The transistor is one of the most vital electronic components, revolutionizing modern electronics. John Bardeen, an American physicist, along with his colleague Walter Brattain, successfully created the world's first semiconductor triode, earning them the Nobel Prize in Physics. The triode's key function is to control a large current using a small current, similar to how martial arts techniques rely on precision and control.
The diagram below illustrates the structure and circuit symbols for two types of transistors: NPN and PNP.
[Image: Deep analysis of the working principle of the triode]
Positive [ˈpɒzətɪv]
Negative [ˈnegətɪv]
Many beginners mistakenly believe that a transistor is simply two PN junctions combined. This is incorrect. While two diodes may share some similarities, they do not function as a single transistor. Let’s take an NPN transistor as an example (see Figure 2). The two PN junctions share a thin P region, typically just a few micrometers thick. These junctions are intricately connected, forming a unified whole. This interaction makes the transistor distinct from two separate PN junctions. When voltage is applied, the transistor generates base, collector, and emitter currents, acting as a current amplifier.
The current amplification capability of a transistor is closely tied to its physical structure. Inside the transistor, complex physical processes occur, which can be challenging for beginners to grasp at first. From an application perspective, it's helpful to think of a transistor as a current divider. Once manufactured, the relationship between the three currents is generally fixed (see Figure 3).
β and α are referred to as the current distribution coefficients of the transistor. β is well-known as the current gain. If one current changes, the others change proportionally. For instance, if the base current ΔIb = 10 μA and β = 50, then ΔIc = 50 × 10 = 500 μA, demonstrating current amplification. However, the transistor itself doesn't convert a small current into a large one; instead, it controls the power supply in the circuit, providing the three currents I_b, I_c, and I_e in a defined ratio.
To better understand this, consider a water flow analogy (see Figure 4). Imagine a thick and thin pipe, where the thick pipe has a gate controlled by the thin pipe. When no water flows through the thin pipe, the gate remains closed. As more water enters the thin pipe, the gate opens wider, allowing more water to flow through the thick pipe. This reflects the principle of "small control over large."
In the transistor, the base corresponds to the thin pipe, the collector to the thick pipe, and the emitter to the junction point. When a voltage is applied, currents I_b, I_c, and I_e are generated. Adjusting the potentiometer RP changes I_b, which in turn affects I_c. Since I_c = βI_b, a small I_b controls a much larger I_c. The current I_c isn't generated by the transistor itself but is supplied by the power source VCC under the control of I_b, making the transistor act as an energy converter.
[Images: Circuit diagrams and graphs illustrating transistor behavior]
As shown in the figure, assume β=100, RP=200K. At this time, Ib=6V/(200k+100k)=0.02mA, Ic=βIb=2mA. When RP=0, Ib=6V/100k=0.06mA, Ic=βIb=6mA. Both cases align with Ic=βIb, indicating the transistor is in the "amplification zone." If RP and Rb are increased significantly, Ube < dead zone voltage (Si tube about 0.5V, Ge tube about 0.3V), leading to a non-conducting state with Ib=0 and Ic=0, placing the transistor in the "cut-off area."
From a broader perspective, a higher β value is generally desirable. However, when connected to a common emitter amplifying circuit, a harmful leakage current called the penetration current I_ceo is generated, proportional to β. A larger β results in a larger I_ceo, which is not controlled by Ib but becomes part of I_c. This parasitic current can affect the stability of the amplifier circuit, especially with temperature changes. Therefore, selecting a transistor with an optimal β value is crucial. Silicon tubes typically have β values between 40-150, while germanium tubes range from 40-80.
At normal temperatures, the penetration current of germanium tubes is relatively high, ranging from tens of microamperes to hundreds of microamperes, whereas silicon tubes have much smaller values, usually a few microamperes. Although I_ceo is small, it is highly sensitive to temperature, doubling every 10°C increase. Measuring I_ceo is straightforward: connect the base open, apply a 6V power supply between the collector and emitter, and measure the current with a multimeter.
