New MOSFETs Target High Voltage Applications With Low Ohmic and Fast Body Diode

December 10, 2021 by Kristijan Nelkovski

The push for better semiconductor components never stops. One company, Alpha and Omega Semiconductor (AOS), is introducing two new MOSFET products featuring low ohmic and fast body diode properties.

Today, Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are the backbone of modern computing and can be found in everything from processors and computer memory to power supply switching systems. 

All in all, engineers have found that some of the properties of MOSFETs are more advantageous when compared to other components, allowing today's computer chips to shrink in size and expand in computational power while also improving energy efficiency. 

In general, MOSFETs are semiconductor components used for switching and amplifying signals in digital and analog circuits and developing integrated circuits (ICs). 


Example diagrams of a p-type (left) and n-type (right) MOSFET.

Example diagrams of a p-type (left) and n-type (right) MOSFET. Image used courtesy of MKS Instruments


Like traditional transistors, these components have three terminals, two ohmic contacts, a source, and a drain, operating as a conducting semiconductor channel controlled by the third contact called the gate, which in MOSFETs is separated from the channel with an insulating silicon dioxide layer.

Despite this technology's wide use and advantages, there is always room for improvement in MOSFET technology for different applications. Companies such as Alpha and Omega Semiconductor (AOS) are working exactly on these types of solutions. 

Today's article will focus on two MOSFET products recently developed by AOS based on a new technology that features improved 'on-resistance' and reverse recovery features named low ohmic and fast body diode properties.


Drain-source On-resistance and the Low Ohmic Property

The MOSFETs total resistance between the drain and source terminals during operation or its 'ON' state is called drain-source on-resistance and is written down as RDS(on). The RDS(on) is an essential characteristic of these semiconductor components as it accounts for the power loss throughout a circuit. 

Usually, MOSFET on-resistance is on the level of one ohm or less, and, when designing a circuit, a lower value for RDS(on) is preferred.

A lower value for this resistance means that the power loss throughout the circuit is smaller since the power consumption by the MOSFET is equal to the product of its on-resistance and the drain current squared. 

Additionally, when lowering the resistance and power consumption of a component, the operating heat dissipation is lowered, thus improving the thermal problems that some transistor systems face.


On-resistance vs. drain current and gate voltage graphs for AOK040A60 (left) and AOK042A60FD (right). Screenshots used courtesy of Alpha and Omega Semiconductor


Now, looking at AOS's newest family of MOSFETs, these feature a low ohmic property where the RDS(on) equals 40 milliohms for its AOK040A60 MOSFET and 42 milliohms for its AOK042A60FD MOSFET. 

Both of these components are 600 V MOSFETs and are designed to use multiple types of switching topologies in the development of power supply systems. Besides server and telecom power supplies, the company states that these components will also target EV charging and other renewable energy inverter applications.


Body Diode Characteristics

One unique MOSFET characteristic is an internal diode between its drain and source, which directly results from the p-n junction itself inherent to the structure of these electronic components. 

This diode is commonly referred to as the 'body diode' of a MOSFET or a parasitic diode. The body diode is an important characteristic that can directly impact the performance of MOSFETS and can inform proper implementation in different applications.

An important parameter of the body diode that relates to the overall characteristics of the MOSFET is the diode's reverse recovery time (ttr). 


Reverse recovery example of a silicon carbide (SiC) MOSFET.

Reverse recovery example of a silicon carbide (SiC) MOSFET. Image used courtesy of ROHM


The ttr is the time in which a current flows in the opposite direction in a circuit because of the forward current switching its direction due to a residual charge in the semiconductor component. 

MOSFETs that have a body diode with a lower ttr are considered fast body diode MOSFETs. Overall, ttr directly impacts the reverse recovery charge (qrr), which is the charge that accumulates in the p-n junction from the forward current switching directions during the reverse recovery time.


Body diode characteristic graphs for AOK040A60 (left) and AOK042A60FD (right).

Body diode characteristic graphs for AOK040A60 (left) and AOK042A60FD (right). Screenshots used courtesy of Alpha and Omega Semiconductor


As for AOS's latest components, one of the new diodes, the AOK042A60FD (FD standing for 'fast diode'), does not only feature the low ohmic characteristic but also features a fast body diode. It's designed to increase the reliability of switching systems, mainly using half-bridge and full-bridge topologies. 

Additionally, the lower qrr of the MOSFET design further reduces the losses during reverse recovery, improving the overall reliability when implemented in a system.


Improving Future High Voltage Power Supply Systems

These two MOSFET characteristics in AOS's newest products aim to improve the overall efficiency of current and future power supply systems without changing their main principles or form factors. 

With a lower internal MOSFET resistance and a shorter reverse recovery time of the MOSFET's body diode, many modern applications such as high power electric vehicle power supplies can benefit from a lower energy requirement, lower heat production, and thus lower electrical, cooling and ventilation costs. These characteristics could also improve a system's reliability and longevity. 

Though the MOSFET is a widely used and beneficial semiconductor component, it is always necessary to keep improving.