Question One: The Issue of Using MOSFETs as Switches

As described in the title: What are the differences between using MOSFETs as switches alone and using them in conjunction with transistors?

Note: A transistor is a three-terminal semiconductor device composed of three differently doped semiconductor materials. Its main structure includes a doped base region of either n-type or p-type, and two doped regions of either p-type or n-type. The base region is sandwiched between the two doped regions, forming two pn junctions, hence the transistor is also known as a bipolar junction transistor (BJT). Transistors control the current flow between the collector and emitter by controlling the base current.

A MOSFET is a four-terminal semiconductor device consisting of two types: the metal-oxide-semiconductor field-effect transistor (MOSFET) and the metal-oxide-semiconductor bipolar transistor (MOS-bipolar transistor). The main structure of a MOSFET includes a metal gate, an insulating layer, and a semiconductor material. The insulating layer between the metal gate and the semiconductor material can control the gate voltage, thereby controlling the drain current. MOSFETs are characterized by high input impedance, low output resistance, fast switching speed, and high reliability.

Transistors are mainly used in amplification and switching circuits, while MOSFETs are mainly used in power amplification and switching circuits, especially in high-frequency and high-voltage applications, where the advantages of MOSFETs are more pronounced.

Question Two: Reasons for MOSFETs Being Easily Damaged in an NPN+P-MOS Switching Circuit

In a simple NPN+P-MOS switching control circuit, with a load of 850nm laser diode and the MCU being driven by STC15F2, there are cases where the MOSFETs are easily damaged.

The parameters of the laser are as follows: this is the schematic of one group of lasers, and the other group is the same, with a total of 10, each with a power of 100mW, model ML101J25.

Previous Answers:

Previous Answer One: The MOSFET will not turn on, but there will be current flowing in the circuit. This is mainly because there is a body diode inside most MOSFETs, and in the current connection method, the body diode inside the MOSFET is forward biased.

Previous Answer Two: In fact, all electronic devices will have leakage current, it's just a matter of how much. NMOS and PMOS themselves also need to consume current, the magnitude of which depends on the parameters of the leakage current IGSS and IDSS determined by the device manufacturing itself. In practical online applications, as long as the leakage current is not greater than the factory parameters, it is already considered extremely low. Even so, it is still necessary to control and adjust it to the best state in the actual circuit production process.

Note: This depends on VGS & VDS, as well as the voltage added by Ron and the hardware combination environment.