Analysis of MOSFET Working Principle I. Overview of MOSFET

MOSFET， Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a semiconductor device widely used in electronic circuits. It has the advantages of high input impedance, low noise, good thermal stability, and simple manufacturing process, and plays a crucial role in digital circuits, analog circuits, and power electronics.

From a historical perspective, the concept of MOSFET was first proposed in the 1920s, but it was not until the 1960s, with the development of semiconductor manufacturing technology, that large-scale production and application were achieved. Nowadays, MOSFETs have become an indispensable component of modern electronic devices, ranging from smartphones and tablets to industrial automation systems and power transmission equipment.

2、 Structure and Classification of MOSFET Basic Structure

MOSFETs are mainly composed of a metal gate, an oxide insulating layer (usually silicon dioxide), a semiconductor substrate, and source and drain electrodes. Among them, the metal gate and the semiconductor substrate are separated by an oxide insulating layer, forming a capacitive structure. The source and drain are high concentration regions formed by impurity doping in the semiconductor substrate.

Classification method

MOSFETs can be classified into N-channel MOSFETs and P-channel MOSFETs based on the type of conductive channel. The conductive channel of N-channel MOSFET is composed of electrons, while the conductive channel of P-channel MOSFET is composed of holes. In addition, MOSFETs can be classified into enhancement and depletion types based on whether there is a conductive channel when the gate voltage is zero. Enhanced MOSFETs do not have a conductive channel when the gate voltage is zero, and a certain gate voltage needs to be applied to form a conductive channel; A depletion MOSFET already has a conductive channel when the gate voltage is zero, and the gate voltage can control the width of the conductive channel.

3、 The basic principle of MOSFET operation and the basis of electric field effect

The working principle of MOSFET is based on the electric field effect. When a voltage is applied to the gate, an electric field is generated in the oxide insulation layer, which affects the distribution of charge carriers in the semiconductor substrate. Taking N-channel enhanced MOSFET as an example, when the gate voltage is zero, there is no conductive channel in the semiconductor substrate, and the source and drain are insulated. When a positive voltage is applied to the gate, the electric field will attract electrons in the semiconductor substrate to gather below the gate, forming a conductive channel composed of electrons. At this time, current can be conducted between the source and drain.

The motion of charge carriers

When MOSFET is conducting, charge carriers (electrons or holes) will flow between the source and drain. For N-channel MOSFETs, electrons flow from the source and pass through the conductive channel to reach the drain; For P-channel MOSFETs, holes flow from the source and pass through a conductive channel to reach the drain. The speed and quantity of carrier movement depend on the magnitude of the gate voltage, the width of the conductive channel, and the voltage difference between the source and drain.

4、 Working characteristics of different types of MOSFETs - N-channel enhanced MOSFET

N-channel enhanced MOSFET is the most commonly used type of MOSFET. Its turn-on voltage (Vth) is usually positive, and when the gate voltage (Vgs) is greater than the turn-on voltage, the MOSFET begins to conduct. As the gate voltage increases, the conductive channel widens, the on resistance between the source and drain decreases, and the current increases. When the gate voltage is large enough, the MOSFET enters the saturation region, and the current no longer increases with the increase of gate voltage.

P-channel enhanced MOSFET

The working principle of P-channel enhanced MOSFET is similar to that of N-channel enhanced MOSFET, but its turn-on voltage is negative. When the gate voltage is less than the turn-on voltage, the MOSFET begins to conduct. In practical applications, P-channel enhanced MOSFETs are commonly used in circuits that require negative power supply, or as complementary devices in conjunction with N-channel MOSFETs.

N-channel depletion MOSFET

The N-channel depletion MOSFET already has a conductive channel when the gate voltage is zero. When a negative voltage is applied to the gate, the conductive channel narrows, the on resistance between the source and drain increases, and the current decreases; When the gate voltage reaches a certain negative value, the conductive channel will completely disappear and the MOSFET will turn off.

P-channel depletion MOSFET

The P-channel depletion MOSFET is opposite to the N-channel depletion MOSFET, as it has a conductive channel composed of holes when the gate voltage is zero. When a positive voltage is applied to the gate, the conductive channel will narrow and the current will decrease; When the gate voltage reaches a certain positive value, the MOSFET turns off.

5、 The application of MOSFET in practical circuits and its application in digital circuits

In digital circuits, MOSFETs are mainly used as switching elements. Due to its high input impedance and fast switching speed, MOSFETs can achieve fast logic operations and signal processing. For example, in CMOS (Complementary Metal Oxide Semiconductor) circuits, N-channel MOSFETs and P-channel MOSFETs are used complementarily to form logic gate circuits, such as AND gates, OR gates, NOT gates, etc. CMOS circuits have the advantages of low power consumption and strong anti-interference ability, and are widely used in fields such as computers and communications.

Applications in Analog Circuits

In analog circuits, MOSFETs can be used as amplifiers. By selecting the appropriate bias circuit and load resistance, MOSFETs can operate in the amplification region, achieving amplification of input signals. MOSFET amplifiers have the advantages of high input impedance and low noise, making them suitable for applications that require high signal quality, such as audio amplifiers, RF amplifiers, etc.

Applications in the field of power electronics

In the field of power electronics, MOSFETs are mainly used for power switching and voltage regulation. Due to its low on resistance and high switching speed, MOSFETs can achieve efficient power conversion. For example, in a switching power supply, MOSFETs serve as switching elements that can convert input DC voltage into output DC voltage of different voltage levels by controlling their conduction and cutoff times. In addition, MOSFETs are widely used in fields such as motor drives and electric vehicles.

6、 Temperature factors affecting the performance of MOSFETs

Temperature has a significant impact on the performance of MOSFETs. As the temperature increases, the on resistance of MOSFET will increase, the turn-on voltage will decrease, and the transconductance will decrease. This can lead to an increase in power consumption, a decrease in efficiency, and even the possibility of thermal runaway in MOSFETs. Therefore, in practical applications, appropriate heat dissipation measures need to be taken to ensure that MOSFETs operate within the appropriate temperature range.

Voltage and current stress

MOSFETs need to withstand certain voltage and current stresses during operation. If the applied voltage or current exceeds the rated value of the MOSFET, it may cause damage to the MOSFET. For example, when the drain source voltage is too high, breakdown may occur; When the drain current is too high, it may cause MOSFET overheating and damage. Therefore, when designing circuits, it is necessary to choose the rated parameters of MOSFETs reasonably and take overvoltage and overcurrent protection measures.

Parasitic parameters

MOSFETs have some parasitic parameters, such as parasitic capacitance, parasitic inductance, etc. These parasitic parameters can affect the switching speed and operational efficiency of MOSFETs. For example, parasitic capacitance can cause charging and discharging delays during the switching process of MOSFETs, reducing switching speed; Parasitic inductance can generate voltage spikes during the switching process, increasing the voltage stress on MOSFETs. Therefore, in circuit design, it is necessary to minimize the influence of parasitic parameters as much as possible.

In summary, MOSFET, as an important semiconductor device, involves multiple aspects such as electric field effects and carrier motion in its working principle. Different types of MOSFETs have different operating characteristics and are suitable for different application scenarios. In practical applications, it is necessary to consider the influence of factors such as temperature, voltage and current stress, parasitic parameters, etc. on the performance of MOSFETs to ensure their stable and reliable operation. With the continuous development of semiconductor technology, the performance of MOSFET is also constantly improving, and it will be widely used in more fields in the future.