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Discrete NMOS Logic in 2020

Optimizing NMOS logic gates made of discrete transistors

TimTim
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NMOS logic was what made higly integrated digital circuits in the 70ies and early 80ies tick. In contrast to logic based on bipolar transistors it never found widespread commercial use as discrete implementation where transistors and resistors in single packages are used.

However, due to it's simplicity and robustness, discrete NMOS logic is an excellent candidate to build discrete logic circuits for educational purposes and fun.

This project explores transistor selection and optimization of discrete NMOS logic.

  • A Quick Survey of Discrete Planar MOSFETs

    Tim09/25/2020 at 21:06 0 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.

  • NMOS Logic and Discrete MOSFET Types

    Tim09/25/2020 at 20:26 0 comments

    The most basic NMOS gate, an inverter, consists of a load resistor and an N-MOSFET as shown below. NAND and NOR gates can easily be formed by using two N-MOSFETS, more complicated gates are formed by combinations thereof.

    Some benefits compared to Resistor-Transistor-Logic are:

    • Lower component count since no base resistor is required
    • Since no current flows into the gate, the fan-out behavior is much more forgiving. It is not necessary to scale the load resistor according to the number of gate connected.
    • Stacking of transistors to form NAND gates is easily possible. In contrast to RTL, no issues with load instability or parasitic current paths are observed.
    • The logic levels are well defined by the threshold voltage of the NMOS.

    In short, it is much easier to realize discrete logic circuits without spending a lot of thought on analog circuit design.

    The Catch

    There is a drawback, isn't there? Two things stand out:

    1. The MOSFET is a four terminal device. To build fany circuits like pass-transistor logic or cross coupled XOR gates we also need to to control the substrate contact.
    2. Somehow it seems that all practical examples of discrete NMOS circuits are very very slow, as evidenced by the Megaprocessor and the Monster6502. Let's figure out what to do about this.

    Types of discrete MOSFETS

    Although often no distinction is made by the circuit symbol, there are actually several different types of MOSFETS.

    Planar MOSFET

    The basic type of MOSFET that is also used in integrated circuits (at least up to the 32 nm node) is called the planar MOSFET. A cross section is shown below.

    By Cyril BUTTAY, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=671256

    Both source and drain contact are accessible from the top of the device, while bulk contact can be established by contacting the subtrate by a p+ region at the bottom or next to the S/D junction.

    Power MOSFET

    DIscrete MOSFET are often used as switch for high currents. These applications are typically addressed by a functionallity similar, but differently constructed device called the Power-MOSFET. A cross-section of the basic structure is shown below. Note that there are many variants and refinements of this devices. Each power-MOS consists of many of the cells as shown below.

    CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=538889

    Compared to the planar device, the current flow through the power-MOS is vertical. The source is at the top and the drain is at the bottom. The bulk is directly connected to the source. Due to this, it is not possible to control the bulk contact independently anymore and a PN-Diode is formed between source and drain. This is given by design and is not a disdavantage for most application. It does, however, prevent us from using these devices in a pass gate configuration.

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