This article thoroughly analyzes the working principle of the triode!
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Regarding triodes, I believe that everyone who works on hardware should have read the basic principles. Now we are reviewing the old and learning the new, so it is best to read it with questions in mind.
I have prepared a few questions here. Let’s read on with these questions in mind.
1. Why does the collector junction undergo reverse conduction and generate Ic? This seems to contradict the unidirectional conductivity of the PN junction emphasized by the diode principle.
2. Why is the collector current Ic in the amplification state only controlled by the current Ib and has nothing to do with the voltage? That is, why is there a fixed amplification factor relationship between Ic and Ib? Although the base region is thin, as long as Ib is zero, Ic is zero.
3. In the saturation state, when the Vc potential is very weak, a large reverse current Ic will still be generated.
Why are these three points mentioned above?
Many textbooks do not properly explain this part of the content, especially popular textbooks for beginners and intermediate students, which mostly adopt an evasive approach, only giving the conclusion without explaining the reasons.
Even for highly professional textbooks, the explanation methods they use are often questionable. These problems are mainly reflected in the inappropriate approach of the explanation method, which makes the explanation content inconsistent and even makes it worse than not explaining it, making it easy for beginners to feel confused after reading it.
Traditional teachings and problems
The traditional explanation is generally divided into three steps, taking the NPN type as an example (all discussions below are based on the NPN type silicon tube as an example), as shown in the figure below.
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The emitter region injects electrons into the base region;
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Diffusion and recombination of electrons in the base region;
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The collector region collects electrons diffused from the base region.
Question 1 : In step 3, when explaining the formation of collector current Ic, this explanation method does not focus on the nature of the carrier to explain the reverse bias conduction of the collector region, thereby generating Ic, but inappropriately emphasizes the high potential effect of Vc, while emphasizing the thinness of the base region. This emphasis can easily lead to misunderstandings. It is believed that as long as Vc is large enough and the base region is thin enough, the collector junction can be reversely conducted, and the unidirectional conductivity of the PN junction will fail. In fact, this is exactly contradictory to the current amplification principle of the transistor. The current amplification principle of the transistor requires that in the amplified state, Ic and Vc must be independent of each other in quantity, and Ic can only be controlled by Ib.
Question 2 : It cannot explain the saturation state of the transistor very well. When the transistor works in the saturation region, the value of Vc is very small or even lower than Vb. At this time, there is still a large reverse saturation current Ic, that is, when Vc is very small, the collector junction will still be reversely conductive. This is obviously inconsistent with the emphasis on the high potential effect of Vc.
Question 3 : The traditional explanation of step 2 overemphasizes the thinness of the base region, which can easily lead to the misunderstanding that the thinness of the base region is enough to support the reverse conduction of the transistor collector junction. As long as the base region is thin enough, the collector junction may lose the unidirectional conductivity of the PN junction. This is obviously inconsistent with the experience of people using the unidirectional conductivity of the two PN junctions inside the transistor to determine the pin name. Even if the base region is very thin, when people judge the pin name, they have not found that the unidirectional conductivity of the PN junction fails due to the thinness of the base region. The base region is very thin, but the unidirectional conductivity of the two PN junctions is still intact, which gives people a way and basis to determine the pin name of the transistor.
Question 4 : In step 2, when explaining why Ic is controlled by Ib and why there is a fixed proportional relationship between Ic and Ib, it is not explained in a vivid way. It only emphasizes the thinness of the base region and the low doping degree from the process, but cannot fundamentally explain why the current amplification factor remains unchanged.
Question 5 : The natural connection between diodes and transistors in principle cannot achieve a natural transition in content. It even leads to contradictory ideas. The diode principle emphasizes that the PN junction is unidirectional and reversely cut off, while the transistor principle requires that the PN junction can be reversely conductive. At the same time, it cannot reflect the historical connection between the crystal transistor and the electronic transistor in the principle of current amplification.
In order to explain the problem naturally, we must choose the right entry point. To explain the principle of triode, we start with the principle of diode. The structure and principle of diode are very simple. A PN junction inside has unidirectional conductivity, as shown in the figure.
It is obvious that the diode is in reverse bias state and the PN junction is cut off. We should pay special attention to the cut-off state here. In fact, when the PN junction is cut off, there is always a small leakage current, that is to say, the PN junction always has the reverse turn-off phenomenon, and the unidirectional conductivity of the PN junction is not 100%.
