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7PHYSICS AND MATHEMATICS
PHYSICAL FEATURES OF BRANCHED CIRCUITS WITH CONTACT POTENTIAL DIFFERENCES. (NEW ABOUT BIPOLAR TRANSISIOR)
Parthentiev N.
Russian State University of Cinematography named after S. Gerasimov (VGIK)
Partfenteva N.
Moscow State University of Civil Engineering (MSUCE or MGSU)
ABSTRACT
Conditions are formulated under which the contact differences of potentials in a branched chain can be neglected.
1) A relatively small current value at which the Peltier and Zeeman effects can be neglected.
2) Constant current in all elements of loop current.
3) The constancy of the energy spectrum of current carriers.
A detailed analysis of contact phenomena in a bipolar transistor is carried out. It is shown that an abrupt increase in the input voltage upon the appearance of a collector voltage is associated with disturbances in the energy balance, when the electron energy spent on the metal-emitter barrier transition cannot be compensated for in the base-metal transition. The balance is disturbed due to the different energy spectra of electrons in the transitions of the transistor. The conventional interpretation of this phenomenon, which assumes feedback between collector and input voltage, is erroneous. The adopted concept, together with the constructive solution of the semiconductor device, the properties of the metals used to actually connect the transistors. Development of this concept to build more accurate transistor models, free from errors associated with inaccurate physical representation of the operation of the transistor.
Keywords: branched chain, contact phenomena, bipolar transistor.
Introduction
A real electrical circuit usually contains components made from different materials.
In classical electrical engineering, this circumstance is rightly not given much attention [1..6]. In real conditions, contact potential differences are inevitably present in a closed circuit, but they completely compensate each other, allowing the physical laws, discovered and formulated several centuries ago, to be used quite reasonably for their calculation. On the other hand, the Peltier and Zeeman effects (associated with contact phenomena) successfully work in technology, allowing both to measure the temperature and change its value in the required direction. The physical nature of the contact potential difference provides an opportunity to both increase the energy of carriers falling into the zone of its action, and to lower it. Naturally, specific changes in energy depend on the polarity of the potential difference, the direction of the carrier velocity, and the polarity of the carriers themselves. These phenomena are of particular importance in semiconductor technology, which has penetrated everywhere and has changed practical life, in which they occupy a large and important place.
However, contact phenomena are not always correctly explained, since, oddly enough, the laws that determine their influence are still not clearly defined. A classic example of where the contact difference manifests itself in a paradoxical way is the bipolar transistor, about which everything from school textbooks to serious fundamental books on electronics seems to be written [1]. The first thing we learn from these sources is that a transistor is a device whose main function is to amplify or convert a weak signal from a measuring sensor into a voltage or current supplied to the device.
The most common way to turn on a transistor is with a common emitter circuit for maximum power gain. In this case, both the input current and the input voltage increase at the output of the circuit. For practical calculations, equivalent circuits of real transistors are used, in which there are models of the input base circuit and the output-collector circuit.
The main element of this circuit is the current source in the collector circuit, and the magnitude of this current is linearly related to the magnitude of the current in this circuit, and the coefficient determining this relationship is much greater than unity. However, some unpleasant phenomenon occurs in the circuit - with the help of the collector current in bipolar transistors of the npn type, it is necessary to increase the voltage supplied to the base. Moreover, this phenomenon, noticeable on real input (basic) characteristics, is of an abrupt nature - it manifests itself immediately after a slight increase in the collector voltage (current).
In classical equivalent circuits, this effect is described by the feedback coefficient between the collector voltage and the base voltage. The purpose of this article is a physical interpretation of this phenomenon, showing the obvious fallacy of the accepted interpretation. The longevity of this misconception is probably due to the fact that the effect itself manifests itself in the "inoperative" region of the transistor, when the required current gain (the main concern of the transistor designer) is far from the maximum value. LTSpace, an excellent electronic component simulator created to advertise products from the famous Analog Device, fails when trying to describe the operation of a transistor in this area.
Analysis of contact phenomena in a bipolar transistor npn.
Let's consider in detail what happens in a real transistor switching circuit. In practice, a device whose basic principles of operation are determined by semiconductors is inevitably connected to the measuring circuit using metal contacts. Naturally, in each such
connection, a contact potential difference arises. In the diagram shown in Fig. 1, they are represented by voltage sources in the form of circles with a polarity designation - the method adopted in the LTSpice program.
Figure. 1: connection diagram with a common emitter of an NPN bipolar transistor. V1 - contact potential difference metal-emitter, V2-np transition emitter-base, V3 - contact potential difference metal-base, V4-pn base-collector junction, V5 pin potential difference collector metal.
The input characteristics of the transistor necessarily include the original curve of the dependence of the current on the input voltage in the absence of the collector voltage, and, consequently, the collector current.
At zero collector current, the contact differences of the metal connections of the transistor fully compensate each other, without exerting any influence on the formation of the input voltage, which is completely dependent on the junction voltage np. Naturally, this characteristic is not linear and corresponds to the current-voltage characteristic of a semiconductor diode.
