Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How does a logic gate permanently maintain its state?

  1. Mar 10, 2012 #1
    Logic gates can be switched to a high state, where the current flow is cutoff and a low state, allowing current to flow through. What electromagnetic processes in the semiconductor material enable this to happen? Since the switching action is purely electronic and there are no moving parts involved, does the control signal alter the electrical conductivity of the logic gate?

    And what electromagnetic processes enable a logic gate relay with multiple output pins to switch from one set of output pins to another every time a pulsed input from the control signal enters the device?
    Last edited: Mar 10, 2012
  2. jcsd
  3. Mar 10, 2012 #2
    To answer your question, you need a few books starting from basic network theorem, to transistors before you can even talk about your question.

    Even though it is EM, but you really don't look at it this way. Just like when you look at a painting, you don't look at the atomic structure of the materials. You look at it as circuit with transistor how they switch and latch to one state or the other. You can do a latch to remember the last state using two transistors in a bi-stable configuration. But until you understand basic electronics and transistor, I don't see the point going an further here.
  4. Mar 10, 2012 #3
    The problem with many textbooks about transistors and electrical networks is that they only explain how to setup circuits using circuit diagrams and the mathematical calculations used to design a circuit to given specifications. They do not explain in detail what is happening on an atomic level in logic gates and logic gate relays. And that is why this question was posted in the first place.

    Though the electromagnetic processes happening in an atomic and subatomic particle scale in transistors implemented as logic gates is outside the scope of electrical engineering subjects where the focus is on how the transistor is used for various applications. The question posted in this thread may be more suited to quantum electrodynamics than electrical engineering.
    Last edited: Mar 10, 2012
  5. Mar 10, 2012 #4
    If that's what you are asking, a book on semi-conductor physics is just what the doctor's order.

    As CMOS is the main stay in digital logics, you may want to look at:

    Design of Analog CMOS Integrated Circuits by Behzad Razavi.

    It is a very good book for CMOS design and it get into the device physics at the beginning. Go on Amazon and take a peak from page 9 to 40.
  6. Mar 10, 2012 #5
    Is there a free online article or e-book that can provide this information?
  7. Mar 10, 2012 #6
    I don't know, I always like to have a book in my hand. Try Amazon, I told you the page number already.
    If e-book means Kindle, I am sure you can find that book as it is a very popular book in this subject.
  8. Mar 10, 2012 #7
    The best way to understand a simple NAND logic gate is to look at and analyze the internal schematic, like on page 2 of


    or page 2 of

    http://www.ti.com/lit/ds/symlink/sn7401.pdf [Broken]

    Both are current sinking logic, with the first being a totem pole output, and the second an open collector output. Once you understand these, you can analyze the SR (set reset) flip flop in

    http://en.wikipedia.org/wiki/Flip-flop_(electronics [Broken])

    Bob S
    Last edited by a moderator: May 5, 2017
  9. Mar 10, 2012 #8


    User Avatar
    Science Advisor
    Gold Member

  10. Mar 13, 2012 #9
    The originator of the thread has read about the operation of a flash memory device. It is comprised of multiple memory cells built into a memory card.

    Each memory cell has an NMOS NPN transistor, a control gate, and a floating gate surrounded by an insulating layer. When a write operation is performed, the control gate is designed to deliver a voltage that is higher than the voltage it uses during a read operation and when this happens the current that the control gate will switch on will be increased. Some of the high energy electrons that result from the higher amount of current passing from the source terminal to the drain terminal will tunnel through the insulating layer and be absorbed by the floating gate which can act like a capacitive plate that can absorb electrical charge. This type of method for placing charge into the floating gate is called hot-electron injection.



    Now there are questions regarding the operation of this device:

    1.) To erase information, an elevated voltage of opposite polarity is applied and this causes the electrons in the floating gate to tunnel out of the gate and go into the NMOS transistor. The question is how can a transistor operate on an opposite polarity when the current it is switching on is coming from a DC power supply that has a fixed polarity? And what physical interaction between the NMOS transistor and the floating gate causes the reverse voltage to cause the electrons from the floating gate to tunnel out of it?

