A basic error I was making about MOSFETs.

MOSFETs are Metal-Oxide, Field Effect Transistors, and are a main type of transistor in modern circuit design. Their properties are well-documented.

I had a basic misconception about their behavior, that concerned how they go into saturation, and to what extent their Drain voltage affects their behavior, even after saturation.

This error started, because I had read a specifications-sheet, about the circuit-simulation program SPICE, according to which MOSFETs are simulated as having parameters, which are denoted by keywords that SPICE recognizes, but the meanings of which can easily be misread by any person, who does not know a lot about circuits. This specifications-sheet names the parameter ‘Lambda’.

What I had mistakenly thought, was that this parameter both defines the Drain voltage at which saturation takes place, as well as the degree to which channel-length modulation takes place afterward. I had justified this idea, with the additional idea, that the Bulk voltage of a MOSFET, plays more strongly near the Source and Drain of a MOSFET, than the relative Gate voltage does. This was factually wrong.

I had quoted another article on the Web, that defines channel-length modulation, and which may explain the phenomenon of saturation while doing so.

The fact is that the established literature now seems 100% accurate in describing both phenomena. And a practical consequence of this is, that the circuits which are labeled ‘Current Mirrors’ in circuit-design, will work very well. Because, either MOSFET that forms part of such a current mirror, will saturate in a trivially easy way.

The following would be my own updated synopsis, of both phenomena. It assumes an n-Channel, enhancement-mode MOSFET:



I added a bit of complexity, to how channel-length modulation sets in, so that ‘exactly at the saturation point’, the two definitions / simulations of Drain current would be equal. But, because (‘λ’) is usually assumed to be small, the impact of this modification, or of its absence, should also be small.

Conclusion:

By default, a MOSFET operates in its saturation-region, in which Gate voltages vary Drain current. The ability of the MOSFET to operate in its ‘triode region’, is as limited as the Drain voltage is in practice, not to exceed the saturation-point.

Dirk

 

Sometimes, we misunderstand things.

One of the facts which I’ve been blogging about, concerns the software called ‘NG-SPICE’, which stands for ‘Simulation Program with Integrated Circuit Emphasis’. When using it, I can sometimes seem to recognize the parameters with which it defines components, those parameters will correspond to concepts which I already know about – such as capacitance, or ‘transconductance’ – but in the modern context, those parameters can stand for the real-world properties of a MOSFET, that do not match old-world properties, by the same name.

So as an example of this, I can demonstrate the following ‘SPICE’ definition, of an old-fashioned, discrete MOSFET, which was named the ‘2N7000′:

http://dirkmittler.homeip.net/text/2N7000.mod.txt

In this specification there is a parameter named ‘CGSO’ , of which I might say, ‘It stands for Gate-Source capacitance’. Its value is close to 1.79·10-7 . If this value was in Farads, it would actually mean that the transistor to be modeled has 180nF input-capacitance, when not active. This would be a very high capacitance-value, which in turn, would lead me to think that the transistor-type was of very low quality. But in reality, this parameter is in Farads /Meter. And so what I would now think, after noticing this detail, is that because the transistor in question only has a width of 100μM, its passive input-capacitance is really only 18pF.

The same goes for ‘CGDO’.

This will make a huge difference, in terms of how fast circuits can become, that use this transistor. And I am back to having faith, in how NG-SPICE simulates its circuits.

At the same time, when such a transistor is functioning in an active circuit, it will exhibit the property of voltage-gain indirectly. This voltage-gain will reduce the apparent capacitance, between the Gate and Common, because it reduces the voltage-changes between the Gate and the Source of the transistor, which determine how much current will flow due to its real Gate-Source capacitance, with respect to how large the changes are, in Gate-to-Common voltages. However, this phenomenon needs to be held in suspicion, if we are expecting it to reduce input capacitance by more than a factor of 1000, because:

  • The real voltage-gain of the transistor-circuit is only rarely that high, and
  • There are additional factors which will increase input-capacitance, so that such an ideal will not be reached.

The transistor needs to be a fast one, with low Gate-Source capacitance, if that’s what the circuit-designer is looking for.

Dirk

 

Another Simple Output-Amplifier, Using Discrete MOSFET Transistors

One of the facts which I’ve been writing about, is that I possess the open-source version of ‘SPICE’, that is named ‘NG-SPICE’, and that this acronym stands for ‘Simulation Program, with Integrated Circuit Emphasis’. The full, associated suite of programs allows me to edit schematic diagrams graphically, but to export ‘Netlists’, so that I can then simulate the circuit – and see if it works.

