The Transistor

A transistor is a device which allows the use of one electrical signal to be used to control another (Lesurf, 2000). A transistor is named from a combination of the words "transfer" and "resistor" and the name indicates how the device works. There are many kinds of transistors, and this paper will look at one called a Junction Field Effect Transistor, or J-FET. A J-FET is a voltage-controlled device, and a small change in input causes a large change in output current (American Semiconductor, 2000).

A typical J-FET transistor is made of a body of semiconductor material which includes a first layer of semiconductor material and a second layer of semiconductor material of one conductivity type of relatively low resistivity contiguous with the fist layer (Delphion, 2000). The first layer has a surface at the outside of the transistor. Several parallel barriers of nonconductive protective material extend into the first layer from the surface. Alternate portions of the first layer between adjacent barriers are the gate zones of the transistor, and the intervening portions of the first layer between adjacent barriers are source zones. A strip of semiconductor protective material underlies each gate zone.

Each strip has edges underlying the adjacent barrier of nonconductive protective material, and is spaced from the adjacent barriers. Several gate regions of the opposite conductivity type are located in the first layer. Each gate region extends from the surface of the firs

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t will be possible for channel electrons to cross the walls and move into the substrate. If this happens, the electrons will not move from the source to the drain, and so the current won't flow. Also, modern FETs have such small gates that even a small channel-gate current will blow up the transistor. The basic properties of a typical J-FET are shown by the curves in figure 2. The curves in the diagram are for J-FETs designed to work with small signals. The curves on the left side of the diagram show how the drain-source current Ids varies with the applied drain-source voltage Vds. The diagram shows curves for four different gate-source voltages. The curves show that if the drain-source voltage is above a minimum value of about two volts, the current doesn't depend on the actual drain-source voltage. When it is below this value, the current does vary with the drain-source voltage. Once the voltage is big enough, the current is almost entirely controlled by the gate-source voltage. This is demonstrated in the diagrams in figure 3 (this page and following page). With a steady gate-source voltage of one volt, there is always one volt across the wall of the channel at the source end. If the drain-source voltage

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