Strictly speaking, β is not a constant. It varies with the collector current I_c and temperature. When I_c is very small or close to the maximum allowable current I_CM, β tends to be lower. For small power transistors, β is relatively higher when I_c is above 1 mA. When debugging amplifier circuits, choosing an appropriate operating current I_c is essential for optimal amplification.
The β value also changes with temperature, increasing by 0.5% to 1% per degree Celsius. For example, with a BC847 transistor, β increases by approximately 57.46% when the temperature rises from -20°C to 50°C. Similarly, the base current Ib also changes with temperature, increasing by around 12.19% over the same range.
Transistors have a limit parameter called the collector maximum allowable current, I_CM. Exceeding this value can lead to overheating and damage. I_CM is specified to prevent excessive β degradation due to high collector currents. Typically, I_CM is set when β drops to about half its maximum value.
The current amplification factor β is also influenced by the operating frequency. Within a certain frequency range, β remains relatively constant. However, as frequency increases beyond a certain point, β decreases significantly. To maintain sufficient amplification at high frequencies, the β cutoff frequency f_β is defined—the frequency at which β drops to 0.707 times its low-frequency value. f_β represents the maximum operating frequency for a common emitter circuit.
For a common base configuration, the α cutoff frequency f_α is used, where α (the current gain) drops to 0.707 times its low-frequency value. f_α reflects the frequency limit of the common base configuration. In practice, low-frequency tubes have f_α < 3MHz, while high-frequency tubes have f_α > 3MHz.
When the frequency exceeds f_β, β continues to decrease until it reaches 1, at which point the transistor loses its amplification capability. The characteristic frequency f_T is defined as the frequency at which β = 1. f_T is a critical parameter for marking a transistor's frequency characteristics. When selecting a transistor, f_T should be 3–5 times higher than the actual operating frequency.
The physical meaning of f_α and f_β is similar, but their configurations differ. Theoretical analysis shows that f_β is much smaller than f_α for the same transistor, with the relationship f_β = (1 - α) * f_α. This indicates that the common emitter circuit has a much lower frequency limit compared to the common base circuit. Hence, high-frequency amplification and oscillation circuits often use a common base configuration.
Understanding the operation of a transistor begins with the basics of a diode. A diode consists of a PN junction with unidirectional conductivity. When reverse biased, the PN junction is turned off, and a small leakage current exists. This leakage current is due to minority carriers, which are easily reversed through the PN junction under reverse bias.
This phenomenon occurs because the P and N regions contain a small number of minority carriers. Under reverse bias, these minority carriers form a leakage current. The magnitude of this current depends on the number of minority carriers. Increasing the number of minority carriers artificially, such as through illumination, can increase the leakage current. This principle is used in photodiodes, where light increases the number of minority carriers, creating a photocurrent.
Photodiodes operate in reverse bias, as illumination increases the number of minority carriers, thereby changing the reverse leakage current. This principle allows photodiodes to convert light into electrical signals.
In summary, understanding the role of majority and minority carriers in PN junctions is essential. Majority carriers contribute to forward current, while minority carriers contribute to reverse current. This explains the unidirectional conductivity of PN junctions and the behavior of transistors in different states.
Continuing the discussion, the reverse bias state of a PN junction can be controlled by injecting electrons into the P region, similar to the method used in photodiodes. This leads to the formation of a transistor, where electrons are injected into the base region, forming a collector current.
The relationship between I_c and I_b is determined by the internal structure of the transistor. In the amplified state, the collector current is primarily controlled by the injection of carriers into the base region, rather than the collector potential. This explains why I_c is independent of the collector potential in the amplified state.
The current amplification mechanism of a transistor is similar to that of an electron triode. The base acts like the grid, intercepting a portion of the current, while the rest flows through to the collector. The ratio of intercepted current to total current determines the β value.
In conclusion, the transistor's behavior is governed by its internal structure, the movement of charge carriers, and the external conditions applied. Understanding these principles is essential for effectively utilizing transistors in various electronic applications.
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