What is Mosfet: Definition and 213 Discussions

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of insulated-gate field-effect transistor that is fabricated by the controlled oxidation of a semiconductor, typically silicon. The voltage of the covered gate determines the electrical conductivity of the device; this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals.
The MOSFET was invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959, and first presented in 1960. It is the basic building block of modern electronics, and the most frequently manufactured device in history, with an estimated total of 13 sextillion (1.3×1022) MOSFETs manufactured between 1960 and 2018. It is the dominant semiconductor device in digital and analog integrated circuits (ICs), and the most common power device. It is a compact transistor that has been miniaturised and mass-produced for a wide range of applications, revolutionizing the electronics industry and the world economy, and being central to the digital revolution, silicon age and information age. MOSFET scaling and miniaturization has been driving the rapid exponential growth of electronic semiconductor technology since the 1960s, and enables high-density ICs such as memory chips and microprocessors. The MOSFET is considered the "workhorse" of the electronics industry.
A key advantage of a MOSFET is that it requires almost no input current to control the load current, when compared with bipolar junction transistors (BJTs). In an enhancement mode MOSFET, voltage applied to the gate terminal can increase the conductivity from the "normally off" state. In a depletion mode MOSFET, voltage applied at the gate can reduce the conductivity from the "normally on" state. MOSFETs are also capable of high scalability, with increasing miniaturization, and can be easily scaled down to smaller dimensions. They also have faster switching speed (ideal for digital signals), much smaller size, consume significantly less power, and allow much higher density (ideal for large-scale integration), compared to BJTs. MOSFETs are also cheaper and have relatively simple processing steps, resulting in high manufacturing yield.
MOSFETs can either be manufactured as part of MOS integrated circuit chips or as discrete MOSFET devices (such as a power MOSFET), and can take the form of single-gate or multi-gate transistors. Since MOSFETs can be made with either p-type or n-type semiconductors (PMOS or NMOS logic, respectively), complementary pairs of MOSFETs can be used to make switching circuits with very low power consumption: CMOS (Complementary MOS) logic.
The name "metal–oxide–semiconductor" (MOS) typically refers to a metal gate, oxide insulation, and semiconductor (typically silicon). However, the "metal" in the name MOSFET is sometimes a misnomer, because the gate material can also be a layer of polysilicon (polycrystalline silicon). Along with oxide, different dielectric materials can also be used with the aim of obtaining strong channels with smaller applied voltages. The MOS capacitor is also part of the MOSFET structure.

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  1. S

    Doubt about MOSFET Body Effect

    I found on my notes that for an NMOS with increase in Vsb voltage Body Effect increases, Vt also increases. But it is mentioned that with Vgs constant id increases which I feel is wrong since id is proportional to (Vgs - Vt)2. Hence I think id should decrease. Please show me the light :-)...
  2. S

    Why is the substrate terminal considered to have no effect on MOSFET operation?

    I will quote my textbook, and further I will explain what I do not understand. "Since, as we shall see shortly, the drain will always be at a positive potential relative to the source, the two pn junctions can effectively be cut off by connecting the substrate terminal to the source terminal...
  3. S

    MOSFET Push Pull Common Drain

    Hi everyone, I have this following problem (uploaded it in the pdf). I have made an attempt at solving it, but now i am just stuck. I have done question one, and have confirmed my answers in PSpice. my Id = 16uA, Vgs = 1.4V and Vds = 4.6V ( 3 + 8/5) and for p-channel the values are...
  4. P

    Zz.What is the theory behind Dual Gate MOSFETs and current controlled diodes?

    Hi, I want to know more about Dual Gate MOSFETs, i.e, their structure, working, advantages etc... Googling "Dual Gate MOSFET" didn't help at all! So please suggest some sources (online) where I can find more information on them. Also, where can I find more information on current...
  5. S

    Using MOSFET in Saturation (as a switch)

    I'm trying to learn about MOSFETs in order to use them as a switch. (note: this is not a homework/school question). On my breadboard, I have the following circuit hooked up: I vary the gate voltage using a potentiometer. Here is what confuses me: according to wikipedia, the MOSFET is in...
  6. genxium

    Calculate the Transconductance for a MOSFET, given W,L,Vg,Vd,u,Cox but not Vth

    Homework Statement This problem is given to me by programme inductor, and I just copy the whole description here so every given element is the same as what he/she gives. I'm confused with the given conditions. 1) What is the transconductance (gm) of a MOSFET biased at W=L=1um, uCox=0.01A/V^2...
  7. A

    Best Mosfet for PIC PWM Motor Control?

