Metal Oxide Field Effect Transistor: What is RDS(on)?

R_DS(on) stands for “drain-source on resistance,” or the total resistance between the drain and source in a Metal Oxide Field Effect Transistor, or MOSFET when the MOSFET is “on.” R_DS(on) is the basis for a maximum current rating of the MOSFET and is also associated with current loss. All things being equal, the lower the R_DS(on), the better.

The voltage applied to the Gate determines if current will flow between the Source and Drain terminals. ID is the continuous drain current. (Image Credit: Kenneth Reese, III).

MOSFETs make perfect switching devices and are often used in power applications. Example applications for power MOSFETs include Switched Mode Power Supplies (SMPS), motor control, automotive, and in any application where a heavy-duty electronic switch is needed, such as a driver. Current flows between the n-channels when a gate-to-source voltage (V_GS) is applied, otherwise the MOSFET behaves like a resistor. When V_GS reaches the threshold voltage V_GS(th), an inversion layer forms that enables current flow. The inversion layer becomes the conductive path (or channel) of the MOSFET between drain and source.

R_DS(on), the total resistance in the path from source to drain, is made up of a series of resistances that traverses the path of current flow. R_N is the source region’s diffusion resistance. R_CH is the channel region’s resistance. R_A is the resistance of an area called the accumulation region. R_J is the resistance of an area called the JFET region. R_D is the drift region resistance and the most important factor in high-voltage MOSFETs. R_S is the resistance of the substrate itself and can be ignored in high-voltage MOSFETs. However, in low voltage MOSFETs it can have a large effect on R_DS(on). See Figure 2 of a vertical structure of a MOSFET and a series of resistances in series forms the total R_DS(on) in the path of current flow from source to drain.

A MOSFET vertical structure, showing the total resistances that make up RDS(on). (Source: AN-9010 MOSFET Basics by ON Semi)

Besides these inherent structural contributors to R_DS(on), imperfect contact between the source and drain metal and even the wiring that connects the die to the leads on the package can also contribute to R_DS(on).[i] The latter can be identified as R_WCML, or the total of the bond wire resistance, contact resistance, and the resistance of the lead frame.[ii]

R_DS(on) increases with increasing temperature (this is also known as a positive temperature coefficient.) This is because of the mobility of the hole and electron decrease with increasing temperature.

R_DS(on) is a function of temperature as defined by the following formula:

R_DS(on) (T) = R_DS(on) x (25°C) x (T/300)^2.3, where T is absolute temperature.[i]

[i] AN-9010 MOSFET Basics. (2000). ON Semi Application Note, 1-17. Retrieved April 30, 2017.

[ii] IDAN0061 Power MOSFET Basics. Abdus Sattar, IXYS Corp. Retrieved April 30, 2017.