And one of the facts which I have also been contemplating, is that by default, SPICE will put transistors, which correspond to micron-sized transistors, which will therefore never be able to drive output-loads, from a hypothetical IC, unless an explicit attempt is made, to design output-buffers, which can. These output-amplifiers have as function, that they should merely follow their input voltage, but draw as little current from their respective inputs as possible – that are outputs of other, more interesting ICs – while allowing low load-resistances to be connected to their own outputs, which correspond to plausible external components, such as 100Ω load-resistors.

I had posted an earlier, conceivable design, of such an output-buffer, which had a major flaw, that I also pointed out in the preceding posting: That amplifier could only produce a range of voltages, which was a direct function of what the Gate-Source threshold voltages would be, of the component transistors used. Hence, because I had also specified low-quality, outdated MOSFET transistors with high threshold-voltages, the output-voltage-range, was also modest but reasonable. But, newer transistors will have lower threshold voltages by design, which would, oddly enough, reduce the voltage-range of that amplifier. This would be an important consideration if the transistors were not in fact discrete, but needed to be incorporated onto the IC, where low-threshold-voltage transistors are already standard. Which means, that I needed to design a better output-buffer.

So below is a better output-buffer, schematic:

buffer_2_i

And these are the SPICE definitions, of the discrete transistors which I decided to base my design on again, both enhancement-mode MOSFETs:

http://dirkmittler.homeip.net/text/2N7000.mod.txt

http://dirkmittler.homeip.net/text/BS250P.mod.txt

The main disadvantage of this latest design would be, that the transistors which I labeled ‘X2′ and ‘X3′, do in fact conduct current to their combined inputs, which makes the additional transistor ‘X1′ necessary, since this amount of current would already be excessive, to connect to an output, of any pre-existing IC circuits. But then, the advantage goes so far, that ‘X2′ now models a level-shift, which exactly mirrors the level-shift of ‘X4′, and the voltage-level-shift of ‘X3′ now mirrors ‘X5′. There is design beauty in this. But one disadvantage now is, that the Gate-Source threshold-voltage of (1) n-Channel MOSFET (2.2V) plus (1) p-Channel MOSFET (3.2V) gets subtracted from the input-voltage, so that the available voltage-range still suffers, with respect to both the supply, and the input-voltage. Input-voltage now ranges from 5.4V to approximately 12.5V, which is closer to the range of supply-voltages than what the previous circuit allowed, and the resulting output-voltages are graphed below:

screenshot_20180618_075248

(Update 06/20/2018, 0h20 : )

There is another observation which I should add:

In the days of vacuum tubes, ‘transconductance’ was measured in Amperes / Volt, and was therefore given in ‘Mhos’, which were the reciprocal of Ohms. Apparently, in modern days, the transconductance of a MOSFET, also given as its ‘KP’, is in Amperes / Volt2 . :-D This conscious design-decision must follow the real-world behavior of MOSFETs, but makes my earlier Math, of multiplying such a component-property by the series-resistance, to arrive at gain, incorrect. Gate-Source voltage-changes lead to current-changes, but greater Drain-Source voltages, lead to greater current-gain. This is good, because the actual gain of a MOSFET, reduces the apparent capacitance at its Gate.

The low-end output-voltage came into being as follows:

Continue reading Another Simple Output-Amplifier, Using Discrete MOSFET Transistors

A Pertinent Question, about Micron-Sized Transistors

If we position two electrodes in free air, 1Centimeter apart, and if we then apply 10000Volts across them, the air’s ability to resist electric current will break down, and an electric arc will appear across it.

Because of this simple observation, the question could (and probably, should) be asked, ‘Can a MOSFET transistor the size of a micron, on an Integrated Circuit, withstand 15Volts of Source-Drain voltage, at all?’

A suggestion to the contrary would be, that 10000Volts /Centimeter, is equal to 1 Volt /Micron. Thus, if the two electrodes were 1Micron apart, and standing free in air, it would take only 1 Volt to cause the air to break down, and for a microscopic arc to appear. Yet, Integrated Circuits are known to exist, which operate at 2 Volts, and which use ‘nanometer technologies’. And so in an effort to answer my own question, I would take two further observations into consideration:

  1. I already recall reading elsewhere, that the breakdown voltage of high-quality, semiconductor silicon, is considerably greater than that of air !
  2. I possess a suite of programs named “SPICE”, which, when performing a Level-8 simulation of MOSFET transistors, only needs to be given the width and the length of a transistor-instance, and which will, on that basis, compute all the other properties of the resulting transistor, making certain assumptions about its design.

This use of SPICE has been commented on, on the following Bulletin-Board:

https://sourceforge.net/p/ngspice/discussion/133842/thread/9a75acf2/#62e7

The part of the thread, which I’ve linked to before, and which I want to call the reader’s attention to, is the part where Holger Vogt writes:

“A 0.18µm process however should run at lower supply voltage, e.g. 1.8-2 V.”

Continue reading A Pertinent Question, about Micron-Sized Transistors