Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

Power MOSFETs are a type of voltage-controlled device that operate with less power than current-controlled devices such as BJTs. These devices are non-latching and require a continuous gate-source voltage to remain on, similar to BJTs. However, MOSFETs are distinguished by their exceptional switching speed, which surpasses that of all other power switches and operates in the order of megahertz.

There are two types of MOSFETs: depletion and enhancement.

The depletion-type MOSFET functions as a normally-on electrical switch, while the enhancement-type MOSFET functions as a normally-off electrical switch.

MOSFET Structure

MOSFETs are electronic devices that possess four terminals, namely the source (S), gate (G), drain (D), and body terminals. The body terminal is frequently linked to the source terminal, thereby reducing the overall number of available terminals.

Figure 13 illustrates the configuration and symbol of an N-channel enhancement-type MOSFET, which comprises a substrate composed of p-type material and heavily doped n+ drain and source regions.

Figure 13. N-channel enhancement type MOSFET

Figure 14 depicts the configuration and emblem of a P-channel enhancement-type MOSFET. This MOSFET comprises an n-type substrate and p+ heavily doped drain and source regions. This is a P-channel MOSFET because the current is propelled by the movement of positively charged holes.

Figure 14. P-channel enhancement type MOSFET

MOSFET Operation and Characteristics

In an N-channel enhancement-type MOSFET, the application of a positive gate-to-source voltage (VGS) results in the attraction of electrons from the underlying p-substrate, leading to their accumulation at the surface beneath the oxide layer. Upon reaching the threshold voltage (VT), a critical number of electrons accumulate to form a virtual n-channel, thereby enabling the flow of current from the drain to the source. The functioning of a P-channel is analogous, with the flow of current being attributed to positively charged holes, and the polarity of current and voltage being reversed.

Figure 15. Output characteristics of N-channel enhancement-type MOSFET

The output characteristics of an N-channel enhancement-type MOSFET are illustrated in Figure 13, which displays three distinct operating regions: cutoff, linear, and saturation (active) region. It is important to note that the concept of the saturation region differs from that of BJTs. When VGS < Vt; VT, the device turns off and operates in the cutoff region. Conversely, for VGS > Vt; VT, the device can operate in either linear mode or saturation mode. In the linear (or ohmic) mode, VDS < VGS – VT, while in the saturation mode, VDS > VGS – VT. In the linear mode, the drain current (ID) varies proportionally with the drain-source voltage (VDS). Power MOSFETs are commonly used in the linear region for switching applications due to their low drain voltage and high drain current. Conversely, the drain current remains almost constant in the saturation mode, regardless of any changes in VDS.

Figure 16. MOSFET switching characteristics

In power electronics, the turn-on delay time (td(on)) is defined as the period between the point at which the gate-source voltage surpasses 10% of VGS and the moment when the drain-source voltage descends to 90% of VDS. The rise time (tr) pertains to the duration required for the voltage between the drain and source to diminish from 90% to 10% of VDS. The turn-on (ton) time is the summation of td(on) and tr. Correspondingly, the turn-off delay time (td(off)) is the interval between the point at which the gate-source voltage falls below 90% of VGS and the moment when the drain-source voltage reaches 10% of VDS. The fall time (tf) is the time span during which the drain-source voltage ascends from 10% to 90% of VDS. The turn-off time is the summation of td(off) and tf.

MOSFET Applications

MOSFETs are widely used in various power electronics applications.

DC-DC Converters

DC-DC converters can be operated through the use of MOSFETs, which are capable of controlling the power flow in various modes of converter operation. This is particularly applicable in buck-boost converters.

Motor Control

MOSFETs have been used in motor control, whereby the conduct of DC or stepper motors can be regulated through the implementation of various methodologies such as Pulse Width Modulation (PWM).

Other Applications

They are also used in inverters, switched-mode power supplies (SMPS), choppers, uninterrupted power supplies (UPS), etc.