Principle and characteristics of transistor

1、 The current amplification principle of transistor

There are two types of transistors based on their materials: storage transistors and silicon transistors.

And each of them has two structural forms, NPN and PnP, but the most commonly used are silicon NPN and PnP transistors. Except for the power polarity, their working principles are the same. The following only introduces the current amplification principle of NPN silicon transistors.

When manufacturing a transistor, it is consciously necessary to make the majority carrier concentration in the emitter region higher than that in the base region. At the same time, the base region should be made very thin, and the impurity content should be strictly controlled. In this way, once the power is turned on, due to the correct emitter junction, the majority carriers (electrons) in the emitter region and the majority carriers (control holes) in the base region can easily cross the emitter structure and diffuse in opposite directions. However, because the concentration of the former is higher than that of the latter, the current passing through the emitter junction is basically an electron current, which is called the emitter current Ie. Due to the thin base region and the reverse bias of the collector junction, most of the electrons injected into the base region cross the collector junction and enter the collector region, forming a collector current. Ic, leaving only a small (1-10%) number of electrons to recombine with the holes in the base region. The recombined holes in the base region are supplied by the base power source Eb By supplementing the memory, the base current Ibo was formed. According to the principle of current continuity, Ie=Ib+Ic means that by adding a small Ib to the base, a larger Ic can be obtained at the collector, which is called current amplification. Ic and Ib maintain a certain proportional relationship, that is, β 1=Ic/Ib. In the formula, β - is called the DC amplification factor, and the ratio of the change in collector current △ Ic to the change in base current △ Ib is: β=△ Ic/△ Ib. In the formula, β - is called the AC current amplification factor. Due to the small difference in values between β 1 and β at low frequencies, sometimes for convenience, the two are not strictly distinguished. The value is about tens to hundreds. A transistor is a current amplification device, but in practical use, it often utilizes the current amplification effect of a transistor, which is converted into voltage amplification effect through resistance.

2、 Characteristic curve of transistor

1. The input characteristic diagram 2 (b) is the input characteristic curve of the transistor, which represents the relationship between Ib and Ube. Its characteristics are: 1) When Uce is in the range of 0-2 volts, the position and shape of the curve are related to Uce, but when Uce is higher than 2 volts, the curve Uce is basically independent. Usually, the input characteristic can be represented by two curves (I and II).

2) When Ube<UbeR, the segment of Ib ≈ O (0~UbeR) is called the "dead zone". When Ube>UbeR, Ib increases with the increase of Ube. When amplified, the transistor operates in a more straight segment.

3) The input resistance of a transistor is defined as: rbe=(△ Ube/△ Ib) Q point, and its estimated formula is: rbe=rb+(β+1) (26 millivolts/Ie millivolts). rb is the base resistance of the transistor, and for low-frequency low-power transistors, rb is about 300 ohms.

2. Output characteristics

The output characteristic represents the relationship between Ic and Uce (with Ib as the parameter). As shown in Figure 2 (C), the output characteristic is divided into three regions: cutoff region, amplification region, and saturation region. When Ube<0 in the cutoff region, Ib ≈ 0, and there are no electron injections into the base region in the emission region. However, due to the thermal motion of molecules, there is still a small amount of current passing through the collector, that is, Ic=Iceo, which is called the penetration current. At room temperature, Iceo is about a few microamperes, and germanium tubes are about tens of microamperes to hundreds of microamperes. Its relationship with the reverse current Icbo of the collector is: Icbo=(1+β) Icbo. At room temperature, the Icbo of silicon tubes is less than 1 microampere, and the Icbo of germanium tubes is about 10 microamperes. For germanium tubes, for every 12 ℃ increase in temperature, The Icbo value doubles, while for every 8 ℃ increase in the temperature of the silicon transistor, the Icbo value doubles. Although the Icbo value of the silicon transistor changes more dramatically with temperature, the germanium transistor is still heavily affected by temperature due to its larger Icbo value. In the amplification region, when the emitter junction of the transistor is in forward bias and the collector junction is in reverse bias operation, Ic changes approximately linearly with Ib, and the amplification region is the area where the transistor operates in an amplification state. When both the emitter and collector junctions are in a positive bias state in the saturation region, Ic basically does not change with Ib and loses its amplification function. Based on the bias of the emitter and collector junctions of the transistor, its working state may be determined.

