When using MOSFETs, there is a phenomenon that often causes headaches, known as the Miller Effect.

First, let's review the turn-on process of a MOSFET.

The turn-on process of a MOSFET can be divided into three stages: cutoff, linear region, and saturation region.

In the cutoff region, the gate-source voltage of the MOSFET is zero, and the MOSFET is in the off state. As the gate-source voltage increases, when it reaches the gate threshold voltage (Vg(th)), the MOSFET enters the linear region.

In the linear region, as the gate-source voltage of the MOSFET continues to increase, it eventually reaches the saturation voltage (Vs(th)), and the MOSFET enters the saturation region.

However, in practical applications, when we apply a voltage to the gate of the MOSFET, we observe an interesting phenomenon: after the gate-source voltage reaches the gate threshold voltage, the conduction current of the MOSFET does not immediately increase but instead exhibits a plateau.

During this plateau period, the conduction current of the MOSFET remains constant, as if it is stuck. This phenomenon is known as the Miller Effect, caused by the presence of the input capacitance that limits the rate at which the gate-source voltage (Vgs) can rise, preventing the MOSFET from immediately entering the on state.

So, how does the Miller Effect occur?

To understand this, we need to look at the structure of the MOSFET.

The MOSFET is made of n-type or p-type semiconductors and includes two electrodes: the source and the drain, as well as a gate.

During the operation of the MOSFET, a capacitance is formed between the gate and the source, known as the Miller capacitance (Cgd).

When the gate-source voltage increases, the gate must first charge the Miller capacitance before the gate can continue to charge, thereby increasing the conduction current of the MOSFET.

While the Miller Effect can be troublesome, there are ways to mitigate its impact:

We can reduce the Miller capacitance, increase the driving current, optimize the driving circuit, etc.

Let's start with a simple model. Suppose we have a MOSFET with an input capacitance of Cgs. When the gate-source voltage (Vgs) rises, this capacitance begins to charge. During the charging process, the rate of Vgs rise gradually slows down because the capacitance charging speed is limited. When Vgs rises to a certain level, the MOSFET enters the on state.

In this way, the MOSFET can serve us better and bring more convenience to our lives.

In conclusion, the turn-on process and Miller Effect of MOSFETs are complex and interesting processes.

By understanding this process, we can better grasp the working principle of MOSFETs and provide more ideas and inspiration for our electronic designs. I hope this article can be helpful and let us explore the mysteries of the electronic world together!