FETs are mono-polar transistors. The current passing from drain to source doesn’t cross a P-N junction, as in a bipolar transistor. Because of this, the parts are resistive when you turn them on with a gate signal. The on-resistance of a FET is a fundamental spec of the transistor. The lower it is, the less heat is generated in the part by the current passing through it. The on-resistance depends on the impurity doping in the silicon die, the metal lead-frame size, and size of the bond wires the manufacturer uses to connect the die to the pins.
A physically large die will have lower on-resistance, all other things being equal. The breakdown rating of the transistor is directly proportional to the on-resistance. A few milliohms of resistance is much more common for low-voltage parts than for parts with a breakdown voltage of 150 V. A 1000-V part will have an on-resistance of 10 Ω.
Due to the slower mobility of holes versus electrons, P-channel FETs will have higher on-resistance than N-channel types. Gate-drive voltage is another factor to consider. Driving the gate terminal with a 10-V signal will give a lower on-resistance than a 5-V drive signal. If you’re operating the transistor from logic, you must read the datasheet carefully to learn the on-resistance at that drive voltage.
One benefit of the resistive nature of MOSFETs is that you can parallel them to reduce the total on-resistance. Bipolar transistors will current-hog; that is, tend to take a little more current, which makes one heat up more and, thus, making it take even more current. While paralleling FETs is fairly straightforward, you have to ensure that the gate traces on the PCB are of equal length and impedance so that the multiple transistors turn on and off at exactly the same. Placing a small series resistor on each gate circuit board trace may help damp oscillations at the expense of slower switching.
To get a sampling of five FETs with low on-resistance, we have focused on one package, the DPAK also called the TO-252 or SC-63. This is a good medium-sized transistor that can take 100 A of current, while still small enough to be used in compact designs. The DPAK also has much better power ratings than TO-92, or SOT-23 packages, while being half the size of the T0-220 package. Here are five FETs in the DPAK package with low on-resistance, sometimes called RDS(on) (resistance from drain to source, on):
1. The IPD100N04S402ATMA1 from Infineon has an RDS(on) of 2 mΩ with a breakdown voltage of 40 V. This is with a 10-V gate-drive signal.
2. The TK55S10N1,LQ from Toshiba has an RDS(on) of 6.5 mΩ with a breakdown voltage of 100 V. This is with a 10-V gate-drive signal.
3. The FDD86250-F085 from ON Semiconductor has an RDS(on) of 22 mΩ with a breakdown voltage of 150 V. This is with a 10-V gate-drive signal.
4. The AOD424 from Alpha and Omega Semiconductor has an RDS(on) of 5.7 mΩ with a breakdown voltage of 20 V. This is with a 2.5-V gate-drive signal.
5 The P-channel SQD50P03-07_GE3 from Vishay Siliconix has an RDS(on) of 7 mΩ with a breakdown voltage of 30 V. This is with a 10-V gate-drive signal.
If you need to control a large current with low power loss in your next circuit design, be sure to check out these parts.