure 11-36.  The basic structure of a field effect transistor and  its electronic symbol.  P P Source Gate Drain Source Gate Drain Diffused P-type material N-type silicon bar Channel  P substrate Source Oxido layer Gate Body Drain Metal contact N N