On-resistance is one of the most important specifications of metal oxide semiconductor field-effect transistors (MOSFETs). This mouthful of a component is what fills the ubiquitous digital integrated circuit, which can contain hundreds of MOSFETs. What makes ICs ubiquitous is that nearly every electronic gadget contains them. Power MOSFETs are used for applications from electric motor control in industrial systems to converting power in the switches and routers that enable the world's communications networks.
Fig. 1: The MOSFET's three terminals: gate, drain, and source.
A MOSFET consists of three terminals: drain, source, and gate (see Fig. 1). The source is the terminal through which the majority carriers enter the channel; the conventional current entering the channel at S is designated as Ig . The drain is the terminal through which the majority carriers leave the channel, and the conventional current leaving the channel at D is designated as ID . On-resistance, measured like all resistance in ohms (Ω), is between the drain and the source, hence the orthography for this term: RDS(on).
Power MOSFETs are known for their superior switching speed and low gate-drive power requirement due to the insulated gate. The main drawback is on-resistance and its strong positive temperature coefficient, so reducing on-resistance is very important. The main components involved in on-resistance include the channel, junction gate field-effect transistor (JFET), accumulation layer, drift region, and parasitics (metallization, bond wires, and package).
Temperature has a strong effect on RDS(on) , with resistance approximately doubling from 25° to 125°C. The temperature coefficient of RDS(on) is the slope of the curve and is always positive because of majority-only carriers. The strong positive RDS(on) temperature coefficient compounds the I2 R conduction loss as temperature increases.
A typical approach for measuring RDS(on) is to force current between the chuck and the probes contacting the top of the wafer. Another approach uses probes instead of a chuck on the back side of the wafer. This method can be accurate down to 2.5 mΩ.
Fig. 2: In MOSFETs, a significant source of error is the contact between the wafer and the chuck. (Source: www.electronicproducts.com/Power_Products/Power_Semiconductors/Accurately_measure_power_MOSFET_RDS_on.aspx)
A significant source of error is the contact between the wafer and the chuck (see Fig. 2). Because there is roughness on the chuck and on the back of the wafer, electrical contact is made in discrete areas. The contact resistance between the wafer and the chuck is large enough to introduce significant error in the RDS(on) measurement. Simply repositioning the wafer on the chuck will change contact areas and change RDS(on) measurement results.
On-state resistance (RDS(on) ) is an important measurement but often a difficult one to make accurately. Typical sources of error in this measurement include the sensitivity (or lack thereof) of the voltage measurement, the noise level, and the difficulty of compensating for drops in the test leads.
Given that RDS(on) could be as low as a few milliohms, very little voltage is generated even when the currents are as high as 50 A or more. Curve tracers simply can’t provide sufficient voltage sensitivity. In some cases, users have attempted to create test solutions by integrating power supplies and sensitive voltmeters, but these solutions are typically limited by synchronization and settling time problems. Most devices can only handle high currents for very short periods, perhaps just hundreds of milliseconds. All it takes is a little extra stray inductance in the test cables to extend the measurement’s settling time, creating inaccurate results.
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