MOSFETs

A MOSFET is a voltage controlled device composed of a gate, a source and a drain. The gate voltage is typically provided by a microcontroller or a driver IC.

By applying the gate voltage to the gate terminal, the current will start to flow from the source to the drain terminals.

To understand the efficiency of the current flow, we can take a look at the R_DS(on) rating of the device. The R_DS(on) is the resistance from the source terminal to the drain terminal when the device is on. This rating is measured in ohms or milliohms.

There are two different MOSFET modes, the enhancement and the depletion. Both of these modes have two channel types, the N-channel, and the P-channel. The enhancement mode means that the device is always off, and needs a gate voltage to turn on.

On the other hand, the depletion mode means that the device is always on, and needs a gate voltage to turn off. Nowadays, the most commonly used MOSFETs are N-channel enhancement-type devices.

MOSFETs can also be lateral or vertical when it comes to their structure, which means that the conducting channel of the MOSFET is either lateral or vertical.

Let‘s use the N-type enhancement mode planar MOSFET to deep dive into the MOSFET structure. The most important element is the metal oxide semiconductor interface because it is responsible for its operation. Here, you can see the detailed structure of an N-type enhancement mode planar MOSFET.

The basis of the MOSFET construction is a silicon material that is lightly doped with Boron atoms to make it slightly P-type in concentration. This is called the p-substrate. To build the source and the drain regions, high-energy and high-concentration implants of N-type atoms are used.

The gate oxide, which is built with thin oxide, is on the silicon region between the source and the drain, partially overlapping these regions. When the gate voltage is applied, the negative atoms are attracted under the gate and the N-channel is formed. At this moment, the current will begin to flow.

Please note that other manufacturing steps may vary depending on the MOSFET type. Nevertheless, these are the basic operational building blocks of a MOSFET.

MOSFETs operate in three different modes:

› Depletion

› Accumulation

› Inversion

These modes refer to the status of the MOSFET channel and are described below:

On the left are typical datasheet curves related to the linear and saturation modes, where we can see the relationship between voltage and the output current.

By analyzing this graph, we can see the relation between the drain-source current and the drain-source voltage, applied for different values of the gate to source voltage.

Here, the dotted line indicates the edge of the saturation boundary, until the current increases linearly with the applied drain voltage. When this limit is exceeded, the drain current remains more or less constant with the increasing drain voltage.

By analyzing this graph, we can see the relation between the drain-source current and the drain-source voltage, applied for different values of the gate to source voltage.

Here, the dotted line indicates the edge of the saturation boundary, until the current increases linearly with the applied drain voltage. When this limit is exceeded, the drain current remains more or less constant with the increasing drain voltage.

Additionally, this graph shows  the relation between the drain current and the applied gate voltage for a fixed value of the drain voltage.

As you can see, the current quickly increases once the gate voltage is higher than the threshold voltage.

Please note that there is some amount of current flow before the threshold voltage is reached. This is called subthreshold current or leakage current.