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A Quick Survey of Discrete Planar MOSFETs

A project log for Discrete NMOS Logic in 2020

Optimizing NMOS logic gates made of discrete transistors

TimTim 09/25/2020 at 21:060 Comments

As outlined before, a planar n-channel MOSFET in a discrete package, such as SOT23, would be the perfect device to build NMOS logic. Unfortunately, it turns at that these are so rare that it does not even make sense to benchmark them. I spend quite some time digging for candidates, so they are listed here for reference.

CD4007

The CD4007 is a 14pin IC from the 4000 series of CMOS logic gates. It contains three pairs for discrete p- and n-channel devices with the substrate connected to the supply.

Suprisingly, the CD4007 is still in production at Texas Instruments (datasheet). It's a nice device to do some experiments with discrete mosfets. There are some nice ideas in the analog devices wiki.

Unfortunately these are really old and slow transistors with switching times in the 100s of ns, possibly still manufacutred in metal gate CMOS. Also the form factor is not very well suited to build discrete logic. Coupled with a relatively high price, the 4007 is not a good candidate to build larger circuits.

Siltronix SD5000 series

This is a family of product with four lateral n-channel transistors in a package with a common substrate contact. I believe some of these are used in the Monster6502 in places where transmission gates are needed.

The performance looks quite good with fast switching times. Unfortunately these devices are out of production and can only be obtained in low numbers for obscene prices. Maybe good for some experimentes but not a larger design.

BSS83

Not to be mixed up with the BSS83P, the BSS83 is a single n-channel mosfet with separate bulk connection in a small SOT23-like SMD package (SOT143). The switching time is <5ns and the input capacatiance only 1.5pF. A very suitable device that was once made by Infineon and Nexperia. But alas, it is out of production...

Linear Systems DMOS Portfolio

Linear Systems is a small semiconductor manufactor that seems to have acquired some of the product lines that were discontinued at Siltronix. They have an entire portfolio of discrete lateral MOSFETs. (LDMOS is a type of planar power device. It's not entirely symmetric in the sense that source and drain are fully exchangable, but it can still be used as a pass-gate when not driven at too high voltage when a separate bulk contact as available.)

One suitable device in their portfolio seems to be the SST214.

Although these devices are technically still available, it seems that linear systems only sells via rather obscure distributors. I can only presume that the prices will also not be very suitable for designs with a high number of transistors.

Conclusions

In summary, the availability of planar n-channel mosfets it rather limited. There are a few options, but apart from the CD4007 they all look rather pricey with uncertain supply availability.

It looks like we have to settle with Power-MOSFETS with integrated body-diode for any practical purposes.

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