N Channel Depletion Mosfet

A N-Channel MOSFET is a type of MOSFET in which the channel of the MOSFET is composed of a majority of electrons as current carriers. When the MOSFET is activated and is on, the majority of the current flowing are electrons moving through the channel.

As opposed to the enhancement-mode MOSFETs, these depletion-mode devices operate in a ‘normally-on’ mode, requiring zero turn-on voltage at the gate terminal. With blocking voltages up to 1700V and low drain-to-source resistances they provide simplified control and reduced power dissipation in systems that are continuously “on. The DN2470 is a low threshold depletion-mode (normally-on) transistor utilizing an advanced vertical DMOS structure and well-proven silicon-gate manufacturing process. N-Channel Depletion MOSFET A voltage value with the negative polarity is applied at the gate. The electrons present in it get repelled and settles down at the dielectric layer. This is the reason due to which the depletion of the charge carriers occurs and results in the reduction of the overall conductance.

D-MOSFET “Depletion MOSFET” Depletion MOSFET or D-MOSFET is a type of MOSFET where the channel is constructed during the process of manufacturing. Therefore, the D-MOSFET can conduct between its drain and gate when the V GS = 0 volts. Therefore, D-MOSFET is also known as normally ON transistor. N-Channel Depletion MOSFET are available at Mouser Electronics. Mouser offers inventory, pricing, & datasheets for N-Channel Depletion MOSFET.

This is in contrast to the other type of MOSFET, which are P-Channel MOSFETs, in which the majority ofcurrent carriers are holes.

Before, we go over the construction of N-Channel MOSFETs, we must go over the 2 types that exist. There are 2 types of N-Channel MOSFETs, enhancement-type MOSFETs and depletion-type MOSFETs.

A depletion-type MOSFET is normally on (maximum current flows from drain to source) when no differencein voltage exists betweeen the gate and source terminals. However, if a voltage is applied to its gate lead, the drain-source channel becomes more resistive, until the gate voltage is so high, the transistor completely shuts off. An enhancement-type MOSFET is the opposite. It is normally off when the gate-source voltage is 0(VGS=0). However, if a voltage is applied to its gate lead, the drain-source channel becomesless resistive.

In this article, we will go over how both N-Channel enhancement-type and depletion-type are constructed and operate.

How N-Channel MOSFETs Are Constructed Internally


An N-Channel MOSFET is made up of an N channel, which is a channel composed of a majority of electron current carriers. The gate terminals are made up of P material. Depending on the voltage quantity and type (negative or positive)determines how the transistor operates whether it turns on or off.

How an N-Channel Enhancement type MOSFET Works


How to Turn on a N-Channel Enhancement type MOSFET

Mosfet

To turn on a N-Channel Enhancement-type MOSFET, apply a sufficient positive voltage VDD to the drain of the transistorand a sufficient positive voltage to the gate of the transistor. This will allow a current to flow through the drain-source channel.

So with a sufficient positive voltage, VDD, and sufficient positive voltage applied to the gate, the N-Channel Enhancement-type MOSFET is fully functional and is in the 'ON' operation.

How to Turn Off an N-Channel Enhancement type MOSFET

To turn off an N-channel Enhancement MOSFET, there are 2 steps you can take. You can either cut off the bias positivevoltage, VDD, that powers the drain. Or you can turn off the positive voltagegoing to the gate of the transistor.

How a N-Channel Depletion-type MOSFET Works


How to Turn on an N-Channel Depletion-Type MOSFET

To turn on an N-channel Depletion-type MOSFET, to allow for maximum current flow from drain to source, the gate voltage should be set to 0V. When the gate voltage is at 0V, the transistor conducts the maximum amount of current and is in the active ON region. To reducethe amount of current that flows from the drain to source, we apply a negative voltage to the gate of the MOSFET. As the negative voltage increases (gets more negative), less and less current conducts across from the drain to the source. Once the voltage at the gate reaches a certain point, all current ceases to flowfrom the drain to the source.

So with a sufficient positive voltage, VDD, and no voltage (0V) applied to the base, the N-channel JFET is in maximum operation and has the largest current. As we increase the negative voltage, current flows gets reduced until the voltage is so high (negative), that all current flow is stopped.

How to Turn Off an N-Channel Depletion-type MOSFET

To turn off the N-channel Depletion-type MOSFET, there are 2 steps you can take. You can either cut off the bias positivevoltage, VDD, that powers the drain. Or you can apply sufficient negative voltage to the gate. When sufficientvoltage is applied to the gate, the drain current is stopped.

MOSFET transistors are used for both switching and amplifying applications. MOSFETs are perhaps the most popular transistors used today. Their high input impedance makes them draw very little input current, they are easy to make, can be made very small, and consume very little power.

N Channel Depletion Mosfet

Related Resources

How to Build an N-Channel MOSFET Switch Circuit
P Channel MOSFET Basics
N Channel JFET Basics
P Channel JFET Basics
Types of Transistors


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Simulation result for formation of inversion channel (electron density) and attainment of threshold voltage (IV) in a nanowire MOSFET. Note that the threshold voltage for this device lies around 0.45 V.

