Describe the different types of FET and JFET transistors.
The concept of a field-effect transistor (FET) is conseptually very simple, and predates even the BJT as a concept. Basicaly, an applied electric field induces excess charge in a channel, which raises the conductivity of the channel and permitts electric current to flow - the "on" state. A n-channel device uses excess electrons, and a p-channel device uses holes.
There are many types of FETs because this concept is not limited to a specific configuration or material; only that an electric field induces excess charge in a well-defined region. For example,
* The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) uses an insulator (typically SiO2) between the gate and the semiconductor substrate.
* The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to separate the gate from the body.
* The MESFET (Metal–Semiconductor Field-Effect Transistor) uses a Schottky barrier to separate the gate from the body.
* HFET (heterostructure FET) uses bandgap engineering in ternary semiconductors to modulate the electrical conductivity between gate and body.
* An ISFET is an Ion-Sensitive Field Effect Transistor used to measure ion concentrations in a solution; when the ion concentration (such as pH) changes, the surface charge on the gate (i.e. applied E-field) changes.
* The chemFET is a specialized FET that acts as a biosensor, by using a gate sensitive to specific chemical reactions to change the surface charge density.