Why does this phenomenon occur?
This is mainly because in addition to the majority carrier "holes" generated by "doping", there are always a very small number of intrinsic carrier "electrons" in the P region. The same is true for the N region. In addition to the majority carrier electrons, there are also a very small number of carrier holes.
When the PN junction is reverse biased, the majority carriers that can conduct forward are pulled toward the power supply, making the PN junction thicker, and the majority carriers can no longer perform the function of carrying current and conducting electricity through the PN junction.
Therefore, the formation of leakage current at this time mainly relies on minority carriers, and it is the minority carriers that play the conductive role.
As can be seen from the above figure, the direction of the internal electric field of the PN junction is from the N region to the P region. This internal electric field promotes the passage of minority carriers through the PN junction.
The reason why the leakage current is very small is that the number of minority carriers is too small. Obviously, the size of the leakage current at this time mainly depends on the number of minority carriers. If you want to artificially increase the leakage current, you only need to find a way to increase the number of minority carriers during reverse bias.
Therefore, as shown in the figure, if the number of minority carriers can be artificially increased in the P region or N region, it is natural that the leakage current will be artificially increased.
In fact, this is the principle of photodiodes.
The photodiode is the same as the ordinary photodiode, its PN junction has unidirectional conductivity. Therefore, the photodiode should be applied with reverse voltage when working, as shown in the figure.
When there is no light, there is also a small reverse saturation leakage current in the circuit, generally 1×10-8 —1×10 -9A (called dark current), which is equivalent to the photodiode being cut off;
When light is irradiated, the vicinity of the PN junction is bombarded by photons, and the bound valence electrons in the semiconductor absorb the photon energy and are triggered to produce electron-hole pairs. The number of these carriers has little effect on the majority carriers, but for the minority carriers in the P and N regions, the concentration of minority carriers will be greatly increased. Under the action of reverse voltage, the reverse saturation leakage current will increase greatly, forming a photocurrent, which changes accordingly with the change of the incident light intensity.
When the photocurrent passes through the load R, a voltage signal that changes with the incident light will be obtained at both ends of the resistor. This is how the photodiode completes the electrical function conversion.
Photodiodes work in a reverse bias state because light can increase the number of minority carriers, so light will cause a change in reverse leakage current. People use this principle to make photodiodes. Since the increase in leakage current is artificial, the increase in leakage current can be easily controlled artificially.
At this point, we must focus on explaining the roles and properties of majority carriers and minority carriers when the PN junction is forward biased or reverse biased. When forward biased, majority carriers conduct current, while when reverse biased, minority carriers conduct current. Therefore, when the forward bias current is large and the reverse bias current is small, the PN junction shows unidirectional electrical properties.
In particular, it is important to point out that it is very easy for minority carriers to pass through the PN junction in the reverse direction when reverse biased, even easier than for majority carriers to pass through the PN junction in the forward direction when forward biased.
Why?
As we all know, there is an internal electric field inside the PN junction caused by the mutual diffusion of majority carriers, and the direction of the internal electric field always hinders the forward passage of majority carriers. Therefore, when majority carriers pass through the PN junction in the forward direction, they need to overcome the effect of the internal electric field, which requires an external voltage of about 0.7 volts, which is the gate voltage for the forward conduction of the PN junction.
When reverse biased, the internal electric field will be strengthened under the action of the power supply, that is, the PN junction will be thickened. When the minority carriers pass through the PN junction in the reverse direction, the direction of the internal electric field is consistent with the direction of the minority carriers passing through the PN junction. That is to say, the internal electric field at this time will not only not hinder the reverse passage of minority carriers, but may even help it.
This leads to the conclusion we mentioned above: it is very easy for minority carriers to pass through the PN junction in the reverse direction when reverse biased, even easier than for majority carriers to pass through the PN junction in the forward direction when forward biased.
This conclusion can well explain the "Problem 2" mentioned above, which is the saturation state of the transistor that will be discussed in the subsequent content of the textbook. In the saturation state of the transistor, the collector potential is very low and may even be close to or slightly lower than the base potential. The collector junction is at zero bias, but a large reverse current Ic of the collector junction is still generated.
Continue to discuss the reverse bias state of the PN junction.
