- Semiconductor Materials: An Introduction to Basic Principles - B. G. Yacobi - Google Книги
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As discussed above, at finite temperatures thermal vibrations will break some bonds. When a bond is broken, a free electron, along with a free hole, results, i. When an electric field is applied to the semiconductor, both the electrons in the conduction band and the holes in the valence band gain kinetic energy and conduct electricity. The electrical conductivity of a material depends on the number of charge carriers i. In an intrinsic semiconductor there exists an equal number of free electrons and free holes.
The electrons and holes, however, have different mobilities—that is to say, they move with different velocities in an electric field. The mobilities of a given semiconductor generally decrease with increasing temperature or with increased impurity concentration. 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 silicon becomes an n -type semiconductor because of the addition of the electron. The arsenic atom is the donor. 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.
Semiconductor Materials: An Introduction to Basic Principles - B. G. Yacobi - Google Книги
When a forward bias is applied to the p -n junction i. 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 p-n junction diode is a solid-state device that has two terminals. Depending on impurity distribution, device geometry, and biasing condition, a junction diode can perform various functions. There are more than 50, types of diodes with voltage ratings from less than 1 volt to more than 2, volts and current ratings from less than 1 milliampere to more than 5, amperes. A p-n junction also can generate and detect light and convert optical radiation into electrical energy.
This type of p-n junction diode is specifically designed to rectify an alternating current— i. Such diodes are generally designed for use as power-rectifying devices that operate at frequencies from 50 hertz to 50 kilohertz. The majority of rectifiers have power-dissipation capabilities from 0.
A high-voltage rectifier is made from two or more p-n junctions connected in series. This voltage regulator is a p-n junction diode that has a precisely tailored impurity distribution to provide a well-defined breakdown voltage. It can be designed to have a breakdown voltage over a wide range from 0. The Zener diode is operated in the reverse direction to serve as a constant voltage source, as a reference voltage for a regulated power supply, and as a protective device against voltage and current transients.
The varactor variable reactor is a device whose reactance can be varied in a controlled manner with a bias voltage. It is a p-n junction with a special impurity profile, and its capacitance variation is very sensitive to reverse-biased voltage.
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Varactors are widely used in parametric amplification, harmonic generation, mixing, detection, and voltage-variable tuning applications. A tunnel diode consists of a single p-n junction in which both the p and n sides are heavily doped with impurities. The depletion layer is very narrow about angstroms. Under forward biases, the electrons can tunnel or pass directly through the junction, producing a negative resistance effect i.
Because of its short tunneling time across the junction and its inherent low noise random fluctuations either of current passing through a device or of voltage developed across it , the tunnel diode is used in special low-power microwave applications, such as a local oscillator and a frequency-locking circuit.
Such a diode is one that has a metal-semiconductor contact e. It is named for the German physicist Walter H.
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Schottky, who in explained the rectifying behaviour of this kind of contact. The Schottky diode is electrically similar to a p-n junction, though the current flow in the diode is due primarily to majority carriers having an inherently fast response. It is used extensively for high-frequency, low-noise mixer and switching circuits. Metal-semiconductor contacts can also be nonrectifying; i. Such a contact is called an ohmic contact. All semiconductor devices as well as integrated circuits need ohmic contacts to make connections to other devices in an electronic system.
The p-i-n diode has found wide application in microwave circuits. It can be used as a microwave switch with essentially constant depletion-layer capacitance equal to that of a parallel-plate capacitor having a distance between the plates equal to the i -region thickness and high power-handling capability. This type of transistor is one of the most important of the semiconductor devices.
It is a bipolar device in that both electrons and holes are involved in the conduction process. The bipolar transistor delivers a change in output current in response to a change in input voltage at the base. A perspective view of a silicon p-n-p bipolar transistor is shown in Figure 4A. 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 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 bipolar transistor is composed of two closely coupled p-n junctions.
The base-collector n -p junction is reverse-biased. It has high resistance, and only a small leakage current will flow across the junction. If the base width is sufficiently narrow, however, most of the holes injected from the emitter can flow through the base and reach the collector. This transport mechanism gives rise to the prevailing nomenclature: 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. 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, types of bipolar transistors for low-frequency operation, with power outputs up to 3, watts and a current rating of more than 1, amperes.
At microwave frequencies, bipolar transistors have power outputs of more than watts at 1 gigahertz and about 10 watts at 10 gigahertz. The operation of thyristors is intimately related to the bipolar transistor, in which both electrons and holes are involved in the conduction processes. The name thyristor is derived from the electron tube called the gas thyratron , since the electrical characteristics of both devices are similar in many respects. Because of their two stable states on and off and low power dissipations in these states, thyristors are used in applications ranging from speed control in home appliances to switching and power conversion in high-voltage transmission lines.
More than 40, types of thyristors are available, with current ratings from a few milliamperes to more than 5, amperes and voltage ratings extending to , volts. 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 p 1 and p 2 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 p 2 region to form the n 2 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 p 1 is called the anode , and that to the outer n layer n 2 is designated the cathode.
An additional electrode, known as the gate electrode, is connected to the inner p layer p 2. The basic current-voltage characteristic of a thyristor is illustrated in Figure 6C. It exhibits three distinct regions: 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 n 1 -p 2 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 n 1 attached to the collector of the other.
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.
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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 bidirectional, three-terminal thyristor is called a triac. This device can switch the current in either direction by applying a small current of either polarity between the gate and one of the two main terminals. The triac is fabricated by integrating two thyristors in an inverse parallel connection. It is used in AC applications such as light dimming, motor-speed control, and temperature control.
There also are many light-activated thyristors that use an optical signal to control the switching behaviour of devices. The metal-semiconductor field-effect transistor MESFET is a unipolar device, because its conduction process involves predominantly only one kind of carrier. It is particularly useful for microwave amplifications and high-speed integrated circuits, since it can be made from semiconductors with high electron mobilities e.
Because the MESFET is a unipolar device, it does not suffer from minority-carrier effects and so has higher switching speeds and higher operating frequencies than do bipolar transistors. 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. He received his doctorate in physics from the Hebrew University of Jerusalem in He has worked in several areas of experimental solid-state physics, including the synthesis, applications and characterization of various electronic and photonic materials and devices.
He is author or co-author of over ninety scientific publications and four previous books. Even the different ages have been defined in relation to the materials used. Some of the major attributes of the present-day age i. Springer Shop Bolero Ozon. An Introduction to Basic Principles.