Principle of transistor

1. The Current Amplification Principle of Transistors:
Crystal transistors (hereinafter referred to as transistors) are divided into two types based on their materials: germanium and silicon. Each type has two structural forms: NPN and PNP. However, the most commonly used are silicon NPN and PNP transistors. Aside from the power supply polarity, their operating principles are the same. The following describes only the current amplification principle of silicon NPN transistors.

Figure 1. Structure of a transistor (NPN)

Figure 1 shows the structure of an NPN transistor. It consists of two N-type semiconductors sandwiched between a P-type semiconductor. As can be seen from the figure, the PN junction formed between the emitter and base regions is called the emitter junction, while the PN junction formed between the collector and base regions is called the collector junction. The three leads are called emitter e, base b, and collector.
When the potential at point b is a few tenths of a volt higher than that at point e, the emitter junction is forward biased. When the potential at point C is a few volts higher than that at point b, the collector junction is reverse biased, and the collector supply voltage Ec must be higher than the base supply voltage Ebo.
During transistor manufacturing, the majority carrier concentration in the emitter region is intentionally higher than that in the base region. The base region is also made very thin, and the impurity content is strictly controlled. This way, once power is applied, the majority carriers (electrons) in the emitter region and the majority carriers (holes) in the base region easily intercept and diffuse across the emitter structure due to the correct emitter junction. However, because the concentration of the former is higher than the latter, the current through the emitter junction is primarily electron current, which is called the emitter current Ie.
Because the base region is very thin and the collector junction is reverse biased, most of the electrons injected into the base region cross the collector junction and enter the collector region to form the collector current Ic, leaving only a small number (1-10%) of electrons to recombine in the holes in the base region. The recombined base holes are replenished by the base power supply Eb, thus forming the base current Ibo. According to the current continuity principle,
Ie=Ib+Ic.
That is to say, by adding a very small Ib to the base, a larger Ic can be obtained on the collector. This is the so-called current amplification effect. Ic and Ib maintain a certain proportional relationship, namely:
β1=Ic/Ib
where: β--called DC amplification factor,
the ratio of the change in collector current △Ic to the change in base current △Ib is:
β= △Ic/△Ib
where β--called AC current amplification factor. Since the values ​​of β1 and β are not much different at low frequencies, sometimes for convenience, the two are not strictly distinguished, and the β value is about tens to more than one hundred.
The transistor is a current amplifier device, but in actual use, the current amplification effect of the transistor is often utilized to convert it into voltage amplification through resistance.

2. Transistor Characteristic Curves
1. Input Characteristics
Figure 2 (b) shows the input characteristic curve of a transistor, which shows how Ib varies with 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. However, when Uce exceeds 2 volts, the curve becomes largely independent of Uce. The input characteristic is usually represented by two curves (I and II).
2) When Ube < UbeR, Ib ≈ 0. The region (0 to UbeR) is called the "dead zone." When Ube > UbeR, Ib increases as Ube increases. During amplification, the transistor operates in a relatively straight region.
3) The transistor input resistance is defined as:
rbe = (△Ube/△Ib) Q-point. Its estimation formula is:
rbe = rb + (β + 1)(26 mV/Ie mV).
rb is the transistor base resistance. For low-frequency, low-power transistors, rb is approximately 300 ohms.
2. Output characteristics
The output characteristics represent the relationship between Ic and Uce (with Ib as the parameter). As can be seen from the output characteristics shown in Figure 2 (C), it is divided into three regions: cut-off region, amplification region and saturation region.
In the cut-off region, when Ube is less than 0, Ib is approximately 0. No electrons are injected into the base region from the emitter region, but due to the thermal motion of molecules, a small amount of current still flows through the collector, that is, Ic=Iceo, which is called the penetration current. At room temperature, Iceo is about a few microamperes, and for germanium tubes, it is about tens to hundreds of microamperes. Its relationship with the collector reverse current Icbo is:
Iceo=(1+β)Icbo.
At room temperature, the Icbo of the silicon tube is less than 1 microampere, and the Icbo of the germanium tube is about 10 microamperes. For the germanium tube, the Icbo value doubles for every 12°C increase in temperature, and for the silicon tube, the Icbo value doubles for every 8°C increase in temperature. Although the Icbo of the silicon tube changes more dramatically with temperature, since the Icbo value of the germanium tube is larger than that of the silicon tube, the germanium tube is still the tube that is more seriously affected by temperature. In the amplification region, when the emitter junction of the crystal triode is in forward bias and the collector junction is in reverse bias, Ic changes approximately linearly with Ib. The amplification region is the region where the triode works in the amplification state.
In the saturation region, when both the emitter and collector junctions are forward biased, Ic essentially does not change with Ib, and the amplification function is lost. The transistor's operating state can be determined by the bias conditions of its emitter and collector junctions.

