Metal-Oxide-Semiconductor Field Effect Transistors

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Presentation transcript:

Metal-Oxide-Semiconductor Field Effect Transistors MOSFETs Metal-Oxide-Semiconductor Field Effect Transistors

Classes of Field Effect Transistors Metal-Oxide-Semiconductor Field Effect Transistor Which will be the type that we will study in this course. Metal-Semiconductor Field Effect Transistor MESFET Junction Field Effect Transistor JFET High Electron Mobility Transistor or Modulation Doped Field Effect Transistor HEMT or MODFET Fast Reverse/Fast Recovery Epitaxial Diode FREDFET DNA Field Effect Transistor The conduction path is through a strand of DNA

Field Effect Transistors The conductivity (or resistivity) of the path between two contacts, the source and the drain, is altered by the voltage applied to the gate. Device is also known as a voltage controlled resistor.

Types of MOSFETS n-channel Enhancement Mode (nMOSFET) p-channel (pMOSFET) Depletion Mode

Cross-Sectional View of n channel planar Enhancement Mode Transistor

p channel Enhancement Mode Transistor

n channel Depletion Mode Transistor

p channel Depletion Mode Transistor

Symbols for n channel Enhancement Mode MOSFET VGS ≥ 0V, VDS ≥ 0V VTN is positive

Symbols for p channel Enhancement Mode MOSFET VGS ≤ 0V, VDS ≤ 0V VTP is negative

Symbols for n channel Depletion Mode MOSFET

Symbols for p channel Depletion Mode MOSFET

PSpice MOSFET Symbols The IRF150 is an nMOS and the IRF9140 is a pMOS. Both are enhancement mode transistors. The body terminal is connected to the source terminal on the FET. “M” is used to denote that the device is a MOSFET.

MOS Capacitor

MOS Capacitor Under Bias: Electric Field and Charge Parallel plate capacitor Accumulation Positive gate bias Electrons attracted to gate

Negative gate bias: Holes attracted to gate Depletion Inversion

MOS Capacitor: p-type semiconductor Accumulation Depletion Inversion

Threshold Voltage The gate voltage that causes the concentration of electrons immediately under the gate oxide is equal to the concentration of holes is called the threshold voltage. Enhancement mode FETs NMOS VG = VTN When enough electrons have been attracted to the oxide-semiconductor interface to create a path for current to flow between the source and drain. PMOS VG = VTP When holes have been attracted to the oxide-semiconductor interface to create a path for current to flow between the source and drain. Depletion mode FETs When holes have been attracted to the oxide-semiconductor interface to stop current from flowing between the source and drain. When electrons have been attracted to the oxide-semiconductor interface to stop current from flowing between the source and drain.

Capacitance http://ecee.colorado.edu/~bart/book/

MOSFETs Enhancement mode Depletion mode Also known as Normally Off transistors. A voltage must be applied to the gate of the transistor, at least equal to the threshold voltage, to create a conduction path between the source and the drain of the transistor before current can flow between the source and drain. Also known as Normally On transistors. A voltage must be applied to the gate of the transistor, at least equal to the threshold voltage, to destroy a conduction path between the source and the drain of the transistor to prevent current from flowing between the source and drain.

Before electron inversion layer is formed After electron inversion layer is formed

Family of ID Versus VDS Curves: Enhancement-Mode nMOSFET Triode/ Nonsaturation VDS < VGS – VTN VDS > VGS – VTN Pinch-off/Saturation Enhancement Mode FETs are also known as Normally Off devices. Voltage must be applied to the Gate to allow current to flow between the Source and the Drain. Cut-off VGS < VTN

Family of iD Versus vDS Curves: Depletion-Mode nMOSFET Depletion Mode FETs are also known as Normally On devices. Voltage must be applied to the Gate to stop current from flowing between the Source and the Drain. Assuming that VTN < -1V

For this discussion I am going to emphasize the operation and applications of n channel enhancement mode FETs

Piecewise Model Cut-off Region VGS < VTN VTN is positive ID = 0 mA

Piecewise Model Nonsaturation/Triode Region VGS > VTN VDS < VGS – VTN ID ≤ 0 mA VTN is positive

Piecewise Model Saturation/Pinch-off Region VGS > VTN VDS > VGS – VTN ID ≤ 0 mA VTN is positive

Summary of I-V Relationships Region NMOS Nonsaturation/ Triode VDS < VDS(sat) Saturation/ Pinch-off VDS > VDS(sat) Transition between triode and pinch-off VDS(sat) = VGS - VTN Enhancement Mode VTN > 0 V, ID ≥ 0 mA, ID = IS, IG = 0 mA For Enhancement Mode FETs

Questions To increase the drain current ID at a particular VDS and VGS, should you use a MOSFET with a larger or smaller W/L ratio? Compare the operation of two FETs, where MOS #1 has a smaller VTN than MOS #2. Sketch the differences on a graph of ID-VDS. The microelectronics industry is working to decrease the channel length L. If W is held constant, how will: the capacitance between the gate and the channel change? the time it takes for an electron to move from the source to the drain be altered? the value of VTN change? this modify RDSon for a particular set of VDS and VGS? The microelectronics industry is also working to decrease the thickness of the gate oxide TOX and is researching high K and low K dielectrics to replace silicon dioxide as the gate dielectric? If TOX decreases, how will the capacitance between the gate and channel change? Should a low K or high K dielectric be used to increase the capacitance?