Chapter 2 Semiconductor DiodesSemiconductor
diode
is
yourself
semiconductor device with one p-n junction and two
conclusions. Most diodes are based on
asymmetrical p-n junctions. At the same time, one of the areas
diode, usually (p+) highly doped and called an emitter,
other
(n)
lightly alloyed
–
base.
P-n junction
placed in the base because it is lightly alloyed.
Structure, symbol and the name of the conclusions
shown in Fig. 3.1. Between each outer area
semiconductor and its output there is an ohmic contact,
which in Fig. 3.1 is shown with a bold line.
Depending on the manufacturing technology, there are:
point diodes, alloy and microalloy, with diffusion
base, epitaxial, etc.
By
functional
purpose
diodes
divided:
rectifier, universal, pulse, zener diodes and
stabistors, varicaps, tunnel and reversed, as well as microwave diodes, etc.

Classification of diodes by functional purpose and their UGO

2.1. Current-voltage characteristic of the diode

Semiconductor diodes

2.2. Diode equivalent circuit

2.3. The influence of temperature on the diode's current-voltage characteristic

2.4. Rectifier diodes

2.5. Pulse diodes

2.7. Zener diodes and stabilizers

2.9. Tunnel and reverse diodes

2.10. Marking of semiconductor diodes

Discipline: Electrical engineering and electronics

Contents.1.
2.
3.
4.
5.
6.
7.
8.
9.
Definition.
Application area.
Principle of operation.
Types of devices and their designation.
CVC.
Rectification factor.
Bridge circuits for switching diodes.
Schottky diodes.

Definition.

Application area.

Operating principle of a rectifier diode

Types of devices and their designation.

Parameters of rectifier diodes.

Parameters of rectifier diodes.

Volt-ampere characteristics

Rectification factor

Bridge circuits for switching diodes.

Single-phase bridge circuit.

Three-phase bridge circuit.

Schottky diodes

Sections: Physics, Competition "Presentation for the lesson"

Presentation for the lesson

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Attention! Slide previews are for informational purposes only and may not represent all the features of the presentation. If you are interested this work, please download the full version.

Lesson in 10th grade.

Subject: R- And n- types. Semiconductor diode. Transistors."

Goals:

• educational: to form an idea of ​​free electric charge carriers in semiconductors in the presence of impurities from the point of view of electronic theory and, based on this knowledge, to find out the physical essence of the p-n junction; teach students to explain the operation of semiconductor devices, based on knowledge of the physical essence of the pn junction;
• developing: develop students’ physical thinking, the ability to independently formulate conclusions, expand cognitive interest, cognitive activity;
• educational: to continue the formation of the scientific worldview of schoolchildren.

Equipment: presentation on the topic:“Semiconductors. Electric current through semiconductor contact R- And n- types. Semiconductor diode. Transistor", multimedia projector.

During the classes

I. Organizational moment.

II. Learning new material.

Slide 1.

Slide 2. Semiconductor – a substance in which the resistivity can vary over a wide range and decreases very quickly with increasing temperature, which means that the electrical conductivity (1/R) increases.

It is observed in silicon, germanium, selenium and in some compounds.

Slide 3.

Conduction mechanism in semiconductors

Slide 4.

Semiconductor crystals have an atomic crystal lattice, where the outer Slide 5. electrons are bonded to neighboring atoms by covalent bonds.

At low temperatures, pure semiconductors have no free electrons and behave like insulators.

Semiconductors are pure (without impurities)

If the semiconductor is pure (without impurities), then it has its own conductivity, which is low.

There are two types of intrinsic conductivity:

Slide 6. 1) electronic ("n" type conductivity)

At low temperatures in semiconductors, all electrons are bound to the nuclei and the resistance is high; As the temperature increases, the kinetic energy of the particles increases, bonds break down and free electrons appear - the resistance decreases.

Free electrons move opposite to the electric field strength vector.

Electronic conductivity of semiconductors is due to the presence of free electrons.

