Intrinsic Silicon

Many people already understand, that two types of silicon exist, N+ -Doped, and P -Doped.

Well I’ve known for some time, that another type of silicon which exists, is called ‘Intrinsic Silicon’. This is a form of silicon, which theoretically contains no dope at all, and which is therefore non-conductive. It’s not even a semiconductor in that state.

This type of silicon might be of some interest in the design of modern Integrated Circuits, especially in the reduction of the capacitance of individual transistors. But there areĀ  essentially two problems with its use:

  1. It’s practically impossible for the silicon to be perfectly pure. The concentrations of Dope, in the N+ or P -Doped silicon, are already extremely low. The concentration of impurities in Intrinsic Silicon is simply lower, industrially, than in the intentionally-doped silicon, not truly zero. And what this means in practice, is that ‘larger pieces’ of Intrinsic Silicon are still partially conductive. In fact, how low the concentrations of N+ or P -Dope can be brought in the industrial process, depends on how low the level of impurities is, in the silicon, to begin with. In either type of intentionally-doped silicon, the concentration of dope must still be at least one order of magnitude greater, than the level of impurities was.
  2. Actually, I think that Intrinsic Silicon is more expensive in bulk, than either type of intentionally-doped silicon, which means, that if the entire wafer needed to be made out of it, since the substrate of the wafer is meant to provide mechanical support as well, then the cost of the manufacturing process would increase.

Yet, small pieces of Intrinsic Silicon, as the following image shows, can still be used to provide lateral insulation, between the P -Doped and the N+ -Doped wells of individual transistors, where a “buried oxide layer” provides vertical insulation between those wells, and the actual wafer:

https://upload.wikimedia.org/wikipedia/commons/e/ee/Cmos-chip_structure_in_2000s_%28en%29.svg

And, it would be my expectation that because Intrinsic Silicon is ‘non-conductive’, larger pieces of it should also be optically transparent, which means that some people might mistake it for glass.

By definition, glass would be ‘amorphous’, which means ‘not crystalline’, which would make actual glass useless as a semiconductor. However, amorphous forms of silicon can readily be used in the design of wafers, as long as they do not need to participate in the actual semiconductive behavior between N+ and P -Doped silicon.

Dirk

 

A First, Complicated Project at Circuit Design with NG-SPICE

One subject which I wrote about in an earlier posting, was that software exists by the name of ‘SPICE’, which stands for “Simulation Program with Integrated Circuit Emphasis”. There are several variants of this software in existence, but the version which I am focusing on for now is the Open-Source ‘NG-SPICE’ system, which needs to be bundled with numerous other packages under Linux, really to be useful. One important package is ‘ngspice-doc’, but there is a whole suite of Linux packages referred to as ‘gEDA’.

Simply having tested a few demo-projects, is not the same thing as actually having designed a circuit, and having witnessed that project ‘work’, at least according to the simulation. Just last night, I did the latter, in order to get a better, working grasp of how to use the software, and also, some idea of the sort of error messages and problems which invariably occur on a first-time basis. What this means is that I actually designed a circuit using the ‘gEDA Schemtic Editor’, which is also known as ‘gschem’, and then ran multiple simulations of the circuit, discovering at first that it had performance issues as I had imagined it, modifying it numerous times, and ending up with a version of the circuit, which I could be satisfied with for now.

The circuit which I was designing, actually involved MOSFETs, because those are the most important components in circuit-design today, and surely enough, I did run into initial problems. One of the tasks which we must complete, when using active components in SPICE, is to define the component, which is as fundamental as the fact that we also don’t just put a resistor, but must also specify what the Value of the resistor is in Ohms. Well with active components, we must do something similar, which also goes under the GUI heading of the Value attribute for the component. Therefore, MOSFETs, be they NMOS or PMOS, also have values, and by default, those values are defined by a Model Card, from which the computer can predict such physical properties about the NMOS or the PMOS transistor, as what its gate-capacitance is, how well it conducts when switched to conductive, conversely when the gate-voltage is zero, etc., etc., etc..

But, because NG-SPICE (v26) is advanced software – though still not the latest version – it may not require that the user defined all these parameters each time he or she considers designing a circuit, because standard component specifications exist.

By default, our MOSFETs have Reference Descriptors that begin with the letter “M”, and not with the letter “Q”, which would stand for a Bipolar Transistor, but which the GUI of ‘gschem’ suggests for the user when he first clicks a MOSFET into his circuit. So we override that, by editing the RefDes into a text-string that has the letter “M” followed without spaces by a number.

What I next proceeded to do, was to put MOSFET-transistors into my circuit, which from the GUI, only had 3 pins. This is a common way in which MOSFETs are often diagrammed, and looked something like this:

diffamp_l

Believing that I could just accept what the GUI had constructed, I next tried to simulate the circuit, and received the error, which roughly stated “Unable to find Definition of Model.” This error-message wasted much of my time trying to solve, because I had in fact created a Model Card for the transistors which I was going to use, and at first, I despaired that NG-SPICE might not be as good as paid-for software. But I soon learned that indeed, the following example is a sufficient Model Card for an arbitrary NMOS transistor, with which circuits can be designed:

http://dirkmittler.homeip.net/text/NMOS1.mod.txt

Similarly, we can conjure a default PMOS transistor like so:

http://dirkmittler.homeip.net/text/PMOS1.mod.txt

In actual circuit-design, we’d drop the .TXT Filename-Extension, that makes the above examples readable in a Web-browser. Not only that, but we can also use the ‘gschem’ GUI, to embed such definitions directly into the Netlist, by giving them as a ‘Model’ attribute. So what was causing this error message, in my example? The fact is that MOSFETs are 4-pin components by nature. They have a hypothetical Source, a hypothetical Drain, a Substrate Electrode, and a Gate. It’s the voltage between the Gate and the Substrate Electrode, that finally determines how conductive the MOSFET is to become. By convention, many practical MOSFET-packages tie the Substrate Electrode together with the Source lead, which also happens to make the Source different from the Drain.

nmos_3_2

By telling NG-SPICE that we’re including a ‘MOSFET_TRANSISTOR‘ in our circuit, we’re telling this program to read 4 Nodes from the Netlist, to parse what the transistor is to be connected to. But, when the GUI only provides 3 arguments, an error ensues, that garbles the attempt of NG-SPICE to parse the Netlist. That’s all. Curiously, the reverse error does not happen. If I conjure a 4-lead MOSFET-symbol from the GUI, but specify a 3-lead MOSFET (more on that below), then I obtain a well-managed error message, that tells me what the problem is.

(Updated 06/14/2018 … )

Actually, the symbols above are also different in another way. In theory, one stands for an enhancement-mode, and the other, for a depletion-mode transistor. But, because under Linux, ‘this software is divided into two departments’, effectively, this does not matter.

The GUI allows schematics to be drawn in such a way, that Netlists result, while the actual NG-SPICE software emulates what these Netlists define. It’s in the emulation of the Netlists, that the decision is also made, as to whether a component is an enhancement-mode or a depletion-mode, or a subcircuit component… ( :1 )

(Updated 06/16/2018 : )

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