Introduction to MOSFETs and Doping (EST 160)

Introduction to MOSFETs

• MOSFETs are a type of field-effect transistor (FET)
• Analog circuits design (EST 160)
• Prepared by Engr. Rochelle M. Sabarillo

What is Doping?

• Doping is adding impurities to a semiconductor to change its electrical properties
• Used to increase electron or hole concentration
• Dopants are impurity elements
• N-type dopants (column V elements) add more electrons
• P-type dopants (column III elements) add more holes

Mobility

• Electron mobility and hole mobility characterize how quickly charge carriers move through a semiconductor under an electric field
• The only difference between electrons and holes is velocity and thus, energy
• Effective mass results from interactions with lattice and phonons
• Hole mobility is lower than electron mobility

Transistors

• Transistors are classified into BJTs, FETs, and MOSFETs
• FETs are further divided into JFETs and MOSFETs
• MOSFETs are sub-categorized into depletion and enhancement MOSFETs (n-channel, p-channel)

Depletion MOSFET vs Enhancement MOSFET

• Depletion MOSFETs are normally on, with a channel present
• Enhancement MOSFETs are normally off, needing a certain gate voltage to turn on a channel for current flow

MOSFET Symbols

• Circuit symbols are provided for NMOS and PMOS transistors

MOSFET as a Switch

• MOSFET can operate as a switch
• When the gate voltage is high, the source and drain connect
• If the gate voltage is low, the source and drain isolate

MOSFET Structure

• MOSFETs are fabricated on p-type substrate with heavily doped n regions for source drain
• Gate is constructed using polysilicon (a thin layer of silicon dioxide (SiO2) isolates gate from the substrate.)

MOSFET Structure

• Substrate potential greatly influences MOSFET characteristics
• For NMOS, the substrate is usually connected to the most negative supply voltage in the circuit.

MOSFET Structure

• In CMOS technology, both NMOS and PMOS transistors are fabricated on the same wafer

MOSFET Structure (cont.)

• PMOS transistors can be placed in a "local substrate" called an n-well

MOSFET Structure (cont.)

• Each PFET can have an independent n-well

Threshold Voltage: V_TH

• Definition and characteristics of threshold voltage (V_TH) for NMOS and PMOS
• When the gate voltage increases, the threshold voltage increases
• When the interface potential reaches a sufficiently positive value, electrons flow to the drain and the transistor turns on (channel is formed)
• The value of the gate voltage(V_GS) where channel is formed is the threshold voltage for the transistor.

MOS I/V Characteristics

• Graph of MOS current-voltage characteristics
• The current rises quickly as voltage increases; a graph (V_DS vs I_D) illustrating this effect

Threshold Voltage: V_TH (PMOS)

• Similar to NMOS (but reverse polarity)
• The turn-on phenomenon for PMOS devices

MOSFET Type

• Table comparing/contrasting the different NMOS and PMOS enhancement and depletion type characteristics.

MOSFET Operating Regions

• Operating regions (cut-off, linear, saturation)

Triode Region

Deep Triode Region

• Linear operation in deep triode region.

Saturation Region

• Description of saturation region behavior in MOSFETs

Saturation Region (cont.)

• How the device conducts current when pinch-off occurs.

Saturation Region (cont.)

• Description of NMOS/PMOS behavior in saturation with pinch-off

Saturation Region (cont.)

• MOSFETs operating as current sources

V_DS-V_GS Plane: Regions of Operation

• Graph illustrating operating regions of NMOS on a Vds-Vgs plane

MOS Transconductance

• Definition and calculation of transconductance (gm)
• Transconductance in various MOSFET regions

MOS Transconductance (cont.)

• Graph illustrating gm behavior

Second-Order Effects: Body Effect

• Description of body effect

Second-Order Effects: Channel Length Modulation

• Length of channel gradually decreases with decrease in V_GS ,
• Modulation coefficient equation.

Second-Order Effects: Channel Length Modulation (cont.)

• Whether channel-length modulation occurs in the triode region

Second-Order Effects: Subthreshold Conduction

• Description of subthreshold conduction
• How to define threshold voltage in subthreshold conduction

Voltage Limitations

• Effect of high terminal voltage differences on MOSFETs

Know More

• Links for videos about MOSFETs

What to Learn Next

Reminder

• Prepare for oral participation

Thank You for Listening

This quiz covers the fundamentals of MOSFETs, including their classification as field-effect transistors, the concept of doping in semiconductors, and the importance of mobility. Dive into the electronic properties that define how charge carriers behave and how transistors are categorized. Perfect for students in Analog Circuit Design (EST 160).