Slide 7.

2) hole (conductivity "p" type)

As the temperature increases, the covalent bonds between the atoms, carried out by valence electrons, are destroyed and places with a missing electron - a “hole” - are formed.

It can move throughout the crystal, because its place can be replaced by valence electrons. Moving a "hole" is equivalent to moving a positive charge.

The hole moves in the direction of the electric field strength vector.

In addition to heating, the breaking of covalent bonds and the emergence of intrinsic conductivity in semiconductors can be caused by illumination (photoconductivity) and the action of strong electric fields. Therefore, semiconductors also have hole conductivity.

The total conductivity of a pure semiconductor is the sum of conductivities of the “p” and “n” types and is called electron-hole conductivity.

Semiconductors with impurities

Such semiconductors have their own + impurity conductivity.

The presence of impurities greatly increases conductivity.

When the concentration of impurities changes, the number of electric current carriers—electrons and holes—changes.

The ability to control current is at the core wide application semiconductors.

Exist:

Slide 8. 1) donor impurities (donating)– are additional suppliers of electrons to semiconductor crystals, easily give up electrons and increase the number of free electrons in the semiconductor.

Slide 9. These are the conductors "n" – type, i.e. semiconductors with donor impurities, where the main charge carrier is electrons and the minority charge carrier is holes.

Such a semiconductor has electronic impurity conductivity. For example, arsenic.

Slide 10. 2) acceptor impurities (receiving)– create “holes”, taking electrons into themselves.

These are semiconductors "p" - type, i.e. semiconductors with acceptor impurities, where the main charge carrier is holes, and the minority charge carrier is electrons.

Such a semiconductor has hole impurity conductivity. Slide 11. For example, indium. Slide 12.

Let's consider what physical processes occur when two semiconductors come into contact with different types conductivity, or, as they say, in the p-n junction.

Slide 13-16.

Electrical properties of the p-n junction

"p-n" junction (or electron-hole junction) is the area of ​​​​contact of two semiconductors where the conductivity changes from electronic to hole (or vice versa).

Such regions can be created in a semiconductor crystal by introducing impurities. In the contact zone of two semiconductors with different conductivities, mutual diffusion will take place. electrons and holes and a blocking electrical layer is formed. The electric field of the blocking layer prevents further passage of electrons and holes across the boundary. The blocking layer has increased resistance compared to other areas of the semiconductor.

The external electric field affects the resistance of the barrier layer.

In the forward (through) direction of the external electric field, the electric current passes through the boundary of two semiconductors.

Because electrons and holes move towards each other towards the interface, then the electrons, crossing the boundary, fill the holes. The thickness of the barrier layer and its resistance are continuously decreasing.

Passport p-n mode transition:

When the external electric field is in a blocking (reverse) direction, no electric current will pass through the contact area of ​​two semiconductors.

Because As electrons and holes move from the boundary in opposite directions, the blocking layer thickens and its resistance increases.

Blocking mode p-n junction:

Thus, the electron-hole transition has one-way conductivity.

Semiconductor diodes

A semiconductor with one p-n junction is called a semiconductor diode.

- Guys, write it down new topic: "Semiconductor diode."
“What kind of idiot is there?” Vasechkin asked with a smile.
- Not an idiot, but a diode! – the teacher answered, “A diode, which means it has two electrodes, an anode and a cathode.” Do you understand?
“And Dostoevsky has such a work - “The Idiot,” Vasechkin insisted.
- Yes, there is, so what? You are in a physics lesson, not literature! Please don't confuse a diode with an idiot anymore!

Slide 17–21.

When an electric field is applied in one direction, the resistance of the semiconductor is high, in the opposite direction the resistance is small.

Semiconductor diodes are the main elements of AC rectifiers.

Slide 22–25.

Transistors are called semiconductor devices designed to amplify, generate and convert electrical oscillations.

Semiconductor transistors - the properties of "p-n" junctions are also used - transistors are used in the circuitry of radio-electronic devices.

