Introduction

What is MOSFET in the first place? First, for those who want to know the basics of MOSFETs, the basic terms and knowledge of MOSFETs are explained in detail below.

・ What is a MOSFET
・ MOSFET operation
・ Diode in MOSFET
・ Electronic symbol for MOSFET
・ MOSFET internal process structure
・ Application of MOSFET
・ On-resistance of MOSFET
・ MOSFET saturation region
・ Parasitic capacitance of MOSFET
・ MOSFET charge capacity

What is a MOSFET

An acronym for Metal Oxide Semiconductor Field Effect Transistor, it is a type of field effect transistor (FET).
MOSFETs are made by combining N-type semiconductors and P-type semiconductors, but depending on their structure, they can be divided into N-channel MOSFETs and P-channel MOSFETs.
A MOSFET has four terminals: Drain, Source, Gate, and Body. Normally, the body terminal is shorted with the source terminal, so commercially available MOSFETs It has three terminals: drain, source, and gate.

MOSFET operation

When a voltage is applied between the gate and body of a MOSFET, a channel with an inverted P-type or N-type semiconductor is formed directly under the gate. Since the body is shorted to the source, we can think of it as applying a voltage between the gate and the source.
This results in conduction between the drain and the source, allowing current to flow from the drain to the source or from the source to the drain. Taking advantage of this feature, MOSFETs are used as electronic circuit components as "switch elements."

Diode in MOSFET

By shorting the source and body of the MOSFET, the internal PN junction will act as a diode.
This parasitic diode is called a "body diode".
Due to this body diode, current flows from the source to the drain in the N-channel MOSFET, and from the drain to the source in the P-channel MOSFET, regardless of the voltage applied to the gate terminal.

Electronic symbol for MOSFET

The electronic symbols for MOSFETs are sometimes expressed with the following electronic symbols including body-source shorts and body diodes.

MOSFET internal process structure

For MOSFETs, especially power MOSFETs that handle large currents, improvements are being made to the process structure to further reduce losses.
Previously, the mainstream was the planar type, in which the gate electrode was attached to the surface of the wafer. However, in order to suppress the loss in power MOSFETs, trench-type products with embedded gate electrodes have appeared.
After that, based on the trench type, the shield gate type power MOSFET appeared in order to further reduce the loss.
Shield-gate MOSFETs have lower on-resistance and smaller gate charge than trench-type MOSFETs. In addition, a parasitic RC snubber is formed internally, which tends to reduce the ringing that occurs during MOSFET switching.

Application of MOSFET

Since MOSFETs work as switching elements, they are often used to switch current ON and OFF.
Specifically, they are used in applications such as DC/DC converter circuits, power switch elements in motor drive circuits, and load switch circuits.

On-resistance of MOSFET

I explained that MOSFETs are used as switching elements, but in reality they can be thought of as variable resistors in which the resistance value between the drain and source varies depending on the voltage between the gate and source.
The graph on the right is ONSEMI N. channel MOSFET”NTMFS5C404N” transfer characteristic.-source-to-source voltage (V _DS) but 10V When the gate-source voltage (V _GS) is changed, the current flowing to the drain is (I _D.) also shows that the
here, V. _DS is fixed against V. _GS with the change of I _D. is changing, MOSFET the drain of-gate between sources-We can say that there is a variable resistance that varies depending on the source-to-source voltage. this MOSFET drain current when-the resistance that exists between the sources MOSFET On-resistance of [R. _DS(on)] called.
from these things, MOSFET is not digitally turned on and off, but it is necessary to have an image that the on-resistance is variable and current flows.

MOSFET saturation region

Earlier, it was explained that the MOSFET drain current I _D is limited by the on-resistance, but as V _DS increases, it reaches the saturation region where I _D does not rise any further.
In the Ohmic region (linear region) of the MOSFET output characteristics graph, R _DS(on) works and the current increases linearly with V _DS, but in the Active region (saturation region) the current is almost independent of V _DS. The same ​_ID is flowing. In this region, the MOSFET can be considered to behave like a constant current source.

Since the linear region widens as V _GS increases, a high gate voltage is applied to the MOSFET​ ​when low on-resistance operation is desired.
When using a MOSFET, it is necessary to consider whether to use it in this saturation region or in the linear region where the on-resistance works according to the requirements of the application.

Parasitic capacitance of MOSFET

MOSFETs have parasitic capacitances between gate-drain (C _GD), gate-source (C _GS), and drain-source (C _DS).
From these three parasitic capacitances, the input capacitance (C _ISS), output capacitance (C _OSS), and feedback capacitance (C _RSS) are defined as follows, and the switching characteristics of the MOSFET can be determined from these characteristics. increase.

Specifically, a large or small C _ISS affects the slope of the MOSFET 's V _GS voltage, which affects the time it takes to turn the MOSFET on and off.
Similarly, large or small C _OSS affects the amount of current that remains when the MOSFET is turned off, which affects the time it takes for the MOSFET output to turn off.
When C _RSS is large or small, a plateau voltage appears in which the gate voltage does not change due to the current flowing between the drain and gate during the transition period of the V _DS voltage due to the turning on and off of the MOSFET. This affects the time it takes to turn the MOSFET on and off.

It can be said that the smaller C _ISS, C _OSS, and C _RSS are, the better the switching characteristics of the MOSFET are, but there is a trade-off relationship between these capacitance characteristics and the on-resistance of the MOSFET.
Also, since C _ISS, C _OSS, and C _RSS fluctuate with the VDS voltage, exact calculations are difficult, and if used to estimate switching loss, the difference from the actual device will be large.

MOSFET charge capacity

In estimating the switching characteristics of a MOSFET, C. _ISS, C. _OSS, C. _RSS gate charge instead of Q. _G. is used.
gate charge Q. _G. teeth 3 can be categorized into one area. MOSFET specified in the datasheet of Q. _G(TOT) Box's 3 is the total charge in one region.

Area ①: Q _GS region
Gate-source-to-source voltage (V _GS) but 0V to the plateau voltage (V _GP) This is the area up to the rise of . During this time, the gate current is used to (C _ISS) to charge the

Area ②: Q _GD region
V. _GS but V. _GP is the area maintained by . Drain during this time-source-to-source voltage (V _DS) teeth I _D. *R _DS(ON) down to

Region ③: all remaining Q _G. region
V. _GS is the region where the voltage rises up to the driver power supply voltage. during this time I _D. and V. _DS both remain at relatively stable values, and MOSFET operates in the linear region where the on-resistance works.

Knowing Q _G(TOT) and target gate voltage rise time (t _r), the required gate drive current (I _G) can be estimated by the following equation:

lastly

Please also refer to the article below, which summarizes the points and methods for selecting a MOSFET.
https://www.macnica.co.jp/business/semiconductor/articles/onsemi/142934

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