The threshold voltage, commonly abbreviated as Vth, of a field-effect transistor (FET) is the minimum gate-to-source voltage VGS (th) that is needed to create a conducting path between the source and drain terminals. It is an important scaling factor to maintain power efficiency.

When referring to a junction field-effect transistor (JFET), the threshold voltage is often called pinch-off voltage instead.[1][2] This is somewhat confusing since pinch off applied to insulated-gate field-effect transistor (IGFET) refers to the channel pinching that leads to current saturation behaviour under high source–drain bias, even though the current is never off. Unlike pinch off, the term threshold voltage is unambiguous and refers to the same concept in any field-effect transistor.

Basic principles[edit]

In n-channelenhancement-mode devices, a conductive channel does not exist naturally within the transistor, and a positive gate-to-source voltage is necessary to create one such. The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this forms a region with no mobile carriers called a depletion region, and the voltage at which this occurs is the threshold voltage of the FET. Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain; this process is called inversion. The reverse is true for the p-channel 'enhancement-mode' MOS transistor. When VGS = 0 the device is “OFF” and the channel is open / non-conducting. The application of a negative (-ve) gate voltage to the p-type 'enhancement-mode' MOSFET enhances the channels conductivity turning it “ON”.

In contrast, n-channel depletion-mode devices have a conductive channel naturally existing within the transistor. Accordingly, the term threshold voltage does not readily apply to turning such devices on, but is used instead to denote the voltage level at which the channel is wide enough to allow electrons to flow easily. This ease-of-flow threshold also applies to p-channeldepletion-mode devices, in which a negative voltage from gate to body/source creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions.

For the n-channel depletion MOS transistor, a negative gate-source voltage, -VGS will deplete (hence its name) the conductive channel of its free electrons switching the transistor “OFF”. Likewise for a p-channel 'depletion-mode' MOS transistor a positive gate-source voltage, +VGS will deplete the channel of its free holes turning it “OFF”.

In wide planar transistors the threshold voltage is essentially independent of the drain–source voltage and is therefore a well defined characteristic, however it is less clear in modern nanometer-sized MOSFETs due to drain-induced barrier lowering.

Depletion region of an enhancement-mode nMOSFET biased below the threshold
Depletion region of an enhancement-mode nMOSFET biased above the threshold with channel formed

In the figures, the source (left side) and drain (right side) are labeled n+ to indicate heavily doped (blue) n-regions. The depletion layer dopant is labeled NA to indicate that the ions in the (pink) depletion layer are negatively charged and there are very few holes. In the (red) bulk the number of holes p = NA making the bulk charge neutral.

If the gate voltage is below the threshold voltage (left figure), the 'enhancement-mode' transistor is turned off and ideally there is no current from the drain to the source of the transistor. In fact, there is a current even for gate biases below the threshold (subthreshold leakage) current, although it is small and varies exponentially with gate bias.

Depletion

If the gate voltage is above the threshold voltage (right figure), the 'enhancement-mode' transistor is turned on, due to there being many electrons in the channel at the oxide-silicon interface, creating a low-resistance channel where charge can flow from drain to source. For voltages significantly above the threshold, this situation is called strong inversion. The channel is tapered when VD > 0 because the voltage drop due to the current in the resistive channel reduces the oxide field supporting the channel as the drain is approached.

Body effect[edit]

Depletion Transistor

The body effect is the change in the threshold voltage by an amount approximately equal to the change in the source-bulk voltage, VSB{displaystyle V_{SB}}, because the body influences the threshold voltage (when it is not tied to the source). It can be thought of as a second gate, and is sometimes referred to as the back gate,and accordingly the body effect is sometimes called the back-gate effect.[3]

For an enhancement-mode nMOS MOSFET, the body effect upon threshold voltage is computed according to the Shichman–Hodges model,[4] which is accurate for older process nodes,[clarification needed] using the following equation:

VTN=VTO+γ(VSB+2ϕF2ϕF){displaystyle V_{TN}=V_{TO}+gamma left({sqrt {left V_{SB}+2phi _{F}right }}-{sqrt {left 2phi _{F}right }}right)}

where VTN{displaystyle V_{TN}} is the threshold voltage when substrate bias is present, VSB{displaystyle V_{SB}} is the source-to-body substrate bias, 2ϕF{displaystyle 2phi _{F}} is the surface potential, and VTO{displaystyle V_{TO}} is threshold voltage for zero substrate bias, γ=(tox/ϵox)2qϵSiNA{displaystyle gamma =left(t_{ox}/epsilon _{ox}right){sqrt {2qepsilon _{text{Si}}N_{A}}}} is the body effect parameter, tox{displaystyle t_{ox}} is oxide thickness, ϵox{displaystyle epsilon _{ox}} is oxide permittivity, ϵSi{displaystyle epsilon _{text{Si}}} is the permittivity of silicon, NA{displaystyle N_{A}} is a doping concentration, q{displaystyle q} is elementary charge.

