Solid-state materials are commonly grouped into three classes: insulators (insulator), semiconductors, and conductors. (At low temperatures some conductors, semiconductors, and insulators may become superconductors.) Figure 1--> shows the conductivities σ (and the corresponding resistivities ρ = 1/σ) that are associated with some important materials in each of the three classes. Insulators, such as fused quartz and glass, have very low conductivities, on the order of 10⁻¹⁸ to 10⁻¹⁰ siemens per centimetre; and conductors, such as aluminum, have high conductivities, typically from 10⁴ to 10⁶ siemens per centimetre. The conductivities of semiconductors are between these extremes.

The conductivity of a semiconductor is generally sensitive to temperature, illumination, magnetic fields, and minute amounts of impurity atoms. For example, the addition of less than 0.01 percent of a particular type of impurity can increase the electrical conductivity of a semiconductor by four or more orders of magnitude (i.e., 10,000 times). The ranges of semiconductor conductivity due to impurity atoms for five common semiconductors are given in Figure 1-->.

Portion of the periodic table of elements related to semiconductorsThe study of semiconductor materials began in the early 19th century. Over the years, many semiconductors have been investigated. The table (Portion of the periodic table of elements related to semiconductors) shows a portion of the periodic table related to semiconductors. The elemental (chemical element) semiconductors are those composed of single species of atoms, such as silicon (Si), germanium (Ge), and gray tin (Sn) in column IV and selenium (Se) and tellurium (Te) in column VI. There are, however, numerous compound semiconductors that are composed of two or more elements. Gallium arsenide (GaAs), for example, is a binary III-V compound, which is a combination of gallium (Ga) from column III and arsenic (As) from column V.

The semiconductor materials treated here are single crystals (crystal)—i.e., the atoms are arranged in a three-dimensional periodic fashion. Figure 2A--> shows a simplified two-dimensional representation of an intrinsic silicon crystal that is very pure and contains a negligibly small amount of impurities. Each silicon atom in the crystal is surrounded by four of its nearest neighbours. Each atom has four electrons in its outer orbit and shares these electrons with its four neighbours. Each shared electron pair constitutes a covalent bond. The force of attraction for the electrons by both nuclei holds the two atoms together.

Electrical conduction in intrinsic semiconductors is quite poor at room temperature. To produce higher conduction, one can intentionally introduce impurities (typically to a concentration of one part per million host atoms). This is the so-called doping process. For example, when a silicon atom is replaced by an atom with five outer electrons such as arsenic (Figure 2C-->), four of the electrons form covalent bonds with the four neighbouring silicon atoms. The fifth electron becomes a conduction electron that is “donated” to the conduction band. The silicon becomes an n-type semiconductor because of the addition of the electron. The arsenic atom is the donor. Similarly, Figure 2C--> shows that, when an atom with three outer electrons such as boron is substituted for a silicon atom, an additional electron is “accepted” to form four covalent bonds around the boron atom, and a positively charged hole is created in the valence band. This is a p-type semiconductor, with the boron constituting an acceptor.

If an abrupt change in impurity type from acceptors (p-type) to donors (n-type) occurs within a single crystal structure, a p-n junction is formed (see Figure 3B--> and 3C-->). On the p side, the holes constitute the dominant carriers and so are called majority carriers. A few thermally generated electrons will also exist in the p side; these are termed minority carriers. On the n side the electrons are the majority carriers, while the holes are the minority carriers. Near the junction is a region having no free-charge carriers. This region, called the depletion layer, behaves as an insulator.

The most important characteristic of p-n junctions is that they rectify; that is to say, they allow current to flow easily in only one direction. Figure 3A--> shows the current-voltage characteristics of a typical silicon p-n junction. When a forward bias is applied to the p-n junction (i.e., a positive voltage applied to the p-side with respect to the n-side, as shown in Figure 3B-->), the majority charge carriers move across the junction so that a large current can flow. However, when a reverse bias is applied (in Figure 3C-->), the charge carriers introduced by the impurities move in opposite directions away from the junction, and only a small leakage current flows initially. As the reverse bias is increased, the current remains very small until a critical voltage is reached, at which point the current suddenly increases. This sudden increase in current is referred to as the junction breakdown, usually a nondestructive phenomenon if the resulting power dissipation is limited to a safe value. The applied forward voltage is usually less than one volt, but the reverse critical voltage, called the breakdown voltage, can vary from less than one volt to many thousands of volts, depending on the impurity concentration of the junction and other device parameters.