The large “family” of semiconductor devices called transistors includes two types: bipolar and field-effect. The first of them, in order to somehow distinguish them from the second, are often called ordinary transistors. Bipolar transistors are the most widely used. We'll probably start with them. The term “transistor” is formed from two English words: transfer – converter and resistor – resistance. In a simplified form, a bipolar transistor is a semiconductor wafer with three (as in a layer cake) alternating regions of different electrical conductivity (Fig. 1), which form two p–n junctions. The two extreme regions have electrical conductivity of one type, the middle one has electrical conductivity of another type. Each area has its own contact pin. If hole electrical conductivity predominates in the outer regions, and electronic conductivity in the middle (Fig. 1, a), then such a device is called a transistor of the p – n – p structure. A transistor with an n – p – n structure, on the contrary, has regions with electronic electrical conductivity along the edges, and between them there is a region with hole electrical conductivity (Fig. 1, b).

When applied to the base of the transistor n-p-n type When a positive voltage is applied, it opens, i.e., the resistance between the emitter and the collector decreases, but when a negative voltage is applied, on the contrary, it closes, and the stronger the current, the more it opens or closes. For transistors p-n-p structures it's the other way around.

The basis of a bipolar transistor (Fig. 1) is a small plate of germanium or silicon with electronic or hole electrical conductivity, that is, n-type or p-type. Balls of impurity elements are fused onto the surface of both sides of the plate. When heated to a strictly defined temperature, diffusion (penetration) of impurity elements occurs into the thickness of the semiconductor wafer. As a result, two regions appear in the thickness of the plate, opposite to it in electrical conductivity. A p-type germanium or silicon plate and the n-type regions created in it form a transistor of the n-p-n structure (Fig. 1, a), and an n-type plate and the p-type regions created in it form a transistor of the p-n-p structure (Fig. 1, b ).

Regardless of the structure of the transistor, its plate of the original semiconductor is called the base (B), the region of smaller volume opposite to it in terms of electrical conductivity is the emitter (E), and another similar region of larger volume is the collector (K). These three electrodes form two p-n junctions: between the base and the collector - the collector, and between the base and the emitter - the emitter. Each of them is similar in its electrical properties to p-n junctions of semiconductor diodes and opens at the same forward voltages across them.

Conventional graphic designations of transistors of different structures differ only in that the arrow symbolizing the emitter and the direction of current through the emitter junction, for a p-n-p transistor, faces the base, and for an n-p-n transistor, it faces away from the base.

Slide 26–29.

III. Primary consolidation.

1. What substances are called semiconductors?
2. What kind of conductivity is called electronic?
3. What other conductivity is observed in semiconductors?
4. What impurities do you now know about?
5. What is the throughput mode of a p-n junction?
6. What is the blocking mode of a p-n junction?
7. What semiconductor devices do you know?
8. Where and for what are semiconductor devices used?

IV. Consolidation of what has been learned

1. How does the resistivity of semiconductors change when heated? Under lighting?
2. Will silicon be superconducting if it is cooled to a temperature close to absolute zero? (no, silicon resistance increases with decreasing temperature).

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1 of 16

Presentation on the topic: Diode

Slide no. 1

Slide description:

Slide no. 2

Slide description:

Slide no. 3

Slide description:

Tunnel diode. The first work confirming the reality of creating tunnel devices was devoted to a tunnel diode, also called the Esaki diode, and published by L. Esaki in 1958. Esaki, in the process of studying internal field emission in a degenerate germanium p-n junction, discovered an “anomalous” current-voltage characteristic: the differential resistance in one of the sections of the characteristic was negative. He explained this effect using the concept of quantum mechanical tunneling and at the same time obtained acceptable agreement between theoretical and experimental results.