By using light to control the number of minority carriers generated, the size of the leakage current can be artificially controlled. In this case, people will naturally think about whether the control method can be changed, and the number of minority carriers in the N region or P region can be increased by electric injection instead of light, so as to control the leakage current of the PN junction.
That is to say, instead of using "light", the "electricity" method is used to control the current (light increases the intrinsic carriers, while the electrical injection discussed later increases the doped carriers. The intrinsic carriers appear in pairs, which are electron-hole pairs, with positive and negative correspondence. This is different from doped carriers).
Next, we will focus on the P region. The minority carriers in the P region are electrons. If you want to inject electrons into the P region using electrical injection, the best way is to add an N-type semiconductor under the P region using a special process as shown in the figure.
In fact, the picture above is the prototype of the NPN type crystal transistor, and the names and functions of its corresponding parts are exactly the same as those of the transistor.
To facilitate discussion, we will use the names corresponding to the transistor (such as "emitter junction", "collector", etc.) for the names of the various parts shown in the figure.
There are a large number of electrons as majority carriers in the N-type semiconductor in the bottom emitter region. Moreover, as shown in the figure, it is very easy to inject or emit the electrons in the emitter region into the P region (base region) as long as the emitter junction is forward biased.
Specifically, a sufficient positive gate voltage (about 0.7 volts) is applied between the base and the emitter. Under the action of the external gate voltage, the electrons in the emitter region will be easily injected into the base region, thus achieving a change in the number of minority carriers "electrons" in the base region.
Formation of collector current IC
After the emitter junction is turned on by applying a forward bias voltage, under the action of the applied voltage, the majority carriers in the emitter region - electrons - will be easily emitted in large quantities into the base region.
Once these carriers enter the base region, their properties in the base region (P region) still belong to the properties of minority carriers. As mentioned above, minority carriers can easily reverse through the PN junction in the reverse bias state, so these carriers - electrons can easily pass upward through the collector junction in the reverse bias state to reach the collector region to form the collector current Ic.
It can be seen that the formation of collector current does not necessarily rely on the high potential of the collector. The size of the collector current mainly depends on the emission and injection of carriers in the emitter region into the base region, and on the degree of such emission and injection. The degree of such carrier emission and injection has nothing to do with the height of the collector potential.
This naturally explains why the collector current Ic of the transistor in the amplification state is independent of the collector potential Vc. In the amplification state, Ic is not controlled by Vc. The main function of Vc is to maintain the reverse bias state of the collector junction to meet the external circuit conditions required in the amplification state of the transistor.
The following conclusion can also be drawn about Ic: the essence of Ic is the "minority carrier" current, which is an artificially controllable collector junction "leakage" current achieved through electron injection, so it can easily pass through the collector junction in the reverse direction.
The relationship between Ic and Ib
Continuing with the above discussion, the collector current Ic has nothing to do with the size of the collector potential Vc, but mainly depends on the degree of emission injection of carriers in the emitter region into the base region.
Through the above discussion, it is now clear that when the transistor is in the current amplification state, the main current inside is formed by the carrier electrons passing through the transistor from the emitter region through the base region to the collector region. That is, the current Ic passing through the transistor is mainly electron flow.
This electron flow is very similar to the electronic triode in history. The figure below is a schematic diagram of the principle of the electronic triode. The current amplification principle of the electronic triode can be explained naturally because of its intuitive structure.
It is easy to understand that the fixed proportional relationship between the triode Ib and Ic mainly depends on the structure of the tube grid (base).
When the external circuit conditions are met, the electronic triode works in the amplification state. In the amplification state, the current passing through the tube is mainly the electron flow from the emitter through the gate to the collector. When the electron flow passes through the gate, it is obvious that the gate will intercept it, and there is a current interception ratio problem when intercepting the current.
The size of the interception ratio is mainly related to the density of the gate. If the gate is made dense, its equivalent interception area is large, the interception ratio is naturally large, and more electrons are intercepted. On the contrary, if the interception ratio is small, less electrons are intercepted.
The electron flow intercepted by the gate is actually the current Ib, and the remaining electron flow that passes through the gate and reaches the collector is Ic. As can be seen from the figure, as long as the structural size of the gate is determined, the interception ratio is determined, that is, the ratio of Ic to Ib is determined.
Therefore, as long as the internal structure of the tube is determined, this ratio will remain fixed. It can be seen that the β value of the current amplification factor is mainly related to the density of the gate. The denser the gate, the greater the interception ratio, and the lower the corresponding β value. The sparser the gate, the smaller the interception ratio, and the higher the corresponding β value.