    Hi guys, I need to use the PWM output of a PIC microcontroller to switch a mosfet to drive a 9v brushed DC motor @ 115 mA. From what I've gathered, the mosfet should be logic level (fully on @ 3.33v since my Pic uses a 3.33v source). It needs to be able to handle...
  8. Y

    Common source MOSFET amplifier: voltage gain

    Hi everyone, I need to find the voltage gain for the following scheme: My book says I need to calculate 'id' first, which is: id = gm.ro.vin/(Rs+RL+ro+(gm.ro.Rs)) Then with the help of vout = -RL.id you get: Av = -gm.ro.RL/(Rs+RL+ro+(gm.ro.Rs)) So I get the last part, but I really have...
  9. L

    Understand Mosfets (NMOS & PMOS): Why the Contradiction?

    This is the way I understand Mosfets (NMOS) The source is grounded, drain is at some positive voltage V, gate is 0v initially. When gate voltage is increased and reaches Vth, electrons from source are attracted to the gate because of the E-field at the gate. When the electrons reach the...
  10. S

    MOSFET output current limited by Inductor Saturation current?

    So the MOSFET's output current in a simple buck converter with a PWm a driver and a MOSFET and LC circuit. The Inductor's saturation or Irms current limits the MOSFET's current output, correct? I heard that if it exceeds saturation current that the MOSFETs could be damaged. is this true...
  11. A

    Operation of a P channel MOSFET

    Why is it that a P channel MOSFET is always operated with Vds < 0 i.e source positive with respect to the drain? The book says that carrier always needs to flow from the source to the drain, but would the reverse operation harm the MOSFET in any way? Please explain. Thanks.
  12. S

    MOSFET Common Source Amplifier with Source Degeneration

    Why does the small signal voltage gain collapse as we increase the AC input voltage? Voltage gain = Av = (output voltage) / (input voltage). Both small signal (AC). When you draw the small signal circuit using T-Model, you can find that Av= (gm * Rd) / (1 + gm Rs) gm being the the...
  13. M

    MOSFET channel length modulation

    Hi there, I'm a bit beffudled by some of the workings of a MOSFET transistor. First of all, in the saturation region of operation, I understand that the inversion channel is "pinched off" as it approaches the drain. In other words the inversion channel thickness tapers off as it approaches the...
  14. C

    Maximizing MOSFET Switching Speed with High Voltage Pulse Techniques

    ok looking at datasheets for mostfet there is source/drain rise and fall time. does this only depend on the voltage rise fall time of the gate? so if you apply a voltage to the gate it will take a wile until the voltage of the actual gate reaches that of the applied voltage because of...
  15. F

    MOSFET Amplifier Configuration

    A Common-Source amplifier utilizes a MOSFET with µnCox = 400µA/V2, W/L = 10, and VA = 10V. It is biased at ID = 0.2 mA and uses RD = 6 kΩ. Find Rin, Avo and Ro. Also, if a load resistance of 10kΩ is connected to the output, what overall voltage gain Gv is realized? Now, what if a 0.2V peak...
  16. N

    BJT vs MOSFET Amplifiers: Understanding the Differences in Gain and Performance

    (This is not homework) Okay I'm writing a lab report and just writing some discussion. Also I am curious myself. I'm a junior in computer engineering. I want to discuss the differences between BJT and MOSFETs as amplifiers and anything like pros/cons for each. Also why is the BJT...
  17. R

    MOSFET (common source) current paths

    Why is the voltage gain of a common source MOSFET negative? I have a schematic below and I get the voltage gain to be -gm(ro||Rd). I say that is what I get, but the truth is that is what my professor said we should get. I do not understand why the Gain (Av) is negative. It will go a long...
  18. T

    Ok to use MOSFET body diode as flyback protection?

    Hello, I am designing a motor controller, and I was curious about the MOSFETs I'm using that have an internal body diode zener for Vds protection that is common in a lot of datasheets. I am switching the coils of the inductor with these MOSFETs, and I would like to know if its unwise to...
  19. R

    Voltage gain for Common Source Degenerative MOSFET

    Homework Statement We are going over gain, Gm, Ro, and Rin for MOSFETS. We are told to know and memorize certain equations and how to get these equations. In this particular problem we are told to find voltage gain by analyzing small signal. My professor gave this as the answer: (-gm*...
  20. W

    MOSFET vs Optocoupler: Which is Best for My needs?

    Hi all, I have a couple scenarios, and need to decide on which component best suits my need: I have two solenoids that I need to drive with 12V, based on a 5V uC signal. Simple on/off switch. I'm very new to using MOSFETs, so I'm not sure if you can drive 12V with a 5V signal (which would...
  21. T

    What are the functional differences between MOSFETs and JFETs?

