Structure and operating principle transistors
pp 80-94
At the end of this chapter the student should
• Does understanding the structure and principle of operation of the bipolar transistor
• Knows the basic connections of transistors and understand the typical power base and collector
• Knows the importance of the equivalent circuit
• Reads and understands technical brochures of the manufacturers
• You can calculate the polarization of a transistor in a circuit-
But, and determine the operation point on the
direct load
• Uses the equivalent circuit, circuits, amplifier
common emitter connection and calculate the gain
• Does understanding the structure and operation principle of tranzistro
transverse field contact (IFET)
• Knows the characteristic parameters and the characteristic
drain current
• Knows the equivalent circuit and to read and understand the
technical brochures of the manufacturers
• Does understanding the structure and operating principle of the MOSFET and
Singles MOSFET MOSFET dilution by condensation
• Knows the characteristic parameters and the characteristic
drain current
• Knows the equivalent circuit and to read and understand the
technical brochures of the manufacturers
• Uses the equivalent circuit, circuits, amplifier
common source connection and calculate the gain
K E F A L A I O
Transistors
Emitter base collector \ 1 /
B (a) A B (b)
Figure 4.1.1 Structure and a transistor circuit symbol (s) and PNP (b) an NPN
The three areas are called a transistor according to their function, emission (emitter), base (base) and collector (collector). The emitter is an intensely rich region is highlighted in sch.4.1.1 two [+ +]. It is intended to transmit loads to the bottom. The base is an area less enriched in sch.4.1.1 is highlighted with a [+], and is very thin. The thin thickness of the base allows more loads which are emitted from the emitter to reach the collector where it is collected. The level of enrichment of the collector is lower than the emitter and from this base. Moreover, because the collector consumed more power than they were in the base and show the area that occupies the collector is greater.
In each transistor formatted two diodes, one between base and emitter and between base and a collector. For this reason it looks like a transistor consisting of two diodes connected
4.1 Structure and principle 4.1.1 Structure of the transistor of the transistor
The transistor is a crystal with three doped regions enriched. Depending on how enrichment, the transistors are divided into two types, the PNP and the NPN (Fig. 4.1.1, b respectively), where the sequence of letters is the type of semiconductor each area.
Emitter base collector
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in the opposite direction. Because then you need to do several times refer to these passages, from now on will be called for brevity: diode <emitter contact (the first) and diode <contact manifold (the second).
Figure 4.1.1 presents, as mentioned above, two possible cases of building a transistor. The PNP transistors are complementary NPN transistor, because most players in the broadcast and the first panel are holes, while the second electron. This implies that the power of the PNP currents and polarizations are opposite to the currents and polarizations of the NPN (sch.4.1.1). The study will follow, to avoid any confusion, we focus on the NPN type transistor.
The circuit symbol for a PNP transistor and an NPN given in sch.4.1.1a and b respectively. The arrow is always in emission and shows the direction of conventional electricity. The direction of the arrow also shows the N-type semiconductor. Thus, in sch.4.1.1a (PNP), the arrow points to the base, which is N-type, while sch.4.1.1v (NPN), the arrow points to the show, which is again N-type.
Is not applied to a polarization NPN transistor, the emitter of the free electrons diffuse towards the base and part of the holes of the base to the show. The same happens with the electrons in a fraction collector and the base of the hole. This creates a space charge region (Stripping) to each contact, ie contact emitter and collector contact. Along each contact develops a potential barrier, which at 23 ° C has a value of 0,3 V if the semiconductor is germanium and 0,7 V if silicon. Germanium transistors have very limited applications, in contrast to silicon transistors, whose use is widespread. This is because silicon transistors have a wider voltage and current characteristics and are less dependent on temperature than those of germanium transistors. For this reason, I will then give special emphasis on silicon transistors.
Since the three regions have different levels of enrichment sites cargo space spanning different depth in each. Thus, the emission occurs less bandwidth cargo space than the base (emitter diode) and a smaller space bandwidth load occurs at the base than at the collector (collector diode) (sch.4.1.2a).
