Introduction to MOSFETs and Doping

Introduction to MOSFETs

• MOSFETs are field-effect transistors
• Part of analog circuit design (EST 160)

What is Doping?

• Doping is intentionally adding impurities to a semiconductor to change its electrical properties
• Increases electron or hole concentration
• Dopants (impurity) are column III or V elements
• Phosphorus (P), Arsenic (As), Antimony (Sb) are common donors (add more electrons)
• Boron (B), Gallium (Ga), Indium (In), Aluminum (Al) are common acceptors (add more holes)

Mobility

• Electron mobility describes how quickly an electron moves through a metal or semiconductor when pulled by an electric field
• Hole mobility is an analogous quantity for holes
• Carrier mobility refers to both electron and hole mobility
• The difference between electrons and holes is energy and (therefore) velocity
• Effective mass results from electron interaction with the lattice (phonons)
• Hole velocity is smaller and thus a hole spends more time interacting with phonons
• Hole mobility is significantly lower than electron mobility due to higher effective mass

Transistors

• MOSFETs are a type of Field Effect Transistor (FET)
• MOSFETs categorized under depletion and enhancement types
• n channel MOSFET and p channel MOSFET

Depletion MOSFET vs. Enhancement MOSFET

• Depletion MOSFETs are "normally on"
• Enhancement MOSFETs are "normally off"
• Diagrams illustrate and explain the differences

MOSFET Symbols

• Circuit symbols for NMOS and PMOS transistors are illustrated

MOSFET as a Switch

• MOSFETs can be operated as switches
• Connecting/isolating source and drain depends on the gate voltage
• High gate voltage = connected source and drain
• Low gate voltage = isolated source and drain

MOSFET Structure

• NMOS fabricated on p-type substrate
• Regions: source, drain, gate, substrate (bulk, body)
• Poly-silicon gate, Silicon-dioxide (SiO2) oxide layer
• The length (L) and width (W) of the gate affect the MOSFET behavior
• Substrate potential influences device characteristics

MOSFET Structure

• In NMOS operation, the substrate is connected to the most negative supply
• An ohmic p+ region provides the substrate connection
• Complementary MOS (CMOS) technologies use NMOS and PMOS transistors on the same wafer

MOSFET Structure

• PMOS transistors can have independent n-wells
• Note the n-well which is part of the structure
• The flexibility of PFETs useful in some analog circuits

Threshold Voltage: VTH

• For NMOS, the threshold voltage (VTH) is the gate voltage at which the device begins to conduct current
• The width of the depletion region and the potential at the oxide-silicon interface increase with increasing gate voltage
• Electrons flow from the source to the drain creating a channel
• This channel is also known as the "inversion layer"
• Threshold voltage VTH is the gate voltage when the inversion layer starts to develop

MOS I/V Characteristics

• Diagrams illustrate the formation of depletion and inversion regions in response to changing gate voltages (Vgs)

Threshold Voltage: VTH

• In PMOS, the behavior is similar but with inverted polarities.

MOSFET Operating Regions

• Three regions of operation: cut-off, linear, and saturation
• Conditions and equations that characterize the regions of operation are given
• The equations and graphs illustrate the relationships between parameters, such as Vgs, Vds and Id

Triode Region

• ID is constant along the channel
• The graph shows how the current capability of the device increases with VGS
• The peak of current occurs at Vds = Vgs – Vth
• The device operates in the triode region if Vds ≤ Vgs-Vth

Deep Triode Region

• ID is linear function of Vds
• Linear operation if Vds < 2(Vgs – Vth)
• MOSFET can behave like a resistor and is a controlled resistor

Saturation Region

• ID is relatively constant in the saturation region
• The inversion layer stops at x < L, and the channel is pinched off
• Vds increases, Vgs remains constant
• Condition for saturation is VDS ≥ (VGS-VTH); the inversion layer is pinched off

Saturation Region

• The presence of pinch-off is described: drain current in saturation region
• The diagrams illustrates pinch off behavior illustrating the region around the drain

Saturation Region

• VGS and VGD conditions create channels for NMOS transistors

Saturation Region

• The saturated MOSFET can be used as a current source
• Current flows from drain to source, only one terminal of the source is floating

VDS-VGS Plane: Regions of Operation

• A graph illustrates regions of operation based on VGS and VDS
• Regions: off, saturation, and triode region
• Line given by Vds =Vgs – Vth

MOS Transconductance

• Transconductance (gm) measures how effectively the device converts a voltage into current
• gm is given by gm = ID/(VGS – VTH)
• larger gm, the more sensitive the device

MOS Transconductance

• Illustrated graph of gm as a function of parameter
• The parameters can remain constant whilst one parameter changes

Second-Order Effects: Body Effect

• The body effect (back-gate effect) describes a change in the MOSFET's threshold voltage (VTH) due to changes in the body voltage (VB)
• Diagrams show the variation of depletion region charge with body voltage

Second-Order Effects: Channel Length Modulation

• Channel length modulation is a phenomenon where the effective channel length of a MOSFET shortens with increasing drain-source voltage (VDS).
• Resulting in non-zero slope in the ID/VDS characteristic, indicating that the Current (ID) changes in response to VDS
• Finite saturation region slope resulting from channel length modulation

Second-Order Effects: Channel Length Modulation

• Channel length modulation does not occur in the triode region
• The channel continuously stretches from the source to the drain without pinch-off
• There is no modulation of channel length in the triode region

Second-Order Effects: Subthreshold Conduction

• Subthreshold conduction occurs when the gate-source voltage (VGS) is less than the threshold voltage (VTH)
• In this weak inversion region, current still flows through the MOSFET, but it exhibits an exponential dependence on VGS
• Current flows, even if VGS is less than VTH

Voltage Limitations

• MOSFETs are susceptible to high gate-source and drain-source voltages
• High gate voltages can cause gate oxide breakdown
• Short-channel devices have punchthrough, causing very large drain current
• Voltage difference exceeding a given value will damage the MOSFET over time

Know More

• Provide links to relevant videos for further learning (MOSFET workings, semiconductor industry videos)

What to Learn Next

• Open ended section indicating further learning opportunities

Reminder

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Thank You for Listening

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