Slide no. 4

Slide description:

Tunnel diode. A tunnel diode is a semiconductor diode based on a p+-n+ junction with heavily doped regions, in the direct section of the current-voltage characteristic of which an n-shaped dependence of current on voltage is observed. As is known, in semiconductors with a high concentration of impurities, impurity energy bands are formed. In n-semiconductors, such a band overlaps with the conduction band, and in p-semiconductors, with the valence band. As a result, the Fermi level in n-semiconductors with a high impurity concentration lies above the Ec level, and in p-semiconductors below the Ev level. As a result, within the energy interval DE=Ev-Ec, any energy level in the conduction band of the n-semiconductor can correspond to the same energy level behind the potential barrier, i.e. in the valence band of a p-semiconductor.

Slide no. 5

Slide description:

Tunnel diode. Thus, particles in n and p semiconductors with energy states within the DE interval are separated by a narrow potential barrier. In the valence band of a p-semiconductor and in the conduction band of an n-semiconductor, some of the energy states in the DE range are free. Consequently, through such a narrow potential barrier, on both sides of which there are unoccupied energy levels, tunneling motion of particles is possible. When approaching the barrier, the particles experience reflection and in most cases return back, but there is still a probability of detecting a particle behind the barrier; as a result of the tunnel transition, the tunnel current density j t0 is also nonzero. Let's calculate the geometric width of the degenerate p-n junction. We will assume that in this case the asymmetry of the p-n junction is preserved (p+ is a more heavily doped region). Then the width of the p+-n+ transition is small: We will estimate the De Broglie wavelength of the electron from simple relations:

Slide no. 6

Slide description:

Tunnel diode. The geometric width of the p+-n+ transition turns out to be comparable to the de Broglie wavelength of the electron. In this case, in a degenerate p+-n+ junction one can expect the manifestation of quantum mechanical effects, one of which is tunneling through a potential barrier. With a narrow barrier, the probability of tunnel seepage through the barrier is non-zero!!!

Slide no. 7

Slide description:

Tunnel diode. Currents in a tunnel diode. At equilibrium, the total current through the junction is zero. When a voltage is applied to the junction, electrons can tunnel from the valence band to the conduction band or vice versa. For tunnel current to flow, the following conditions must be met: 1) energy states on the side of the junction from which electrons tunnel must be filled; 2) on the other side of the transition, energy states with the same energy must be empty; 3) the height and width of the potential barrier must be small enough for there to be a finite probability of tunneling; 4) the quasi-momentum must be conserved. Tunnel diode.swf

Slide no. 8

Slide description:

Tunnel diode. Voltages and currents that characterize special points of the current-voltage characteristic are used as parameters. The peak current corresponds to the maximum current-voltage characteristic in the region of the tunneling effect. Voltage Up corresponds to current Ip. The valley current Iв and Uв characterize the current-voltage characteristic in the region of the current minimum. The solution voltage Upp corresponds to the current value Iп on the diffusion branch of the characteristic. The falling section of the dependence I=f(U) is characterized by a negative differential resistance rД= -dU/dI, the value of which can be determined with some error by the formula

Slide no. 9

Slide description:

Reversed diodes. Let us consider the case when the Fermi energy in electron and hole semiconductors coincides or is at a distance of ± kT/q from the bottom of the conduction band or the top of the valence band. In this case, the current-voltage characteristics of such a diode at reverse bias will be exactly the same as those of a tunnel diode, that is, as the reverse voltage increases, there will be a rapid increase in the reverse current. As for the current under forward bias, the tunnel component of the current-voltage characteristic will be completely absent due to the fact that there are no completely filled states in the conduction band. Therefore, when forward biasing such diodes to voltages greater than or equal to half the bandgap, there will be no current. From the point of view of a rectifier diode, the current-voltage characteristic of such a diode will be inverse, that is, there will be high conductivity with reverse bias and low with forward bias. In this regard, tunnel diodes of this type are called reverse diodes. Thus, a reverse diode is a tunnel diode without a section with negative differential resistance. The high nonlinearity of the current-voltage characteristic at low voltages near zero (on the order of microvolts) allows this diode to be used for detection weak signals in the microwave range.

