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I have a a question on forming of the inversion layer in NMOS. More specifically, please refer to the following figures

enter image description here

The negative ions (in-mobile) are due to the the accumulation of the positive charges at the gate metal plate that pushes the holes (mobile carriers) downward. This is the first figure. However, in the second figure, there is electrons moving on top of the negative ions. And I question is, is the mobile electrons forming the inversion layers (connect the source to the drain), or does the in-mobile negative ions forming the inversion layer?

This confuses me if you look at another diagram (below). If you look at the place underneath the oxide, marked as "Induced n-type channel", it is hard for me to tell which is which

enter image description here

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It's the mobile electrons that form the channel. When a positive voltage is applied to the gate, the electrons (which are minority carriers in a p-type substrate) are drawn to the surface. When you then apply a voltage between source and drain, an electric field is formed and a drift current flows. The ions could not do this, since, as you say, they are fixed in the lattice and immobile.

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  • \$\begingroup\$ Yes, isn't that the in-mobile ions provide a channel for the mobile electrons to travel and get through them? \$\endgroup\$ – kuku Sep 14 '14 at 2:29
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    \$\begingroup\$ The immobile ions are present throughout the lattice. The 'channel' is what the inverted layer of electrons close to the surface is called. \$\endgroup\$ – sobremesa Sep 14 '14 at 15:06
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Assuming an n-type substrate, and applying a positive voltage to the gate, the following will happen:

  • The positive voltage on the gate will produce an electric field between the gate and the substrate.
  • Hence, the positive charged particles "holes" will be pushed down-ward, leaving behind it uncovered negatively charged acceptor atoms. (they won't contribute, they are fixed.)
  • Also, the negative charged particles "electrons" will be attracted up-ward.
  • If a sufficient voltage (a voltage that exceeds the threshold) is applied, a channel will be formed by the free electrons.
  • Now, any voltage applied to the drain will result in an electric field between the source and the drain.
  • The current will be made up by the movement of electron going from the source to the drain, in the channel formed.
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    \$\begingroup\$ Thank you very much; so, the negative ions they don't provide the channels; only when the mobile (free-charge carriers) electrons are cross from the source to the drain, then they are forming the channel? \$\endgroup\$ – kuku Sep 14 '14 at 4:45
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    \$\begingroup\$ Ions are static, making up the crystal lattice structure and cannot move. They are just there to balance the positive bias voltage on the gate. They don't help in creating the channel or establishing a current. The free electrons pulled from the substrate creates the channel, and the free electrons drawn from the source are the current itself. \$\endgroup\$ – Meshal Sep 14 '14 at 4:55
  • \$\begingroup\$ Can I ask one more question. Why does the transistors need to be kept in reversed biased? I can't really understand why this is a good thing and that the forward biased is the bad thing. \$\endgroup\$ – kuku Sep 14 '14 at 5:14
  • \$\begingroup\$ Not sure what you mean by keeping the transistor in reverse bias. The source and drain diffusions (for an nmos) are n diffusions in a p-type substrate. Their junction with the substrate needs to be reverse biased. If it was forward bias, current would flow from drain/source to substrate and the transistor wouldn't work. If they are reverse biased that can't happen (only a very small reverse current flows). Current can then only flow between source and drain when the channel is formed, as desired. \$\endgroup\$ – sobremesa Sep 14 '14 at 15:27
  • \$\begingroup\$ @kuku: forward biasing means the NP junctions between S and B or between D and B are forward biased. That means there is current flowing between S and B or between D and B without any way to control it by the gate. In a transistor you don't want just current flowing, you want current being controllable by another current (BJT) or voltage (MOSFET). \$\endgroup\$ – Curd Oct 14 '14 at 10:58

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