As mentioned above, the destination of the emitter is transmitting loads to the bottom. To make this possible, the diode emitter must be properly polarized. Moreover, to be collected by the collector load, the diode should be reverse collector polarized, as in most applications.
(A)
Figure 4.1.2 Areas cargo space NPN transistor OE (a) without bias and (b) with polarization
Figure 4.1.2 shows a transistor which has been applied polarization. As we see, in a polarized transistor width of space charge region of the emitter is reduced compared to that without bias, while the width of space charge region of the collector increases.
4.1.2 Principle of operation of the transistor
The operation of a transistor, the simplified approach, based on the emission of bodies from the emitter and collected by the collector. To do this better understood, we will consider an NPN type transistor (sch.4.1.3), where we use the conventional direction of currents. Initially we consider that emitter diode is properly biased. When the base-emitter voltage (nVE) is less than 0,7 V (for silicon transistors) practically does not pass current from the base to the emitter. If the base-emitter voltage exceed 0,7 V should be no appreciable flow of free electrons from the emitter to the base and free holes from the base to the emitter.
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The base, as already mentioned, consists of a thin layer of semiconducting type-P, which is less enriched than the emitter. Besides the contact of the collector, as shown in sch.4.1.3 is reverse biased and significantly reduces the width of the base. These result in an increase in the percentage of electrons which will remain at the base, but will enter the world of the collector contact. The electric field in the collector space charge is such a time to push the electrons that have entered for the collector. Then these electrons are collected by the collector contact and give the collector current (IC). We should not forget that there is the reverse bias current of the diode module, which is very small at this stage would consider insignificant.
• the electron hole
Figure 4.1.3 Biasing of the transistor and flow of free electrons and holes
NCB
The free electrons, which remain at the base, along with free holes, which enter the region of emission current of the base pay (IB). Because the stream resulting from allilexoudeterosi free electrons with free holes, and stream called reconnection (recombination current) in the bipolar transistor (bipolar or BJT).
In the more transistors, more than 95% of loads, emitted by the emitter, collector and arrive in less than 5% remain in the base and contribute to stream base.
For the operation of transistors, the reader should remember the following:
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1. In normal operation, the emitter contact is always properly polarized and the collector contact is reverse biased. There are some special cases where the collector contact is properly polarized.
2. The collector current is approximately equal to the emitter current of
3. The current base is very small.
Considering the transistor node as we see from the first law of Kirchhoff, that the current emissions are equal to the sum of the base and collector currents
IE = IB + Ic 4.1.1
The percentage of free bodies emissions, which reaches the collector and gives the collector current is expressed by the coefficients a ^ (DC alpha), defined as the quotient of the current collector to the emitter current of
IC
• DC = 1C 4.1.2 'e
• dc is the dimensionless size, ie "clean" number. The higher the coefficient a ^, the more electrons reach the collector and the smaller is the current base. The power base can be reduced if the base be thinner and reduce the enrichment. Certainly, a ^ can not be equal to the unit because the base current of the contributors and free holes in the base, the NPN transistor type, and the free electrons of the base, the PNP type transistor. This contribution has the effect of increasing the power base without a corresponding increase in collector current.
If one transistor to increase the voltage of the diode emitter will increase the base current of 'B and this will result in the increase of collector current IC. Experimentally it is found that there is a relationship (ratio) between power base and collector current. For this reason there is a possibility to control the collector current through the power base. Because the current of the base is small and the collector current of the large, set a rate which is called the DC
'CW = ^ 4.1. 3
'N
This, DC is a dimensionless value, ie "clean" number. O ratio, DC indicates how many times the current can not control the collector circuit of a small current through the circuit of the base. For a given base current of L, the collector current equal to the ratio
NC = BOA '' B 4.1. 4
Particular attention should be given to the use of ex.4.1.4. The above equation applies only when the circuit on the collector-
allows the passage of current METERS or greater than that which results from the ex.4.1.4. If the circuit of the collector allows the passage of smaller streams, then the results obtained using Eq. 4.1.4 is not applicable for our circuit. Generally, DC shows in practice the effectiveness of the control current collector.