Dependence on oxide thickness[edit]

In a given technology node, such as the 90-nm CMOS process, the threshold voltage depends on the choice of oxide and on oxide thickness. Using the body formulas above, VTN{displaystyle V_{TN}} is directly proportional to γ{displaystyle gamma }, and tOX{displaystyle t_{OX}}, which is the parameter for oxide thickness.

Depletion

Thus, the thinner the oxide thickness, the lower the threshold voltage. Although this may seem to be an improvement, it is not without cost; because the thinner the oxide thickness, the higher the subthreshold leakage current through the device will be. Consequently, the design specification for 90-nm gate-oxide thickness was set at 1 nm to control the leakage current.[5] This kind of tunneling, called Fowler-Nordheim Tunneling.[6]

N channel mosfet equations
Ifn=C1WL(Eox)2eE0Eox{displaystyle I_{fn}=C_{1}WL(E_{ox})^{2}e^{-{frac {E_{0}}{E_{ox}}}}}

where C1{displaystyle C_{1}} and E0{displaystyle E_{0}} are constants and Eox{displaystyle E_{ox}} is the electric field across the gate oxide.

Before scaling the design features down to 90 nm, a dual-oxide approach for creating the oxide thickness was a common solution to this issue. With a 90 nm process technology, a triple-oxide approach has been adopted in some cases.[7] One standard thin oxide is used for most transistors, another for I/O driver cells, and a third for memory-and-pass transistor cells. These differences are based purely on the characteristics of oxide thickness on threshold voltage of CMOS technologies.

Temperature dependence[edit]

As with the case of oxide thickness affecting threshold voltage, temperature has an effect on the threshold voltage of a CMOS device. Expanding on part of the equation in the body effect section

ϕF=(kTq)ln(NAni){displaystyle phi _{F}=left({frac {kT}{q}}right)ln {left({frac {N_{A}}{n_{i}}}right)}}

where ϕF{displaystyle phi _{F}} is half the contact potential, k{displaystyle k} is Boltzmann's constant, T{displaystyle T} is temperature, q{displaystyle q} is the elementary charge, NA{displaystyle N_{A}} is a doping parameter and ni{displaystyle n_{i}} is the intrinsic doping parameter for the substrate.

We see that the surface potential has a direct relationship with the temperature. Looking above, that the threshold voltage does not have a direct relationship but is not independent of the effects. This variation is typically between −4 mV/K and −2 mV/K depending on doping level.[8] For a change of 30 °C this results in significant variation from the 500 mV design parameter commonly used for the 90-nm technology node.

Dependence on random dopant fluctuation[edit]

N Channel Depletion Mosfet Rate

Random dopant fluctuation (RDF) is a form of process variation resulting from variation in the implanted impurity concentration. In MOSFET transistors, RDF in the channel region can alter the transistor's properties, especially threshold voltage. In newer process technologies RDF has a larger effect because the total number of dopants is fewer.[9]

Research works are being carried out in order to suppress the dopant fluctuation which leads to the variation of threshold voltage between devices undergoing same manufacturing process.[10]

See also[edit]

References[edit]

N Channel Depletion Mosfet Datasheet

  1. ^'Junction Field Effect Transistor (JFET)'(PDF). ETEE3212 Lecture Notes. This is called the threshold, or pinch-off, voltage and occurs at vGS=VGS(OFF).
  2. ^Sedra, Adel S.; Smith, Kenneth C. '5.11 THE JUNCTION FIELD-EFFECT TRANSISTOR (JFET)'(PDF). Microelectronic Circuits. For JFETs the threshold voltage is called the pinch-off voltage and is denoted VP.
  3. ^Marco Delaurenti, PhD dissertation, Design and optimization techniques of high-speed VLSI circuits (1999))Archived 2014-11-10 at the Wayback Machine
  4. ^NanoDotTek Report NDT14-08-2007, 12 August 2007
  5. ^Sugii, Watanabe and Sugatani. Transistor Design for 90-nm Generation and Beyond. (2002)
  6. ^S. M. Sze, Physics of Semiconductor Devices, Second Edition, New York: Wiley and Sons, 1981, pp. 496–504.
  7. ^Anil Telikepalli, Xilinx Inc, Power considerations in designing with 90 nm FPGAs (2005))[1]
  8. ^Weste and Eshraghian, Principles of CMOS VLSI Design : a systems perspective, Second Edition, (1993) pp.48 ISBN0-201-53376-6
  9. ^Asenov, A. Huang,Random dopant induced threshold voltage lowering and fluctuations in sub-0.1 μm MOSFET's: A 3-D “atomistic” simulation study, Electron Devices, IEEE Transactions, 45 , Issue: 12
  10. ^Asenov, A. Huang,Suppression of random dopant-induced threshold voltage fluctuations in sub-0.1-μm MOSFET's with epitaxial and δ-doped channels, Electron Devices, IEEE Transactions, 46, Issue: 8

External links[edit]

  • Online lecture on: Threshold Voltage and MOSFET Capacitances by Dr. Lundstrom

N Channel Depletion Mosfet Symbol

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