MOS as a switch

Discussion in 'General Electronics Chat' started by PG1995, Sep 15, 2011.

  1. PG1995

    Thread Starter Well-Known Member

    Apr 15, 2011
    Hi :)

    In one of the queries about the course Logic Design and Switching Theory I was told that the part "Switching Theory" refers to the fact MOS used as a switching device which turns on/off by sensing the input voltage or whatever. MOS stands for 'Metal Oxide Semiconductor". Is this "MOS" a transistor, diode, or what? I think it's a transistor because a transistor can be used as a switch. What is so special about the "Metal Oxide" prefix? Do we use only those semiconductors to switch on/off the circuits which have been made from metal oxide? Please help me with it. Thank you.

    Last edited: Sep 15, 2011
  2. colinb

    Active Member

    Jun 15, 2011
    You must be referring to a MOSFET, a type of field effect transistor (FET), which itself is a specific type of transistor--other common types being bipolar junction transistor (BJT) and junction gate field-effect transistor (JFET).

    A MOSFET's gate is voltage-controlled, as opposed to a BJT where the output current is proportional to the input current (this holds under certain conditions, at least). The first obvious difference between a BJT and MOSFET is that the MOSFET has a very high input impedance, meaning that the MOSFET gate essentially consumes no current at all. On the other hand, a BJT base needs some current flowing to the emitter to turn on the transistor.
  3. colinb

    Active Member

    Jun 15, 2011
    P.S. The term metal oxide refers to the use of a metal oxide as the insulating layer between the gate terminal and the source substrate. See WP: MOSFET for a useful diagram of MOSFET structure. MOSFETs are a common name for insulated-gate field-effect transistor (IGFET), where IGFET also suggests the “insulated” nature of the gate and thus explaining the high impedance of the IGFET/MOSFET gate terminal.
  4. Crispin


    Jul 4, 2011

    Why are there dedicated drivers then? If I have a MOSFET which has a threshold voltage (think that is the term?) of 5V, is there any reason I could not drive it from a MCU?
    Are drivers used when either the required voltage is higher than you have (3.3?) or is it to "help out" with the gate capacitance?
    On the latter point, is that anything to worry about other than when switching very fast as that would load the driver a bit and slow down the switch time.

    Hope questions are seen as an addition and not a hi-jack :cool:
  5. colinb

    Active Member

    Jun 15, 2011
    You are right: when you want to switch MOSFETs at a high rate (e.g., for high power PWM control) then you want to drive transitions fast so a MOSFET driver can use a higher voltage and supply the higher transient currents required to charge the gate capacitor.
  6. colinb

    Active Member

    Jun 15, 2011
    I drive some logic-level MOSFETs with PWM for high-power LED control from microcontroller output pins and from 74HC595 outputs, so you don't always need a MOSFET driver, but it really depends on the application.
  7. CraigHB


    Aug 12, 2011
    Depends on gate charge and frequency. For a power MOSFET switching higher currents at higher frequency, a driver may be required. In that situation, you need a low output impedance to source the currents required in overcoming the larger gate capacitance. A micro-controller may not be able to handle it. You could overload the controller or get unsatisfactory switching response. You have to look at what you're trying to do and determine if a driver is required or not.

    I try to avoid loading output pins too much on MCUs. I have a pretty low threshold there. If currents are going to get into the milliamps, I usually start looking at different ways to drive stuff. Things like opto-isolators and small signal MOSFETs come in handy for that. I'm pretty quick to use a driver of some kind to drive power MOSFETS at any considerable frequency.
  8. kubeek


    Sep 20, 2005
    There is a big reason why it can´t. Threshold voltage is NOT the voltage that makes the transistor fully open. It is usually specified as the voltage that lets 1mA or 250uA flow between D-S, look carefully and you will see some footnotes in your datasheet.
  9. Adjuster

    Late Member

    Dec 26, 2010
    For this you would require a "logic level" MOSFET, that is to say a device which has a threshold voltage a good deal less than 5V, and which is specified to be reasonably well turned on by that voltage.