From ex.4.1.1. - 4.1.3, it emerges that the relationship which connects the two sizes, DC and aDC are:
gain power, DC, as the ratio of collector current to direct current base.
4.1.5
4.1.6
O ratio, DC, for low-power transistors, has prices ranging from 100 to 300. Power transistors for prices ranging from 30 to 150. From Eq. 4.1.6 concluded that the more the a ^ approaches the unit so the afsanetai, DC. Therefore the methods used to increase the
• dc and achieve the simultaneous afsisi, DC.
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Example 4.1.1
In a transistor count Collector current IC = 8,15 mA and a current
emitter IE = 8,20 mA. To calculate the base current of and conducive-
IC = 8,15 mA
17 = 8,20 mA
and the coefficient, DC will be
0 IC = 8,15 mA
im = 0,05 mA
Free reading:
To further reduce the contribution of free institutions in the region of the base current of the base and thereby raise the coefficient alpha, the current technology has resorted to using various semiconductors for the emitter and base. The transistors are called bipolar transistor eteroepafis (heterojunction bipolar
transistor). This compound semiconductors are used as solid solutions of silicon-germanium (SiGe) or gallium arsenide-aluminum
(AlGaAs). At present, these transistors have relatively limited applications.
Because the base penetrates two space charge regions, the diode emitter diode and the collector, free of charges, electrons in sch.4.1.3, limited to a very thin channel. The resistance of this channel is called a distributed resistance rb and plays an important role in the operation of the transistor at high frequencies. Also important role in the operation of the transistor at high frequencies play the path length of the channel loads the base and parasitic capacities of the collector and emitter diodes.
Solution
The Eq. 4.1.1 gives
IB = IE-IC
So
IB = 8,2 mA - 8,15 mA = 0,05 mA
On the basis of Eq. 4.1.2 and 4.1.3, O factor a ^ is
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Transistors
4.2 Basic transistor configurations
Figure 4.2.1 Basic transistor configurations (a) common emitter (CE), (b) common
base (CB) and (c) common collector (CC)
In the simplest case, the polarization for a transistor requires two voltage sources, one for the loop proper polarization of the emitter diode and a reverse bias to the diode loop collector (Figure 4.1.3). Because in this connection is the common base junction of the two loops wiring connections called Common Ground (Common Base). The common base connections described shortly as KB and the CB is shown in Figure 4.2.1, where, for special emphasis, the base is grounded. These sources are characterized by indicators of the electrode and the common terminal to which they pertain. Thus we have the VEB and VCB, which are connected between the base and emitter and collector respectively.
Another case of connection of the sources is when the loops are a common emitter. This connection is called the common emitter connection (Common Emitter, CE The KE) and is shown in Figure 4.2.1. The respective sources denoted as VBE and VCE.
In the third case is the common collector connection (Common Collector, CC heart rate), in which two loops are common in all the manifold. The connection is shown in Figure 4.2.1g, and the sources denoted as VBC and VEC.
In all joints should be given special attention to that, at least in analog electronics, diode emitter must always be properly polarized. Moreover, should the collector diode is reverse biased.
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4.2.1 Connection of common emitter
The change of connectivity in a common base of transistor in common emitter or common collector, does not change the internal function. The currents in all three cases the times and maintain their measures, provided they comply with the foregoing.
Figure 4.2.2 shows a common emitter connection. In this circuit is the source for powering the VBB loop of the diode base and source VCC to power the loop of the collector diode. The source VBB should be biases correctly emitting diode and the diode VCC collector. The resistors RB and RC serves to limit the currents in each loop. Among the terminal displays the base-emitter voltages VBE and collector-emitter VCE.
+
Figure 4.2.2 Connection of common emitter
The characteristic curve of current and voltage base-base emission bu taken when changing the VBB and measured the IB. The typical power base resembles the same characteristic as any passage between base and emitter formed the equivalent of a simple diode. The diode begins to conduct when the voltage
0,7 V
Figure 4.2.3 Typical power base
Transistors
edges (VBE) exceed 0,7 V, transistors on silicon (sch.4.2.3), and control of this fact is used often hoping to first determine the proper functioning of a transistor.