A perspective view of a silicon p-n-p bipolar transistor is shown in Figure 4A-->. Basically the bipolar transistor is fabricated by first forming an n-type region in the p-type substrate; subsequently a p⁺ region (very heavily doped p-type) is formed in the n region. Ohmic contacts are made to the top p⁺ and n regions through the windows opened in the oxide layer (an insulator) and to the p region at the bottom.

An idealized, one-dimensional structure of the bipolar transistor, shown in Figure 4B-->, can be considered as a section of the device along the dashed lines in Figure 4A-->. The heavily doped p⁺ region is called the emitter, the narrow central n region is the base, and the p region is the collector. The circuit arrangement in Figure 4B--> is known as a common-base configuration. The arrows indicate the directions of current flow under normal operating conditions—namely, the emitter-base junction is forward-biased and the base-collector junction is reverse-biased. The complementary structure of the p-n-p bipolar transistor is the n-p-n bipolar transistor, which is obtained by interchanging p for n and n for p in Figure 4A-->. The current flow and voltage polarity are all reversed. The circuit symbols for p-n-p and n-p-n transistors are given in Figure 4C-->.

The current gain for the common-base configuration is defined as the change in collector current divided by the change in emitter current when the base-to-collector voltage is constant. Typical common-base current gain in a well-designed bipolar transistor is very close to unity. The most useful amplifier circuit is the common-emitter configuration, as shown in Figure 5A-->, in which a small change in the input current to the base requires little power but can result in much greater current in the output circuit. A typical output current-voltage characteristic for the common-emitter configuration is shown in Figure 5B-->, where the collector current I_C is plotted against the emitter-collector voltage V_EC for various base currents. A numerical example is provided using Figure 5B-->. If V_EC is fixed at five volts and the base current I_B is varied from 10 to 15 microamperes (μA; 1 μA = 10⁻⁶ A), the collector current I_C will change from about four to six milliamperes (mA; 1 mA = 10⁻³ A), as can be read from the left axis. Therefore, an increment of 5 μA in the input-base current gives rise to an increment of 2 mA in the output circuit—an increase of 400 times, with the input signal thus being substantially amplified. In addition to their use as amplifiers, bipolar transistors are key components for oscillators and pulse and analog circuits, as well as for high-speed integrated circuits. There are more than 45,000 types of bipolar transistors for low-frequency operation, with power outputs up to 3,000 watts and a current rating of more than 1,000 amperes. At microwave frequencies, bipolar transistors have power outputs of more than 200 watts at 1 gigahertz and about 10 watts at 10 gigahertz.

Figure 6A--> provides a perspective view of a thyristor structure. An n-type wafer is generally chosen as the starting material. Then, a diffusion step is used to form the p1 and p2 layers simultaneously by diffusing the wafer from both sides. (Diffusion is the movement of impurity atoms into the crystalline structure of a semiconductor.) Finally, n-type impurity atoms are diffused through a ring-shaped window in an oxide into the p2 region to form the n2 layer.

A cross section of the thyristor along the dashed lines is shown in Figure 6B-->. The thyristor is a four-layer p-n-p-n diode with three p-n junctions in series. The contact electrode to the outer p layer (p1) is called the anode, and that to the outer n layer (n2) is designated the cathode. An additional electrode, known as the gate electrode, is connected to the inner p layer (p2).

The basic current-voltage characteristic of a thyristor is illustrated in Figure 6C-->. It exhibits three distinct regions: the forward-blocking (or off) state, the forward-conducting (or on) state, and the reverse-blocking state, which is similar to that of a reverse-biased p-n junction. Thus, a thyristor operated in the forward region is a bistable device that can switch from a high-resistance, low-current off state to a low-resistance, high-current on state, or vice versa.

In the forward off state, most of the voltage drops across the centre n1-p2 junction, while in the forward on state all three junctions are forward-biased. The forward current-voltage characteristic can be explained using the method of a two-transistor analog—that is, to consider the device as a p-n-p transistor and an n-p-n transistor connected with the base of one transistor (n1) attached to the collector of the other. As the voltage V_AK in Figure 6C--> increases from zero, the current I_A will increase. This in turn causes the current gains of both transistors to increase. Because of the regenerative nature of these processes, switching eventually occurs, and the device is in its on state. The maximum forward voltage that can be applied to the device prior to switching is called the forward-breakover voltage V_BF. The magnitude of V_BF depends on the gate current. Higher gate currents cause the current I_A to increase faster, enhance the regeneration process, and switch at lower breakover voltages. The effect of gate current on the switching behaviour is shown in Figure 6C--> (dotted line).

