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Understanding the current flow in the semiconductor device

Pure silicon is the most important material for integrated circuit application, and III-V binary and ternary compounds are most significant for light emission. Prior to the invention of the bipolar transistor in 1947, semiconductors were used only as two-terminal devices, such as rectifiers and photodiodes. During the early 1950s, germanium was the major semiconductor material. However, it proved unsuitable for many applications, because devices made of the material exhibited high leakage currents at only moderately elevated temperatures.

Since the early 1960s, silicon has become a practical substitute, virtually supplanting germanium as a material for semiconductor fabrication. The main reasons for this are twofold: Silicon technology is now by far the most advanced among all semiconductor technologies, and silicon-based devices constitute more than 95 percent of all semiconductor hardware sold worldwide.

1.09 Current flow in semiconductor materials.

Many of the compound semiconductors have electrical and optical properties that are absent in silicon. These semiconductors, especially gallium arsenide, are used mainly for high-speed and optoelectronic applications. Electronic properties The semiconductor materials treated here are single crystals —i. 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. Three bond pictures of a semiconductor. At low temperatures the electrons are bound in their respective positions in the crystal; consequently, they are not available for electrical conduction.

At higher temperatures thermal vibration may break understanding the current flow in the semiconductor device of the covalent bonds. The breaking of a bond yields a free electron that can participate in current conduction.

Once an electron moves away from a covalent bond, there is an electron deficiency in that bond. This deficiency may be filled by one of the neighbouring electrons, which results in a shift of the deficiency location from one site to another.

This deficiency may thus be regarded as a particle similar to an electron. This fictitious particle, dubbed a holecarries a positive charge and moves, under the influence of an applied electric fieldin a direction opposite to that of an electron. For an isolated atom, the electrons of the atom can have only discrete energy levels.

When a large number of atoms are brought together to form a crystal, the interaction between the atoms causes the discrete energy levels to spread out into energy bands. When there is no thermal vibration i. The highest filled band is called the valence band. The next higher band is the conduction bandwhich is separated from the valence band by an energy gap. This energy gap, also called a bandgap, is a region that designates energies that the electrons in the semiconductor cannot possess.

Most of the important semiconductors have bandgaps in the range 0. The bandgap of silicon, for example, is 1. 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 understanding the current flow in the semiconductor device 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 2Cfour 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. The p-n junction 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 layerbehaves as an insulator. A Current-voltage characteristics of a typical silicon p-n junction. B Forward-bias and C reverse-bias conditions. D The symbol for a p-n junction. 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. However, when a reverse bias is applied in Figure 3Cthe 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.

  1. During the early 1950s, germanium was the major semiconductor material.
  2. Typical common-base current gain in a well-designed bipolar transistor is very close to unity.
  3. Computing Current Gain The first study can be used to generate a graph known as a Gummel plot. An example of the axisymmetric modeling of a cylindrical field-effect transistor can be found here.
  4. Because of the regenerative nature of these processes, switching eventually occurs, and the device is in its on state.
  5. When no voltage is applied to the gate, the source-to-drain electrodes correspond to two p-n junctions connected back to back.

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. Two-terminal junction devices 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.

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There are more than 50,000 types of diodes with voltage ratings from less than 1 volt to more than 2,000 volts and current ratings from less than 1 milliampere to more than 5,000 amperes. A p-n junction also can generate and detect light and convert optical radiation into electrical energy. Rectifier 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.

  • Gummel plot showing the collector and base currents as a function of the base voltage when a voltage of 0;
  • With semiconductors, we need to be able to understand conduction both in terms of free electrons and also in terms of virtual mobile charge carriers, i.

Zener diode 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. Varactor diode 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.

Tunnel diode 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 100 angstroms. Under forward biases, the electrons can tunnel or pass directly through the junction, producing a negative resistance effect i.

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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 itthe tunnel diode is used in special low-power microwave applications, such as a local oscillator and a frequency-locking circuit.

Schottky diode Such a diode is one that has a metal-semiconductor contact e. It is named for the German physicist Walter H. Schottky, understanding the current flow in the semiconductor device in 1938 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.

Bipolar transistors 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. A Perspective of a p-n-p bipolar transistor; B idealized one-dimensional transistor; C symbols for p-n-p and n-p-n bipolar transistors E is an emitter, B is a base, and C is a collector.

Semiconductor device

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.