Because the base current of rising steeply for VBE> 0,7 V, I, like the contact PN, the voltage between base emissions remain constant at a price VBE = 0,7 V, when the emitter diode conducts.
Base of the loop can be calculated using the equation
RF
4.2.1
The collector characteristic curves obtained when varying the VCC and measure the collector current IC maintaining constant base current of IB throughout the measurement. Because the base current of a variable parameter, the result is to have a flock of curves (Fig 4.2.4) and each curve corresponds to a value of L, which appears above the corresponding curve.
Figure 4.2.4 Characteristic curves collector
Each characteristic collector current for nonzero current of base, presents the following general characteristics:
• When the collector-emitter voltage is zero (VCE = 0 V), the collector diode is reverse biased and the collector current is 0 and that (IC = 0 mA).
• For values of collector-emitter voltage between 0V and approximately 1 V, the
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collector current increases sharply and then becomes almost constant value. This behavior is directly related to the diode reverse bias collector.
• For values of collector-emitter voltage of 1 V larger the collector current remains practically constant and independent of the collector emitter voltage. In this region the transistor behaves more as a source of constant current.
• When the collector emitter voltage exceeds a value, in the case of Fig 4.2.4 to 30 V, collector current of sharply increasing. This is due to degradation of the diode so that the collector device ceases to function as transistors. The value of the collector-emitter voltage of the diode collapses collector refers to technical brochures as breakdown voltage collector-emitter.
The characteristic curves of a transistor collector current set four areas where the operation of the transistor varies considerably from one to another ..
Transistor regions of operation
• The first area is the region of small voltages VCE in which the collector current increases sharply (collector-emitter voltage between 0 V and about 1 V, fig 4.2.4). This steeply sloping area of the region called saturation curves (saturation region). In this region the collector diode is reverse biased and the collector-emitter voltage denoted VCESAT.
• The second area is that which corresponds to a collector voltage between 1 V and 30 V (sch.4.2.4). In this region the emitter diode is properly biased, while the collector diode is reverse biased. In this region, which is the most important, the collector current is determined only by the current base. This area represents the normal operation of the device and is therefore called the active region (active region). In this device behaves as a power source (IC = DC · IB).
• The third area is that in which the collector voltage has exceeded the breakdown voltage collector-emitter. In this region, as mentioned, the device ceases to function as transistors, called region decomposition (breakdown region).
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Transistors
• Finally, there is the characteristic which corresponds to a base current of 0. In this the collector current is very small but not zero. This divergence from ex.4.1.4 because the transistors are not ideal device, the diodes are real and
consequently there are leakage currents. This characteristic determines the collector current cutoff current of collector (collector cutoff current, Iceo, resulting from heat produced bodies and leaks). The area defined by this region is called the characteristic cutoff (cutoff region).
Flows through the transistor by a not negligible collector current and the edges of the VCE develops a tendency which can be significant. For this reason the strength which consumes can be significant and should not exceed the maximum price that the manufacturer in technical brochures. The power which is consumed in a transistor is given by
Pd = VCE · IC 4.2.2
The exact calculation should include the power that is consumed by transistors in the base current of the loop. Due to the small voltage drop in emitting diode (~ 0,7 V) and very small current base, this power is very small and considered negligible.
4.2.2 Equivalent circuit
The analysis and design of transistor circuits requires knowledge of their characteristics and an understanding of the behavior of transistors in a circuit. To better understand the behavior and easier computing a response to a transistor circuit, using the equivalent circuit. The equivalent circuit of a transistor, as was the passage in chapter 3, approximates the behavior of the circuit layout. Obviously, a simple equivalent circuit is well suited to simple functions of the circuit layout. The more requirements that the more we want the equivalent
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namo circuit close to the actual operation of the device (eg the response at low and high frequencies, noise and transient behavior) so what is the complex equivalent circuit of the transistor.
Figure 4.2.5 (a) Equivalent circuit of a transistor and the ideal characteristic current, (b) base and (c) collector
The simplest approach is a transistor, which covers at least part of the characteristic of the preceding paragraph, is shown in sch.4.2.5a. This circuit includes the diode emitter and collector current source.