Figure 2. Input and output characteristics of a transistor

The cut-off region and saturation region are the regions where the transistor works in the 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 cut-off region
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1. DC parameters
(1) Collector-base reverse saturation current Icbo: The collector reverse current when a specified reverse voltage Vcb is applied between the base and collector when the emitter is open (Ie=0). It is only related to temperature and is a constant at a certain temperature, so it is called the collector-base reverse saturation current. For a good transistor, Icbo is very small. The Icbo of a low-power germanium tube is about 1 to 10 microamperes, and the Icbo of a high-power germanium tube can reach several milliamperes. The Icbo of a silicon tube is very small, in the nanoampere level.
(2) Collector-emitter reverse current Iceo (penetration current): The collector current when a specified reverse voltage Vce is applied between the collector and emitter when the base is open (Ib=0). Iceo is approximately β times Icbo, that is, Iceo=(1+β)Icbo. Icbo and Iceo are greatly affected by temperature. They are important parameters for measuring the thermal stability of the tube. The smaller the value, the more stable the performance. The Iceo of a low-power germanium tube is larger than that of a silicon tube.
(3) Emitter-base reverse current Iebo When the collector is open, the emitter current is the current when a specified reverse voltage is applied between the emitter and the base. It is actually the reverse saturation current of the emitter junction.
(4) DC current amplification factor β1 (or hEF) This 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, that is:
β1=Ic/Ib
2. AC parameters
(1) AC current amplification factor β (or hfe) This refers to the ratio of the change in collector output current △Ic to the change in base input current △Ib when the common emitter connection is used, that is:
β= △Ic/△Ib
The β of a general transistor is approximately between 10-200. If β is too small, the current amplification effect is poor. If β is too large, the current amplification effect is large, but the performance is often unstable.
(2) Common base AC amplification factor α (or hfb) This refers to the ratio of the change in collector output current △Ic to the change in emitter current △Ie when the common base is connected, that is:
α=△Ic/△Ie.
Because △Ic＜△Ie, α＜1. If the α of a high-frequency transistor is greater than 0.90,
the relationship between α and β can be used :
α = β/(1+β)
β = α/(1-α) ≈ 1/(1-α)
(3) Cut-off frequency fβ, fα When β drops to 0.707 times the frequency at low frequency, it is the cut-off frequency fβ of the common emitter; when α drops to 0.707 times the frequency at low frequency, it is the cut-off frequency fα of the common base. fβ and fα are important parameters that indicate the frequency characteristics of the tube. The relationship between them is:
fβ≈(1-α)fα
(4) Characteristic frequency fT Because β decreases when the frequency f increases, when β drops to 1, the corresponding fT is an important parameter that fully reflects the high-frequency amplification performance of the transistor.
3. Limit parameters
(1) Maximum allowable collector current ICM When the collector current Ic increases to a certain value, causing the β value to drop to 2/3 or 1/2 of the rated value, the Ic value at this time is called ICM. Therefore, when Ic exceeds ICM, although the tube will not be damaged, the β value will drop significantly, affecting the amplification quality.
(2) Collector-base breakdown voltage BVCBO When the emitter is open, the reverse breakdown voltage of the collector junction is called BVEBO.
(3) Emitter-base reverse breakdown voltage BVEBO When the collector is open, the reverse breakdown voltage of the emitter junction is called BVEBO.
(4) Collector-emitter breakdown voltage BVCEO When the base is open, the maximum allowable voltage applied between the collector and emitter. If Vce>BVceo during use, the tube will be broken down.
(5) Maximum allowable collector power dissipation PCM When the collector current exceeds Ic, the temperature will rise. The maximum collector power dissipation when the parameter change caused by the heat does not exceed the allowable value is called PCM. The actual power dissipation of the tube is the product of the collector DC voltage and current, that is, Pc=Uce×Ic. When in use, it is best to make Pc<PCM.
PCM is related to heat dissipation conditions. Adding heat sinks can improve PCM.