A perspective view of a MESFET is given in Figure 7A-->. It consists of a conductive channel with two ohmic contacts, one acting as the source and the other as the drain. The conductive channel is formed in a thin n-type layer supported by a high-resistivity semi-insulating (nonconducting) substrate. When a positive voltage is applied to the drain with respect to the source, electrons flow from the source to the drain. Hence, the source serves as the origin of the carriers, and the drain serves as the sink. The third electrode, the gate, forms a rectifying metal-semiconductor contact with the channel. The shaded area underneath the gate electrode is the depletion region of the metal-semiconductor contact. An increase or decrease of the gate voltage with respect to the source causes the depletion region to expand or shrink; this in turn changes the cross-sectional area available for current flow from source to drain. The MESFET thus can be considered a voltage-controlled resistor.

A typical current-voltage characteristic of a MESFET is shown in Figure 7B-->, where the drain current I_D is plotted against the drain voltage V_D for various gate voltages. For a given gate voltage (e.g., V_G = 0), the drain current initially increases linearly with drain voltage, indicating that the conductive channel acts as a constant resistor. As the drain voltage increases, however, the cross-sectional area of the conductive channel is reduced, causing an increase in the channel resistance. As a result, the current increases at a slower rate and eventually saturates. At a given drain voltage the current can be varied by varying the gate voltage. For example, for V_D = 5 V, one can increase the current from 0.6 to 0.9 mA by forward-biasing the gate to 0.5 V, as shown in Figure 7B-->, or one can reduce the current from 0.6 to 0.2 mA by reverse-biasing the gate to −1.0 V.

There are basically four different types of MESFET (or JFET), depending on the type of conductive channel. If, at zero gate bias, a conductive n channel exists and a negative voltage has to be applied to the gate to reduce the channel conductance, as shown in Figure 7B-->, then the device is an n-channel “normally on” MESFET. If the channel conductance is very low at zero gate bias and a positive voltage must be applied to the gate to form an n channel, then the device is an n-channel “normally off” MESFET. Similarly, p-channel normally on and p-channel normally off MESFETs are available.

Figure 8--> shows a cross section of a heterojunction FET. The heterojunction is formed between a high-bandgap semiconductor (e.g., Al_0.4Ga_0.6As, with a bandgap of 1.9 eV) and one of a lower bandgap (e.g., GaAs, with a bandgap of 1.42 eV). By proper control of the bandgaps and the impurity concentrations of these two materials, a conductive channel can be formed at the interface of the two semiconductors. Because of the high conductivity in the conductive channel, a large current can flow through it from source to drain. When a gate voltage is applied, the conductivity of the channel will be changed by the gate bias, which results in a change of drain current. The current-voltage characteristics are similar to those of the MESFET shown in Figure 7B-->. If the lower-bandgap semiconductor is a high-purity material, the mobility in the conductive channel will be high. This in turn can give rise to higher operating speed.

A perspective view for an n-channel MOSFET is shown in Figure 9-->. Although it looks similar to a MESFET, there are four major differences: (1) the source and drain of a MOSFET are rectifying p-n junctions instead of ohmic contacts; (2) the gate is a metal-oxide-semiconductor structure, meaning that there is an insulator—silicon dioxide (SiO₂)—sandwiched between the metal electrode and the semiconductor substrate, while for the MESFET the gate electrode forms a metal-semiconductor contact; (3) the left edge of the gate electrode must be aligned or overlapped with the source contact to facilitate device operation, while in a MESFET there is no overlapping of gate and source contact; and (4) the MOSFET is a four-terminal device, so that there is a fourth substrate contact in addition to the source, drain, and gate electrode, as in the case of a MESFET.

One of the key device parameters is the channel length, L, which is the distance between the two n⁺-p junctions, as indicated in Figure 9-->. When the MOSFET was first developed, in 1960, the channel length was longer than 20 micrometres (μm). Today channel lengths less than 1 μm have been fabricated in volume production, and lengths less than 0.1 μm have been created in research laboratories.

The current-voltage characteristic of a MOSFET is similar to that shown in Figure 7B-->. There are also four different kinds of MOSFETs, depending on the type of conducting layer. The four are n-channel normally off, n-channel normally on, p-channel normally off, and p-channel normally on MOSFETs. They are similar to MESFET varieties.