    I am a beginner with respect to MOSFETs and have just begun studying them at college 2 weeks back. What I am actually struggling with is the basic functional differences between MOSFETs and JFETs. Two of them are- dielectric and substrate, both of which are there in MOSFETs. However, I...
  22. M

    Period 0<time<0.5 the MOSFET is OFF

    Hi all, I have been looking at this question and I am having trouble sketching the wave forms. I know that during the period 0<time<0.5 the MOSFET is OFF the diode is ON i = 4amps v =0 id=0 VDS=? Then during 0.5<time<1 period the MOSFET is ON the diode is OFF. i=0 v=...
  23. U

    Troubleshooting P-Channel MOSFET Switching in Electrical Power Systems

    Hi all, I'm using 7 P-channel mosfets to switch different loads using 1 microcontroller on an electrical power and distribution system. My circuit is as follows for each of the 7 Mosfets. Gate: through a resistor to ground on microcontroller and through a High value (600 kilaohms) Drain...
  24. M

    Anybody solve this mosfet transconductance problem

    anybody please solve this mosfet transconductance problem ... hello friends...i can't understand about this question totally...can anybody help me regarding this...its answer is option (C). .i have attached image for this question ,question no. 48... thanks...
  25. R

    P-channel Mosfet Design for switching of LED display as source

    Can anyone kindly help me in using the P channel mosfet as source for switching of LED display? As I've connected it like 16 Mosfets with common source giving 5V from diode b/w power supply, drain is for the16 rows of 32 LEDs. Gate is connected to the shift register. The problem is the...
  26. jegues

    Engineering Boolean expressions for larger MOSFET circuits

    Homework Statement See figure. Homework Equations The Attempt at a Solution I tried breaking the circuit down into a bunch of small logic gates but I'm having a tough time putting things back together into one big circuit. (One boolean expression) So far I've determined that...
  27. jegues

    Another Simple MOSFET Question

    Homework Statement See figure Homework Equations V = IR The Attempt at a Solution I have a few questions about notation before I begin with an attempt at the problem. Are there 2 voltage sources in this circuit diagram? One on the left, one ontop? Also it says that Vo = 5v...
  28. jegues

    Solve Simple MOSFET Question | Homework Equations & Tips

    Homework Statement See figure Homework Equations P=IV The Attempt at a Solution I need to clear something up before I even attempt the question. In the question is states the the R_{on} = 100 ohm is that the 100 ohm resistor I see in the circuit or is it literally inside the...
  29. N

    Switching MOSFET from MCU with VDS of +12v

    Hi all, I have been trying to get a MCU to send a +5v signal and in turn switch a N channel MOSFET with a VDS of +12v. I used a P channel BJT, tied the gate to +12v through a resistor, the top to +12v straight, and the output to the gate of the MOSFET. Then by grounding the gate, the...
  30. F

    Engineering Understanding the Equivalent Circuit Model for a MOSFET Circuit

    hi i have a mosfet circuit question since my prof. didn't explained much about the mosfet I am pre lost.. during the lecture..he show us two circuit diagram.. the mosfet circuit and this regular circuit.. and he ask us why the above equivalent circuit model is a valid representation of...
  31. C

    Finding MOSFET threshold voltage

    Homework Statement I have a MOSFET with a Si/SiO_{2} interface. I need to find the threshold voltage. I'd also like to know whether this puts the transistor in the "on" state or "off" state (I know this is simple; I'd just like a little verification). I know the values for \Phi_{ms}, Q_{i}...
  32. D

    Floating gate potential of a MOSFET

    Hi, First of all, I posted a similar question in [1] but I think it is the wrong forum and wrong question (maybe the reason why I get no answer...). I have to solve this (I think easy?) exercise: http://img705.imageshack.us/img705/8241/floatingmosfet.png I think (3.1) is easy and I solved...
  33. R

    Solving an n-channel MOSFET Homework Problem

    Homework Statement http://img195.imageshack.us/img195/3056/mosfetj.jpg An n-channel enhancement MOSFET for which Vt = 2 V and K = 100 uA/V2 is operated from a 9 V supply using a drain resistor RD of 60kohms and the feedback bias scheme depicted. What drain current results? What is VD What...
  34. G

    MOSFET Driver for Boost Converter Design: A Look into IRF8707PBF and MAX4420

    Dear Oracle, I'm designing a boost converter where the input is a current limited (~500mA) voltage source that varies from 3.5V to 6V. The output (load) varies from 7V to 9V. I intend to drive a MOSFET, IRF8707PBF [1] from IR, with a 250kHz PWM control signal via Maxim's driver IC MAX4420...
  35. M

    Discriminating Between MOSFET Drain & Source Terminals

    If the MOSFET is a symmetrical device, i.e the current can flow either from the drain to the source, or vice versa, depending on wheather you apply the voltage VDD to the drain or to the sourse, then, what is the point of "discrimination" between these two terminals? why not just mark the gate...
  36. S

    Why does a MOSFET heat up with slow rise/fall times at gate?