    2.) When an operation is performed on a specific memory cell to either set the floating gate to a logical '0' or '1', how does the memory card know that that cell is not already at logical '0' or '1'? Does it read the cell first to check the information contained within it?

    3.) A floating gate that is charged with electricity will block the current coming from the control gate so that it cannot switch on the transistor unless it surpasses the now increased threshold voltage. Does the electric field of an electrically charged material interfere with the flow of electrical current?

    4.) How is information read in a flash memory device? Does it use an IC implemented as a jump counter that can switch connections from one cell to another every time an electrical pulse enters it, enabling multiple cells to be read one by one to generate an output signal?

    5.) The thread originator has built a circuit that uses an IC implemented as a jump counter where every time an electrical pulse enters the control terminal of the device, it switches the electrical connections from one output terminal to another. To verify the operation of the circuit, LED's were connected to each output pin so that every time a connection was switched, a different LED was lit. So the thread originator knows how to connect a circuit that can implement a jump counter but how does a jump counter work? What type of electromagnetic phenomena causes the jump counter to electronically switch from one output terminal to the next one?

    6.) Does the RAM card use the same type of technology as flash memory but with a different type of circuit configuration?
    Last edited: Mar 13, 2012
  11. Mar 13, 2012 #10
    @ yungman
    What mathematical background is necessary for the textbook on semiconductors that you recommended?
  12. Mar 13, 2012 #11


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    A basic logic gate (NOT) is not much more than an amplifier with a high gain and a 'decision level that's about half way between the two states.

    I suspect that you are not describing a simple logic gate but a flip-flop or bistable of some kind. First get to know about NOR and NAND gates and then learn about positive feedback and bistables.
    Google Logic Gate Basics ?
  13. Mar 14, 2012 #12
    The problem is that the originator of the thread already knows the functions of the logic gates and how they process information. The thread originator also understands the mathematical principles of discrete mathematics and Boolean algebra needed to design combinatorial and sequential logic circuits. The thread originator's questions, which are posted in post #9, refer to the quantum physics involved in the operation of logic gates, not the logic that the gates can process. The problem is that electrical and computer engineering resources only focus on the computer science and mathematical aspect of these devices and not the physics that enables the devices to work in the first place. Additionally, many electrical and computer engineering resources focus on the design of networks of components rather than providing information about the physical principles that are utilized in the operation of electrical components and the use of physics to design individual electrical components.
    Last edited: Mar 14, 2012
  14. Mar 16, 2012 #13
    It has been many days since the questions in this thread were posted. Can a consultant on this site please answer the questions on post #9?
  15. Mar 16, 2012 #14
    with feedback, have the output of a hex inverter connected to its input, it will maintain state until it is pulled back high or low.
  16. Mar 16, 2012 #15
    hex inverter wrong, that would do something but not what you want, a buffer would work.

    edit: i totally misunderstood the question, a logic gate maintains state through a capacitor or it has a structure like a mosfet with a very thin gate that has a high resistance, that gate is kept charged with a much bigger doped p-junction (or n) next to or on the gate besides the source and drain that you would see in a fet. fets gates are in Mohms so thier is low leakage of this stored voltage.
    Last edited: Mar 16, 2012
  17. Mar 16, 2012 #16

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    I'm having difficulty understanding just what is the question. I think it's been answered already.

    A transistor is pretty much a transistor whether it's a switch in a logic circuit or the transdiode in a logarithmic current amplifier.

    I remember digital computers built with triode tube logic gates, 12AU7's they were*.

    Sophie said it - they're all amplifiers.

    * too bad they weren't 7199 pentodes, they'd have been better for our hi-fi preamps.
  18. Mar 17, 2012 #17
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook