MOSFETs Boost Threshold Voltage Yet Maintain Low On-Resistance

Apr 13, 2005 5:51 PM
By David Morrison, Editor, PETech

Siliconix has introduced a series of n-channel MOSFETs that combine a high 3.4-V threshold voltage (V_TH) with on-resistance (R_DS(ON)) as low as 2.7 mΩ. Available in 40-V and 60-V versions, the 10 MOSFETs are intended for use in high-temperature, high-current applications with inductive loads in the automotive, industrial and fixed telecom industries. Some of the specific uses include high-side switches, motor drives and 12-V board nets.

The 10 new models numbers and their key specifications are listed in the table below. Many of the devices in this MOSFET family carry a maximum junction temperature rating of 175°C as opposed to the more common 150°C rating.

When MOSFETs operate in high-temperature, high-current environments, they can turn on spontaneously if their threshold voltage, impacted by heat, starts to approach 0 V. Until now, this posed a dilemma for designers. One solution was the addition of a negative voltage driver to the circuit, but the drawbacks were increased circuit size, cost and complexity. Another solution is to use a device with a high threshold voltage, but the effect in this case is an undesirable increase in on-resistance.

To illustrate this effect, consider two n-channel MOSFETs that are manufactured in the same process, have the same die size, and are housed in the same package. One MOSFET is a logic-level device with a V_TH of 2 V. This device specifies a maximum R _DS(ON) of 2.3 mΩ. In the other device, VTH is 3 V and maximum R_DS(ON) is 2.8 mΩ. Siliconix’s new power MOSFET family provides a solution to this dilemma with high-density silicon technology that allows the same device to deliver both low on-resistance and a high threshold voltage. This combination prevents the problem of spontaneous turn-on at elevated temperatures.

The key to raising V_TH while lowering R_DS(ON) is an underlying advance in MOSFET process technology that boosts cell density from 45 million cells to 178 million cells. Such increases in cell density are typically used to lower R_DS(ON) in a given sized die, or to achieve the same R_DS(ON) in a smaller die.

Table. Specifications for N-channel MOSFETs with 3.4-V threshold voltage.

However, with these new MOSFETs, Siliconix used its process improvement to offset the expected increase in R_DS(ON) that naturally occurs when V_TH is raised. Although, these MOSFETs are not the first devices manufactured in Siliconix’s 178-million-cell process, they are the first to tradeoff on-resistance and threshold voltage so that the devices are optimized for automotive and other high-temperature applications. Furthermore, the new process more than compensates for the expected increase in R_DS(ON) at the higher V_TH it actually allows for some additional reduction in on resistance. Nevertheless, that reduction in R_DS(ON) is less than it would be if V_TH was maintained at the original value.

Samples and production quantities of the MOSFETs are available now. In 100,000-piece quantities, prices start at $1.35 per piece.

For more information, visit www.vishay.com/mosfets/.

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