Explain in detail the working principle of triode amplification state
There are two carriers of different polarity charges in the transistor participating in conduction, so it is called a bipolar transistor (BJT). It is a current-controlled semiconductor device with current amplification function. Its main function is to amplify weak input signals into larger amplitude electrical signals. It is the core component of many commonly used electronic circuits.
There are two main types of schematic symbols for triodes, as shown in Figure 1.
Q1 is an NPN tube, Q2 is a PNP tube, and the direction of the E-pole arrow represents the actual direction of the current when the emitter junction is forward biased. Their corresponding basic structures are shown in Figure 2.
The basic structure of a triode is formed by superimposing three adjacent and different impurity semiconductors. An electrode is drawn from each of the three impurity semiconductor regions, which we call the emitter (Emitter), collector (Collector), and base (Base) respectively; and the corresponding areas are called the emitter region, collector region, and Base region; two adjacent impurity semiconductors of different types will form a PN junction. We call the PN junction between the emitter region and the base region an emitter junction, and the PN junction between the base region and the collector region. It is a collector junction.
Transistors may have three working states in practical applications:
· Cutoff: The emitter junction is reverse biased and the collector junction is reverse biased.
· Amplification: The emitter junction is forward biased and the collector junction is reverse biased.
· Saturation: The emitter junction is forward biased and the collector junction is forward biased.
Below we take the NPN transistor as an example to explain in detail the working principle of the transistor amplification state.
Speaking of the general trend of the world, if we divide for a long time, we will unite, and if we unite for a long time, we will divide. In this small area composed of three semiconductors, a history of hegemony has also been staged. The story takes place in this area shown in Figure 3.
After applying two voltages to the NPN transistor without any processing, as shown in Figure 4.
To put the NPN tube in an amplified state, the voltage Vce applied across the CE junction is greater than the voltage Vbe applied across the BE junction. Therefore, the potentials of the three poles of the NPN tube are: VC>VB>VE, (the emitter potential Ve is the reference potential 0V). In this way, the emitter junction of the transistor is forward biased, while the collector junction is reversely biased. Bias, this is the basic condition for the triode to be in the amplified state.
At the moment when the voltage is connected, assuming that the base-emitter (emitter junction) bias voltage Vbe=5V, and the collector-emitter bias voltage Vce=12V, two N-type semiconductors and P-type semiconductors form two PN junctions, The BE junction (emitter junction) is forward voltage biased and turned on, limiting the base potential to 0.7V (silicon tube), while the collector potential is 12V (instantaneous potential) due to the reverse bias cutoff of the PN junction. At this time, the collector current), as shown in Figure 5.
Okay, everything is ready, a war is about to begin! When a forward voltage Vbe (forward bias) is applied to the emitter junction, since the doping concentration in the emitter region is very high (the highest among the three regions) and the doping concentration in the base region is the lowest, the majority of carrier electrons in the emitter region Will continuously diffuse through the emitter junction to the base region (the transfer of carriers from the high concentration area to the low concentration area due to the concentration difference is called diffusion), forming the emitter junction electron diffusion current Ien (the direction of the current is consistent with the electron diffusion The direction of movement is opposite).
At the same time, the majority of carrier holes in the base region also diffuse to the emitter region, forming a hole diffusion current Iep (the current direction is the same as Ien). Obviously, Iep is very small compared to Ien. However, the revolutionary There is no size in power! Ien and Iep add up the emitter current Ie, as shown in Figure 6.
The majority carrier electrons that diffuse from the emitter region to the base region have the highest concentration near the emitter junction, and the farther away from the emitter junction, the lower the concentration, thus forming a certain electron concentration difference. This concentration difference causes the electrons that diffuse to the base region to continue Diffusion towards the collector junction. During the electron diffusion process, a small part of the electrons recombine with the majority carrier holes in the base region, thus forming the base current Ibn. We know that the base region is very thin and the doping concentration is low. Therefore, there are few opportunities for electrons and holes to recombine, and the base current Ibn is also very small. Most electrons will be diffused to the collector junction, as shown in Figure 7.
Since the collector junction is reverse biased, the internal electric field in the space charge region is further strengthened (the PN junction becomes wider), which in turn exerts a strong attraction to the carrier electrons from the base region that diffuse to the boundary of the collector junction ( Electrons are negatively charged, and like-like repels and opposite-like attract), causing them to quickly drift through the collector junction (the movement of carriers caused by the attraction or repulsion of the electric field is called drift), thus forming the collector current Icn (the direction is consistent with the direction of electron drift). on the contrary). Obviously, Icn=Ien-Ibn, because a small part of the million-strong army is in the base area, and most of the rest is in the collector area, as shown in Figure 8.
After the majority carrier electrons enter the collector area, the minority carrier holes in the collector area (N-type) and the minority carrier electrons in the base area (P-type) will also drift, forming a current Icbo , while others will cross the base area and reach the emission area to form Iceo, as shown in Figure 9.
Icbo represents the collector-base reverse saturation current, and Iceo represents the collector-emitter reverse saturation current (also collectively referred to as penetration current). They are not controlled by the emitter junction voltage Vbe and do not contribute to the amplification of the current. It only depends on the temperature and the concentration of minority carriers, of course the smaller the better. Under the same conditions, the penetration current of silicon tubes is smaller than that of germanium tubes. In some high-power applications, an external penetration current release resistor must be connected to prevent the triode from overheating and damage due to penetration current.
In the amplified state of the transistor, as long as the external emitter junction voltage Vbe is controlled, the base current IB will also change accordingly, thereby controlling the number of most current-carrying electrons in the emitter region, and ultimately the collector current IC. It can be seen from the principle of transistor amplification that the so-called "amplification" does not amplify the base current IB, but uses a smaller base current IB value to control a larger collector current IC value. From the external circuit It looks like IB has been magnified, which is also the same principle as "four ounces can pull out a thousand catties".
In short, if the above process seems too troublesome, I can summarize it in three sentences:
1) Apply forward voltage to the emitter junction, and diffusion movement forms the emitter current Ie. 2) The recombination motion system of free electrons and holes in the diffusion base region forms the base current Ib. 3) When a reverse voltage is applied to the collector junction, the drift movement forms the collector current Ic.
DC amplification characteristics
Just like bearing in mind the unidirectional conduction characteristics of diodes, whenever we talk about triodes, we must think of "current amplification".
The conclusion is: the triode is a device with a current amplification function, and the small current on the B pole of the triode can control the large current on the C pole.
In order to make this boring concept more vivid, we use a painting to metaphor the current amplification effect of a triode.
Comparing the triode to a water tank, its drainage pipe is controlled by a valve. The flow of the drainage pipe can be controlled by fine-tuning the valve. The water tank is like the C pole of the triode, the valve is like the B pole, and the drain pipe is equivalent to the E pole. When the b-electrode of the transistor obtains the tiny bias voltage (+0.7V) as shown in the figure, it is as if the valve is opened, and water can quickly flow downward from the water tank, and a current flows from the c-electrode to the e-electrode. And the bias voltage of the B pole of the transistor disappears, just like the valve is closed, and there is no current from the C pole to the E pole.
Review Editor: Huang Fei
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