In fact, the current amplification relationship of the crystal triode is similar to that of the electronic triode.
The base of a transistor is equivalent to the gate of an electron transistor, and the base region is equivalent to the grid, except that the grid of the transistor is dynamic and invisible. In the amplified state, when the electron flow that runs through the entire tube passes through the base region, the base region acts similarly to the grid of the electron tube and intercepts the electron flow (when the electron passes through the base region, it will recombine with the base region holes and disappear).
If the base region is made thin and has a low doping degree, the number of holes in the base region will be small, and the amount of holes intercepting electrons will be small. This is equivalent to the grid of the electron tube being sparse. Otherwise, the amount of interception will be large.
Obviously, as long as the internal structure of the transistor is determined, the current cutoff ratio is also determined. Therefore, in order to obtain a larger current amplification factor and make the β value high enough, the base region is often made very thin when making the transistor, and its doping degree is also controlled to be very low.
Unlike electron tubes, the cutoff of transistors is mainly achieved by the positively charged "holes" distributed in the base region neutralizing the negatively charged "electrons" in the electron flow passing through. Therefore, the effect of cutoff mainly depends on the number of holes in the base region.
Moreover, this process is a dynamic process. The "holes" are constantly neutralized with the "electrons", and at the same time, the "holes" are constantly replenished by the external power supply. In this dynamic process, the total equivalent number of holes is constant. The total number of base region holes mainly depends on the doping degree and the thickness of the base region. As long as the transistor structure is determined, the total quota of base region holes is determined, and its corresponding dynamic total amount is determined.
In this way, the cutoff ratio is determined, and the value of the transistor's current amplification factor is a constant. This is why there is a fixed proportional relationship between the transistor's current Ic and Ib in the amplified state.
Explanation for the cut-off status
The proportional relationship shows that in the amplified state, the current Ic is controlled by the current Ib at a fixed ratio, and this fixed control ratio mainly depends on the internal structure of the transistor.
For the cut-off state when Ib is equal to 0, the problem is even simpler. When Ib is equal to 0, it means that the external voltage Ube is too small and does not reach the gate voltage value of the emitter junction. There is no carrier "electron" injection from the emitter region to the base region, so there will be no current Ib at this time, and it is even more impossible to have current Ic.
In addition, it is easier to draw the conclusion from the purely mathematical current amplification formula: Ic=βIb. If Ib is 0, then obviously Ic is also 0.
Issues that need attention in the new teaching
In the above, we have used a new approach to explore the principle of triodes in terms of explanation methods. In particular, we have focused on why the collector junction conducts in the reverse direction to form the collector current when the crystal triode is amplified. At the same time, we have also made an in-depth analysis of why the current amplification factor of the triode is a constant.
The key to this explanation method is to emphasize the principle connection between diodes and transistors.
In fact, it is easy to see from the reverse cutoff characteristic curve of the diode PN that as long as this characteristic curve is rotated 180 degrees, as shown in the figure, its situation is very similar to the output characteristics of the transistor.
This shows that there is an inevitable connection between diodes and transistors in principle. Therefore, it is very appropriate to choose such an entry point in the explanation method, starting from the reverse bias state of the PN junction to explain the transistor. Moreover, such an explanation will make the problem easy to understand and vivid, and the previous and subsequent contents will be naturally harmonious and logical.
The shortcoming of this explanation is that it starts with the leakage current of the PN junction, which easily causes conceptual confusion between intrinsic leakage current and amplified current.
Intrinsic carriers do not contribute to current amplification. The current of intrinsic carriers often has a negative impact on the characteristics of transistors, which needs to be overcome. The current amplification of transistors is mainly achieved by doping carriers. It is important to make a conceptual distinction.
In addition, it should be noted that the issue of carriers inside the crystal is not simple in nature. It involves the analysis of the energy level and band structure of the crystal, as well as the analysis of the potential barriers for carrier movement, etc. Therefore, it is not possible to make a PN junction or a transistor by simply finding one or two conductors or semiconductors with carriers. The actual manufacturing process of transistors is not that simple either.
This method of explanation is mainly to try to simplify the problem as much as possible without violating the principles of physics, and to make it as easy to understand and accept as possible. This is the main significance of this method of explanation.
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