What is a semiconductor? by Elmer Epistola : p-n junctions, diodes, transistors

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The p-n Junction

The Junction Diode

The Bipolar Transistor

The MOSFET

The JFET

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Semicon Manufacturing

What is a semiconductor? (Page 3)

The MOS Field Effect Transistor (MOSFET)

The Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) or MOS transistor is a type of transistor that consists of a metal layer, an oxide layer, and a semiconductor layer. The semiconductor layer is usually in the form of single-crystal silicon substrate doped precisely to perform transistor action. The oxide is usually in the form of a silicon dioxide layer that insulates the semiconductor layer from the metal layer. The metal layer is used as contact for providing voltage inputs to the MOS transistor.

The MOS transistor consists of three terminals: a gate, a source, and a drain. These are equivalent to the base, emitter, and collector of a bipolar transistor. The metal layer of the MOS transistor serves as the gate, while the source and drain are fabricated on the silicon substrate.

Like a bipolar transistor, the current flowing through a MOS transistor is controlled by the input at its gate. However, unlike a bipolar transistor which is controlled by the amount of current into its base, a MOS transistor is controlled by the voltage level at its gate.

The source and drain of a MOS transistor are created on the silicon substrate in such a way that they are 'sandwiching' the gate. The source and drain are doped to be of the same material type, which should be different from the doping received by the substrate. A MOS transistor is referred to as a P-channel MOSFET, or PMOS, if the source and drain are p-type, and the substrate is n-type. It is an N-channel MOSFET, or NMOS, if the source and drain are n-type, and the substrate is p-type.

The area under the gate is known as the channel. The conductivity of the channel may be controlled through the voltage level applied to the gate. For instance, in an NMOS, the major carrier is the electron, so the channel becomes more conductive by applying a positive voltage at the gate, which tends to attract more electrons from the substrate into the channel. The layer formed by these attracted electrons is known as the 'inversion layer', since electrons are the minority carriers of the p-substrate.

If the source of the NMOS is more negative than the drain while a sufficiently positive voltage is applied to the gate, current would pass through the transistor. Removing the positive voltage at the gate would significantly decrease the conductivity of the channel, constricting the flow of electrons. A MOS transistor operating in this manner is known as an enhancement-mode MOS transistor, because it is normally open and conducts only when the channel is 'enhanced.' On the other hand, a normally conducting transistor is known as a depletion-mode transistor, since its conduction is controlled by 'depleting' the normally-present channel.

Figure 3. Structure of an Enhancement MOSFET

The MOS transistor is extensively used in digital circuits because it is a very good and efficient switch. It practically consumes no current at the gate because the gate is isolated from the channel by the oxide layer, and the channel conductivity is dependent only on the potential at the gate.

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The Junction Field Effect Transistor (JFET)

The Junction Field Effect Transistor (JFET) is another type of FET, a basic structure of which may consist of a semiconductor bar with ohmic contacts at the end and heavily doped regions on its opposite sides. If the semiconductor bar is made of n-type material, then it is an n-channel JFET. The JFET is p-channel if the bar is made of p-type material. The terminals at the ends of the bar correspond to the source and drain of the JFET. The heavily doped regions on the sides of the bar are connected to serve as the gate of the JFET. Needless to say, the gate regions are doped to be of opposite type with respect to the channel, so that a p-n junction is formed between the channel and the gate regions.

By applying a voltage across the source and the drain of a JFET, current consisting of majority carriers (electrons for an n-channel and holes for a p-channel) is caused to flow through the channel. The current flowing through the channel is controlled by applying a gate voltage V_gs that reverse biases the p-n junction formed by the gate with respect to the source. The higher the Vgs is, the more the p-n junction is reverse-biased, and the wider the depletion region across the channel becomes. The wider depletion region results in a narrower channel, consequently constricting the flow of current through the channel. Varying Vgs therefore varies the current through the channel for any given voltage across the source and the drain.

The JFET structure described above is no longer practical to use because of the difficulty with having to diffuse dopants from two opposite sides of a bar. Most JFETs built onto IC's nowadays involve single-ended geometries that require doping for the gate from only one side of the channel, i.e., the surface of the wafer. This is achieved by building the JFET on an epitaxially grown channel over a doped substrate that acts as the second gate.

The current through the channel of a MOSFET or JFET consists of only the majority carriers, which is why FETs are referred to also as unipolar transistors.

Figure 4. Structure of a single-ended-geometry junction FET

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