Figure 2: Input and output characteristics of a transistor

The cutoff region and saturation region are the areas where the transistor operates in a switching state. When the transistor is turned on, the operating point falls in the saturation region, and when the transistor is turned off, the operating point falls in the cutoff region.

3、 Main parameters of transistor

1. DC parameters

(1) The reverse saturation current Icbo between the collector and base, when the emitter is open (Ie=0) and a specified reverse voltage Vcb is applied between the base and collector, is only related to temperature and remains constant at a certain temperature. Therefore, it is called the reverse saturation current between the collector and base. A good transistor has a very small Icbo. The Icbo of a low-power germanium transistor is about 1-10 microamperes, while the Icbo of a high-power germanium transistor can reach several milliamps. The Icbo of a silicon transistor is very small, at the nanoampere level.

(2) When the collector emitter reverse current Iceo (penetration current) is open at the base (Ib=0), the collector current is determined by applying a specified reverse voltage Vce between the collector and emitter. Iceo is approximately β times that of Icbo, i.e. Iceo=(1+β). Icbo and Iceo are greatly affected by temperature and are important parameters for measuring the thermal stability of tubes. The smaller the value, the more stable the performance. The Iceo of low-power germanium tubes is larger than that of silicon tubes.

(3) When the collector is open and a specified reverse voltage is applied between the emitter and base, the current at the emitter is actually the reverse saturation current at the emitter junction.

(4) The DC current amplification factor β 1 (or hEF) refers to the ratio of the DC current output by the collector to the DC current input by the base when there is no AC signal input in the common emitter connection, that is: β 1=Ic/Ib

2. Communication parameters

(1) The AC current amplification factor β (or hfe) refers to the ratio of the change in collector output current △ Ic to the change in base input current △ Ib in a common emitter connection, that is, β=△ Ic/△ Ib. Generally, the β of a transistor is between 10-200. html "target=" -blank "title=" 10-200 ">10-200. If β is too small, the current amplification effect is poor. If β is too large, although the current amplification effect is large, the performance is often unstable.

(2) The common base AC amplification factor α (or hfb) refers to the ratio of the change in collector output current △ Ic to the change in emitter current △ Ie when connected in a common base configuration, i.e., α=△ Ic/△ Ie. Since △ Ic<△ Ie, α<1. If the alpha of a high-frequency transistor is greater than 0.90, the relationship between alpha and beta can be used: alpha=beta/(1+beta) beta=alpha/(1- alpha) ≈ 1/(1- alpha)

(3) The cut-off frequencies f β and f α are 0.707 times the frequency when β drops to low frequencies, which is the cut-off frequency f β of the common emitter; When α decreases to 0.707 times the frequency of low frequency, it is the cut-off frequency of the common base, f α of β. f α is an important parameter indicating the frequency characteristics of the tube, and their relationship is: f β ≈ (1- α) f α

(4) The characteristic frequency fT decreases as the frequency f increases, and when β decreases to 1, the corresponding fT is an important parameter that comprehensively reflects the high-frequency amplification performance of the transistor.

3. Limit parameter

(1) When the collector current Ic increases to a certain value, causing the β value to decrease to 2/3 or 1/2 of the rated value, the maximum allowable current ICM of the collector is called ICM. So when Ic exceeds ICM, although it does not cause damage to the tube, the β value significantly decreases, affecting the amplification quality.

(2) When the emitter is open, the reverse breakdown voltage of the collector junction is called BVEBO.

(3) When the collector is open, the reverse breakdown voltage of the emitter junction is called BVEBO.

(4) Collector emitter breakdown voltage BVCEO is the maximum allowable voltage applied between the collector and emitter when the base is open. If Vce>BVCEO during use, the transistor will be broken down.

(5) The maximum allowable dissipated power of the collector PCM when the collector current exceeds Ic and the temperature increases, and the parameter changes of the tube due to heating do not exceed the allowable value, is called PCM. The actual dissipated power of the tube is the product of the collector DC voltage and current, i.e. Pc=Uce × Ic. When used, Pc<PCM. PCM is related to heat dissipation conditions, and adding heat sinks can improve PCM.