The equivalent diode base shows behavior similar to that of the equivalent circuit of the diode seen in Chap.3, which begins to conduct when the voltage base - emitter exceed 0,7 V and the current increases very sharply as shown in fig 4.2. 5b. This behavior serves the designer in analyzing a circuit in
filaments, and a generally accept that the voltage between base and emitter is VBE = 0,7 V.
The equivalent current source collector approached with the help of the ideal characteristics of Fig 4.2.5g. They illustrate well the cut area and active area of operation, but do not cover the region and saturation region division. Clearly, however, that these last areas of operation are not relevant. Indeed in the splitting device does not function as a transistor. Also, saturation in the collector current is not controlled by the power base. So, given that the voltage collector - emitter is very small, we conclude that a collector current source with its characteristic sch.4.2.5 adequately describes the behavior of the transistor.
BCVVBECE
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Example 4.2.1
If in figure 4.2.2 we VBB = 15 V, VCC = 15 V, RB = 470 KO and RC = 3,3
KO to calculate the base current of the collector current and voltage
collector - emitter, given that, DC = 100.
Solution
We use the equivalent circuit of Fig 4.2.5. With the help
of ex.4.2.1. reading the current database:
I VBB-VBE 15V-0, 7V 14,3 V 301u.
Im = Rb = 4.7 x = 470KO 105O = 3 °, 4mA
The collector current is given by ex.4.1.4
IC =, dc · IB = 100 mA × 30.4 = 3,04 mA
The collector-emitter voltage will be calculated from the collector loop
sixth, giving
VCE = VCC - IC · RC = 15V - (3,04 CO) x (3.3 KO) = 15V - 10,3 V = 4,97 V
The latter value confirms that the transistor operates in
active region.
4.2.3 Reading the technical characteristics
In the manufacturers data sheet indicating the code number and type of transistor. Following are some key applications for which provision is made, eg high-frequency amplifiers, oscillators and converters. Here are the absolute maximum estimates, which are commonly referred to at 25 ° C, unless otherwise indicated. These are a barrier to applications where
a designer or conservator intends to use its transistors. We must pay special attention to this point because the manufacturer does not assume responsibility for the operation of the device when outside these limits. Moreover, the recognition
Joined should note that from a set of transistors, eg type
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Can some 2N3904, whose numbers are too small to operate beyond these limits. But this is not a generalization of the random selection rule. Absolute maximum estimates of the trends that are split by size:
VCB: diode breakdown voltage collector-base (eg 60 V)
VCEO: breakdown voltage collector-emitter with (eg 40 V) on the open
VEB: breakdown voltage base-emitter diode (eg, 6 V)
As for the maximum current and power consumption figures are given by:
IC: maximum collector current for ambient temperature (Ta)
PDA: consumed power for maximum ambient temperature (Ta)
PDC: consumed power for maximum shell temperature (TC)
The coefficient of the DC current gain,, DC, and met with a different form and, instead of b, manufacturers use the h (hybrid parameters) (h parameters). So in the technical brochures, DC found in hFE. In practice, the DC current gain does not remain constant with collector current. That's why manufacturers give values of hFE for various values of collector current. It then shows the type of housing with the terminals and follow the so-called static (static) and dynamic (dynamic) characteristics.
SUMMARY 4.2
• The transistor consists of three layers of different semiconducting type (NPN or PNP) and with different levels of enrichment.
• The operation of transistors and are considering participating electrons and holes and so called bipolar
• To properly operate a transistor emitter diode must be properly polarized, while the collector diode reverse
• The rate aDC expresses the percentage of load-emitting
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be provided by the emitter and reach the collector. The a ^ is always less than one.
• The rate of gain DC current, DC indicates how many times larger currents can be in control of the collector circuit via the power base.
• The basic transistor configurations is a three and out the name of the terminal which is common. These connections are common emitter (CE), the common wiring base (CB) and common collector connection (CC).
• In a transistor distinguished four areas, the saturation region, the active region, the area around the cut and split.