At first glance, the appearance of the collector current does not change the situation in any way: the contact differences remain the same, and the weak base
current, being part of the total emitter current flowing to the base, should not be influenced by the contact differences on the transistor electrodes. In this case, in both cases, the contact potential difference V1, which arises during the metal-semiconductor connection due to the different electron concentration in the metal and semiconductor, is the first barrier for electrons.
As a result, electrons penetrate into the emitter, the average energy of which is less than the average energy of electrons in the metal contact. Let us consider in more detail what happens in the transitions of the bipolar transistor itself in Figure 2.
Figure 2: Scheme of a bipolar transistor n-p-n (circuit with a common emitter)
Electrons entering the base layer through the emitter move in crossed electric fields. The relatively weak base voltage (Eb) forces them to move along the thin base layer, while the high collector voltage can drag them into the collector junction zone. For the most part, electrons with a relatively large kinetic energy get there. Such electrons have a high mobility and are likely to fall on the collector, avoiding the possibility of getting into the base contact.
As a result, out of the total number of electrons that passed into the collector through the metal-emitter contact junction, most of them have a higher energy than the electrons entering the base. As a result, the energy spectrum of electrons going to the base differs from the energy spectrum of electrons reaching the collector. The average energy of these electrons is less than the average energy of the electrons entering the emitter. The contact difference V3, which increases their energy, is nevertheless not able to restore its value to the value that they had in the metal in contact with the emitter. Thus, the base-collector branching circuit is a device that sorts electrons by the amount of their energy. The base current does not change, it coincides with the fraction of the current flowing from the metal to the emitter, but the energy spectrum of electrons changes. Thus, when the collector current appears in the base circuit, irreplaceable energy losses occur. The kinetic energy of electrons that have lost energy during the transition of the metal-emitter contact cannot be restored to their original value when passing through the
base-metal contact. Thus, an abrupt change in the input voltage with the appearance of even a small collector voltage, or rather the collector current, turns out to be associated with the contact potential difference of the metal-semiconductor. Based on the above, it is possible to formulate the conditions under which the contact potential differences do not affect the current in the branched circuit:
1) A relatively small current value at which the Peltier and Zeeman effects can be neglected.
2) Equal current value in all circuit elements.
3) The invariability of the energy spectrum of current carriers.
In the base circuit of the transistor, to make up for energy losses when a collector current appears and a concomitant change in the energy spectrum of electrons, it is necessary to increase the input voltage of the transistor. Figure 3 shows the dependence of the ratio of the average energy Em of electrons in a metal to the minimum energy of free electrons E0 , depending on the value of the minimum energy of the conduction band, expressed in meV, calculated at normal temperature.
As follows from the graph, the presence of a contact potential difference that decreases the average energy of electrons is equivalent to an increase in their minimum energy and, consequently, a greater inhomo-geneity of the energy spectrum of electrons.
Figure 3:. Dependence of the ratio of the average energy of electrons in a metal to the minimum energy on
the minimum energy at normal temperature.
A feature of the type of input characteristics of the n-p-n transistor is the high constancy of the amplitude of the input voltage jump. Figure 4 shows a sample of
a typical input characteristic (n-p-n) of an Analog Device MAT-02 transistor.
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Vbe
It is easy to see that the same value of the input current is achieved by increasing the input voltage by an almost constant increase in the input voltage by an amount equal to 0.2 volts. The constancy of this value is an additional argument in the erroneous interpretation of the input voltage jump from the appearance of the collector voltage. Of course, the accepted interpretation of the effect creates additional computational difficulties, since apart from the properties of the transistor itself, the properties of the metal used for contact with the terminals of the transistor must be taken into account.
Studying the characteristics of p-n-p transistors, one can notice manifestations of the opposite effect -when the collector current appears, the absolute value of the input voltage decreases.
This phenomenon is also easily interpreted within the framework of the proposed analysis and is associated with contact phenomena that arise when the transistor is actually turned on in the working circuit.
Conclusions
1. Conditions have been formulated under which contact potential differences in branched electrical circuits can be neglected.
2. The error of the classical interpretation of the input voltage jump when the collector current appears is shown.
ФИЗИЧЕСКИЕ ОСОБЕННОСТИ КОСМОЛОГИЧЕСКОЙ ЭВОЛЮЦИИ ВСЕЛЕННОЙ
Кошман В.С.
канд. техн. наук, доцент, Пермский государственный аграрно-технологический университет,
Пермь, Россия
PHYSICAL FEATURES OF THE COSMOLOGICAL EVOLUTION OF THE UNIVERSE
Koshman V.
Cand. Tech. Sci., Associate Professor, Perm State Agrarian and Technological University,
Perm, Russia
3. A physical interpretation of the effect of a step change in the input voltage of bipolar transistors is carried out.
4. The ways of analyzing the operating mode of bipolar transistors are discussed, which make it possible to eliminate the shortcomings of the existing calculation models.
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