Differences between MOS transistors and bipolar transistors

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MOSFETs are used in high-frequency and high-speed circuits, applications with large currents, and in situations where the current control at the base or drain is sensitive. Transistors operate based on the movement of charge carriers. Taking an npn transistor emitter follower as an example, whether a voltage is applied to the base or not, the PN junction formed by the base and emitter region prevents the diffusion of majority carriers (holes in the base region and electrons in the emitter region). After explaining the uses of these two types of components, what is the difference between MOS transistors and transistors? 1. Operating principle: Transistors are controlled by current, while MOS transistors are controlled by voltage. 2. Cost: Transistors are inexpensive, whereas MOS transistors are more expensive. 3. Power consumption issue: High loss in transistors. 4. Different control principles: MOSFETs are controlled by voltage, while transistors are controlled by current. 5. Different cost structures: MOS transistors are more expensive, while triodes are less costly. 6. Different power consumption: MOSFETs have low power consumption, while transistors have high power consumption. 7. Different driving capabilities: MOSFETs are commonly used in power switches and switching circuits involving large currents, while transistors are typically used in power switches and switching circuits involving small currents. 8. Driving capacity: MOSFETs are commonly used as power switches, as well as in switching circuits that handle large currents. In fact, transistors are relatively inexpensive and easy to use, and they are commonly employed for switch control in digital circuits. MOSFETs are used in high-frequency and high-speed circuits, applications with large currents, and in situations where the current control at the base or drain is sensitive. MOSFETs should be used when only a small current can be drawn from the signal source; transistors, on the other hand, are appropriate when the signal voltage is low and a larger current can be drawn from the signal source. MOS transistors can operate at very low currents and voltages, and their manufacturing process allows many of them to be easily integrated onto a single silicon chip; as a result, MOS transistors are widely used in large-scale integrated circuits. A MOSFET is a voltage-controlled current source, with the control voltage and current belonging to separate circuits; as a result, it is generally not difficult to determine the voltage. The current value can then be found using the expression for the drain current, after which model analysis can be performed to determine the transconductance and output resistance. In contrast, for a BJT, a model must first be established followed by circuit analysis, and the calculation process is quite complex and prone to errors. Overall, I think the analysis of MOSFETs is simpler than that of BJTs.

Differences between BJTs and MOSFETs:
1. In a BJT, both holes and free electrons contribute to conduction, making it a bipolar device denoted as BJT; whereas in a MOSFET, only one type of charge carrier contributes to conduction, classifying it as a unipolar device denoted as FET. Since the concentration of the charge carriers is not affected by external factors such as temperature, light, or radiation, FETs are more suitable for use in environments with drastic changes. This is why MOSFETs are considered more stable.
2. When operating in amplification mode, a BJT has a forward-biased emitter junction and base current, making it a current-controlled device with a relatively low input resistance of around 10³ Ω. In contrast, a FET has no gate current when in amplification mode; it is a voltage-controlled device with a very high input resistance—JFETs have an input resistance greater than 10⁷ Ω, while MOSFETs have an input resistance greater than 10⁹ Ω.
3. The source and drain of a MOSFET are structurally symmetrical and can be used interchangeably (though note that sometimes manufacturers short-circuit the source and substrate within the MOSFET, rendering them non-interchangeable). For depletion-mode MOSFETs, the VGS can be positive, negative, or zero, allowing for greater flexibility in usage. The collector and emitter of a BJT generally cannot be used interchangeably.
4. In low-voltage, low-current applications, a FET can function as a voltage-controlled variable resistor or a contactless electronic switch with very low on-resistance.
5. MOSFETs have a simple manufacturing process and low power consumption, making them suitable for large-scale integration. BJTs, on the other hand, offer high gain, low nonlinear distortion, and stable performance. They still remain dominant in discrete component circuits and medium-to-small scale integrated circuits.
6. The transfer characteristic of a BJT (the relationship between ic and vbe) follows an exponential pattern, while that of a field-effect transistor follows a quadratic pattern. As a result, the nonlinear distortion of MOSFETs is greater than that of BJTs.
7. The three basic configuration circuits for MOSFETs—common-source, common-drain, and common-gate—can be compared to the common-emitter, common-collector, and common-base circuits of BJTs. Since there is no gate current in a field-effect transistor, its input resistance R’i is approximately infinite. The transconductance gm of a MOSFET is one order of magnitude smaller than that of a BJT, and it can be determined by taking the derivative of the transfer characteristic. 8. A transistor can be considered a device that controls a current source using current, with the current being determined by the value of the input resistance; whereas a MOSFET is a device that controls a current source using voltage. 9. Method to remember the characteristic curves of the four types of MOS transistors: Just remember the curve of the N-channel EMOS transistor – its Vgs is greater than 0, and the curve shows an increasing trend. For the P-channel EMOS transistor, Vgs is less than 0, with a decreasing trend in the curve. The Vgs of the DMOS transistor can be both greater than 0 and less than 0; by recalling that N-channel transistors have increasing curves while P-channel transistors have decreasing curves, one can remember the characteristic curve of the DMOS transistor as well. 10. MOS transistors are voltage-controlled devices, whereas triodes are current-controlled devices. MOS transistors use majority carriers for conduction, which is why they are called unipolar devices, while triodes use both majority and minority carriers for conduction, hence they are known as bipolar devices. MOS transistors offer greater flexibility compared to triodes; moreover, their manufacturing process is more suitable for integrated circuits.