What is Drain Source Resistance of a MOSFET?

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Editorial Team - everything PE

Jul 14, 2023

Drain-Source On-State Resistance (RDS(on)) is defined as the resistance between the drain and source terminals of a MOSFET when it is fully turned on. It is an essential parameter that characterizes the conduction behavior of the MOSFET and is usually expressed in ohms (Ω).

When a MOSFET is in its conducting state (fully turned on), the current flowing through it generates heat due to the resistance offered by RDS(on). This resistance causes power loss and reduces the overall efficiency of the system. Therefore, minimizing RDS(on) is crucial for maximizing power efficiency and reducing thermal stress.

Factors Affecting RDS(on):

Several factors influence the RDS(on) value of a power MOSFET. These factors include:

  • Gate to Source Voltage (VGS): The gate-to-source voltage (VGS) directly impacts the on-state resistance (RDS(on)) of a MOSFET. Increasing VGS beyond the threshold voltage enhances channel conductivity, and reduces RDS(on).

  • Channel Length and Width: RDS(on) is inversely proportional to the channel width and directly proportional to the channel length. A wider channel offers lower RDS(on).
  • Device Geometry: MOSFET designs with optimized structures, such as a trench or super junction, can achieve lower RDS(on) values by improving charge carrier flow and reducing resistance.
  • Doping Concentration: Higher doping concentrations in the device structure can reduce RDS(on) by enhancing carrier mobility and lowering resistivity.
  • Technology Advancements: As semiconductor technology advances, smaller feature sizes and improved material properties enable MOSFET manufacturers to achieve lower RDS(on) values.
  • Temperature: The temperature of a MOSFET significantly affects its RDS(on) characteristics. As the temperature rises, the resistance increases, leading to higher power losses and reduced efficiency. 
  • Switching Frequency: The switching frequency of a power MOSFET has significant effects on its RDS(on). Higher switching frequencies can lead to dynamic increases in RDS(on) due to voltage variations during switching transitions and the time required for the channel to establish conductivity. Additionally, higher frequencies increase switching losses caused by charging and discharging parasitic capacitances, influencing the effective RDS(on). Adequate gate drive is crucial at higher frequencies to minimize RDS(on), while optimized gate charge and characteristics are preferred. On the positive side, higher frequencies reduce conduction losses, resulting in lower power dissipation and cooler operating temperatures. However, higher frequencies can also generate more electromagnetic interference (EMI), requiring additional measures for mitigation. Balancing these factors is essential to optimize the performance and efficiency of power MOSFETs.

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