Slide no. 10

Slide description:

Transient processes. With rapid changes in voltage across a semiconductor diode based regular p-n transition, the current value through the diode corresponding to the static current-voltage characteristic is not immediately established. The process of current establishment during such switchings is usually called the transient process. Transient processes in semiconductor diodes are associated with the accumulation of minority carriers in the base of the diode when it is directly turned on and their resorption in the base with a rapid change in the polarity of the voltage on the diode. Since there is no electric field in the base of a conventional diode, the movement of minority carriers in the base is determined by the laws of diffusion and occurs relatively slowly. As a result, the kinetics of carrier accumulation in the base and their resorption affect the dynamic properties of diodes in the switching mode. Let's consider changes in current I when the diode switches from forward voltage U to reverse voltage.

Slide no. 11

Slide description:

Transient processes. In the stationary case, the current value in the diode is described by the equation. After the completion of transient processes, the current value in the diode will be equal to J0. Let us consider the kinetics of the transition process, that is, the change current p-n transition when switching from forward voltage to reverse. When a diode is forward biased based on an asymmetrical pn junction, nonequilibrium holes are injected into the base of the diode. The change in time and space of nonequilibrium injected holes in the base is described. continuity equation:

Slide no. 12

Slide description:

Transient processes. At time t = 0, the distribution of injected carriers in the base is determined from the diffusion equation and has the form: From general provisions it is clear that at the moment the voltage in the diode switches from direct to reciprocal the reverse current will be significantly greater than the thermal current of the diode. This will happen because the reverse current of the diode is due to the drift component of the current, and its value in turn is determined by the concentration of minority carriers. This concentration is significantly increased in the base of the diode due to the injection of holes from the emitter and is described at the initial moment by the same equation.

Slide no. 13

Slide description:

Transient processes. Over time, the concentration of nonequilibrium carriers will decrease, and therefore the reverse current will also decrease. During time t2, called the reverse resistance recovery time, or resorption time, the reverse current will reach a value equal to the thermal current. To describe the kinetics of this process, we write the boundary and initial conditions for the continuity equation in the following form. At time t = 0, the equation for the distribution of injected carriers in the base is valid. When a stationary state is established at a moment in time, the stationary distribution of nonequilibrium carriers in the base is described by the relation:

Slide no. 14

Slide description:

Transient processes. The reverse current is caused only by the diffusion of holes to the boundary of the space charge region of the p-n junction: The procedure for finding the kinetics of the reverse current is as follows. Taking into account the boundary conditions, the continuity equation is solved and the dependence of the concentration of nonequilibrium carriers in the base p(x,t) on time and coordinates is found. The figure shows the coordinate dependences of the concentration p(x,t) at different times. Coordinate dependences of concentration p(x,t) at different times

Slide no. 15

Slide description:

Transient processes. Substituting the dynamic concentration p(x,t), we find the kinetic dependence of the reverse current J(t). The dependence of the reverse current J(t) has the following form: Here is an additional error distribution function equal to The first expansion of the additional error function has the form: Let us expand the function into a series in cases of small and large times: t > p. We obtain: From this relationship it follows that at the moment t = 0 the magnitude of the reverse current will be infinitely large. The physical limitation for this current will be the maximum current that can flow through the ohmic resistance of the diode base rB at reverse voltage U. The value of this current, called the cutoff current Jav, is equal to: Jav = U/rB. The time during which the reverse current is constant is called the cutoff time.

Slide no. 16

Slide description:

Transient processes. For pulsed diodes, the cutoff time τav and the recovery time τv of the reverse resistance of the diode are important parameters. There are several ways to reduce their value. Firstly, it is possible to reduce the lifetime of nonequilibrium carriers in the diode base by introducing deep recombination centers in the quasi-neutral volume of the base. Secondly, you can make the diode base thin so that nonequilibrium carriers recombine on the back side of the base. perpr_pn.swf Dependence of the reverse current on time when switching the diode

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