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Posted on 11 October 2019

Introduction to Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

 

 

 

 

 

 

 

A Metal Oxide Semiconductor Field Effect Transistor is a transistor used for amplifying or switching electronic signals. The body of a MOSFET is usually connected to the source terminal which makes it a three-terminal device similar to other Field Effect Transistors (FET). Field effect transistors form a large family of switchable devices. They are widely used in computer and communications technology. Current flowing within field effect transistors between the main electrodes (source and drain) is controlled by a voltage at the gate electrode. The gate voltage opens or closes a conducting channel between source and drain.

N Channel V Gate Mosfet

Figure 1. N-Channel V-Gate MOSFET

The gate electrode is insulated using silicon monoxide. The length of the conductive channel (when positive woltage is applied to the gate) can be deduced accurately using the difference between the depth of the p- layer and the p+ layer.

In the transferring current from one point to the other, only one type of charge carrier is involved. For example, for N-channel MOSFETs, electrons are the only charge carriers involved in current flow. So MOSFETs, like Schottky diodes, are unipolar components. Since essentially only a gate voltage is needed to switch on and switch off the MOSFET, and almost no current is required, a MOSFET can therefore be controlled without use of very little power. Both current and voltage are required in order to regulate a bipolar transistor, hence power is required for control.

MOSFETs used for low reverse voltages have excellent transmission properties. For high reverse voltages, the transmission properties characterized by the drain source on resistance RDSon worsen since the thickness and specific resistance of the silicon must be increased. Just like for all other unipolar components, MOSFETs are also chaacterized by low switching losses. Cool-MOS is a special type of MOSFET which has very good transmission properties at high reverse voltages. The production of Cool-MOS is, however, rather intensive and requires a lot of resources.

Current flow

Figure 2. Current flow through a planarvertical MOSFET

The MOSFET chip consists of a large number of identical cells (20 to 30 million cells per square inch) which are connected in parallel. The failure of a single cell can cause the entire MOSFET to fail.

The gate consists of very highly doped silicon or metal polycrystalline (Al). The gate, oxide, and silicon form a capacitor whose capacitance increases or decreses depending on the thickness of the gate oxide. The p-substrate, the source, and the n-type epitaxial layer form a P-N junction. So a parasitic diode (body diode) is integrated into each MOSFET. For trench MOSFETs, resistance associated with the channel length d can be very accurately and reproducibly set by the difference between the implantation depth of p and n+.

resistance

Figure 3. Resistance linked to the Suface Area

The smaller cells and the more controlled channel length reaches the trench MOSFET better values ​​of the forward resistance RDSon than the planar MOSFET whose resistance is improved continuously. In the reverse direction not at the MOSFET gate controlled one diode characteristic (parasitic or body diode). The switched MOSFET acts as a gate voltage of the controllable resistance. This resistance is refered to as drain source on resistance RDSon .

 R_{DSon} = \frac {\Delta V}{\Delta I} = \text{Slope of the characteristic}

Drain Source Voltage

Figure 4. MOSFET drain source voltage and drain current

The drain current is limited to a specific value that depends on gate voltage.

Cross over characteristics of MOSFETs, which are the characteristics of the MOSFET during the trasition between on and off states, are considerably worse at high temperatures than at room temperatures. For bipolar components such as diodes, thyristors and IGBTs however, cross over characteristics improve in most cases as temperature increases.

 

For more information, please read:

Thyristor Basics

Bipolar Junction Transistor

P-N Junction Diode

Criteria for Successful Selection of IGBT and MOSFET Modules

 

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