    I have a pulse width modulator that's composed of a triangular wave generator and a comparator. This outputs a pulse width modulated signal, whose duty cycle I set by changing the comparator's V- voltage. I have measured the rise and fall times of my output waveform and they are 27uS (for...
  37. D

    Troubleshooting Mosfet Lock on DC Motor Remotely

    I have a hobby project that involves controlling a dc motor remotely. I used a pwm circuit to control motor speed. I also have a rf circuit that sends a signal that can be used to turn the motor on/off. Both circuits work when used independently, that is the rf signal will turn a motor on/off...
  38. P

    MOSFET threshold voltage from characteristic curve

    Hi, I have Ids vs Vds characteristic curves for a MOSFET at Vgs = 2V and Vgs = 2.5V in both the linear (ohmic) and saturated regions. I wanted to try to figure out the threshold voltage for the MOSFET from this information only. I had two ideas of how to do this, but they differ by a factor...
  39. J

    Measuring MOSFET temperature w. sensor

    Hello All, This is a new thread related to my output stage setup. Only this time, rather than determining if something went wrong, I simply want to monitor the temperature of the MOSFET on the output stage. I have been doing a bit of research to see what kinds of sensors are available out there...
  40. J

    Detecting Shorted MOSFETs in Output Stage

    Hey Everyone, The title of this thread may be a bit ambiguous but what I am wondering is if there is a way to determine if a MOSFET in an output stage has shorted. The scenario I want to catch is the case where the FET drain is exposed to too much voltage (from a collapsing spike or some...
  41. S

    Using a MOSFET and comparator together as a switch

    Hello all, I was advised to use a MOSFET and a comparator together in a solar charger that I am building to stop the solar panel from charging the batteries when full. Seems logical, right? The only problem is I've never used MOSFETs or comparators, so I'm not really sure how to. I need a...
  42. A

    In what region (linear,saturation) does a MOSFET act as a variable resistor

    I'm trying to use a MOSFET as a voltage controlled resistor, and am under the impression that it acts this way when it is in the linear region. However, our VDS will be 2.5V-20V, and our VGS will be 3.6V-5V. That means our VDS will most likely be greater than our VGS - VT, which means it will...
  43. E

    What Are the Major Ratings and Acronyms for MOSFETs?

    Homework Statement List the major device ratings for a MOSFET. The Attempt at a Solution Simple question yeah? Data sheets list about 20 ratings for each device, and they don't explain the acronyms. VBRDSS = Vegemite BReaD Salad Sandwich? Can someone help me trim them...
  44. E

    MOSFET Device Ratings Overview

    What are the major device ratings for a MOSFET?
  45. A

    How to determine unCoxW/L value of a MOSFET?

    So I'm going to be using a MOSFET as a voltage controlled resistor, but to do so,i need to know the value of the unCoxW/L, also known as k is that something that's on datasheets? or is it generally the same for all MOSFETs? (this is the mosfet i plan on using..)...
  46. A

    Need help choosing a Power MOSFET

    I need help finding a MOSFET to buy that can handle max power of 30W, and that can achieve a resistance of 1-25ohms (using it as a voltage controlled resistor). Microsemi originally pointed me to these 2 of theirs: http://www.microsemi.com/catalog/part.asp?ID=21208...
  47. A

    Power MOSFET as a voltage controlled resistor

    We need to have a load resistance to match an internal resistance for peak power.. so we've decided to use a Power MOSFET as the varying load resistance, but my question is.. when I'm looking for which power MOSFET to buy, to use, what should I be looking for in terms of properties? This is...
  48. S

    Designing a Mosfet Switch for DAQ Simulation

    Hi Guys, I need to design something that will isolate our DAQ (Data Acquisition Card) for a basic simulation we need to do. I was thinking about designing a mosfet switch, where the DAQ sends some voltage to the gate above the threshold voltage to turn the MOSFET on and 0 V to turn it off...
  49. V

    Doubt about NMOS Transistor in Microcap: Varunag

    I was making a model of a NMOS transistor in Microcap. It was part of an assignment. I was supposed to make graph with changing value of V_{BS}. But after a certain value of V_{BS}, I got negative current. It came to my mind it is not possible. But, I couldn't think of a reason instantly, as to...
  50. E

    Help with mosfet and resistors

    Hi I am trying to figure out how to determine the size of the resistors I need to put in line of the gate wire to turn on and off the mosfet in a circuit for an airsoft gun I have several mosfets laying around the 1 I want to use is a 50a 100v that said what size resistors do I need to put in...
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