Non-Linear Two Port Devices

• Module covers Non-Linear Two Port Devices

Basic Concepts

• Transistors are non-linear two port devices
• Transistor types are compared

MOSFET Operation and Response

• Device structure and operation outlined
• Depletion MOSFET vs. Enhancement MOSFET are covered
• MOSFET Characteristics explained

MOSFET Models

• Analysis of MOSFET Circuits is performed
• Focus on MOSFET Based Amplifier Circuits
• Includes Small Signal Model
• Includes High Frequency Model

One-Port and Two-Port Components

• A two terminal electrical network in which current enters through one terminal and leaves through another terminal is a one-port component.
• Resistors, Inductors, and Capacitors are examples of linear one-port networks.
• Diodes are an example of nonlinear one-port networks.
• A two port electrical network in which current enters through one port and leaves through another port is a two-port component.
• Transistors, Transformers, Op-Amps, Amplifiers, and Transmission lines (power or communication) are examples of two-port networks.

Examples of Two-Port Circuits

• Transformers as Two-port Circuit
• Transistors as Two-port Circuit
• Op-Amps as Two-port Circuit
• Amplifiers as Two-port Circuit
• Transmission Lines as Two-port Circuit

Diodes vs. Transistors

• Illustrates basic functionality of a diode
• Outlines the configuration of a BJT transistor
• Depicts the configuration of a FET transistor

Transistor Types

• BJT
  • NPN
  • PNP
• Field effect transistor
  • Junction FET
    • Depletion Mode
      • N-Channel
      • P-Channel
  • Metal Oxide Semiconductor FET
    • Depletion Mode
      • N-Channel
      • P-Channel
    • Enhancement Mode
      • N-Channel
      • P-Channel

MOSFET vs BJT vs IGBT

• MOSFET: Metal Oxide Semiconductor Field Effect Transistor
• BJT: Bipolar Junction Transistor
• IGBT: Insulated Gate Bipolar Transistor

Comparison of Transistors (BJT, MOSFET, IGBT)

Benefits and Applications of MOSFETs

• Preferred over BJTs offering higher switching speed, greater efficiency, and lower power loss.
• Used for switching and amplifying signals in electronic circuits/devices.
• Applications:
  • Switching: Fast and low-power switches in digital circuits.
  • Amplification: Used in audio and RF amplifiers, particularly for high frequency amplifiers.
  • Logic Circuits: Used in CMOS for building logic gates and high density VLSI circuits.
  • Power Electronics: Used in power supply regulation circuits and converters like SMPS and DC Buck/Boost converters.
  • Microprocessors & Memory: Core of CPUs, GPUs, and memory chips.
  • Signal Processing: Used in ADCs and DACs.
  • RF & Communication: Used in high-frequency circuits.
  • Protection: Used as electronic relays for over-voltage protection.
  • Motor Control (Motor Drive): Controls DC motors in robotics and automation.
  • Automotive: Used in EVs, ignition, and control systems.

MOSFETs

• MOSFET is a metal oxide semiconductor field effect transistor
• Three terminal device (gate, source, drain)
• Voltage between gate and source controls the current through the drain

NMOS

• N-channel enhancement-mode MOSFET cross-section
• P-type substrate is used, with two heavily doped n-type regions
• A thin, electrically insulating, oxide layer is grown over the substrate surface between the two n-type regions.
• Layers of metallization or poly-silicon allow connection to external conductors.
• Applying sufficient positive voltage from gate to source induces an n-type channel in the p-type body.
• The voltage to form a conducting channel is the threshold voltage.
• Additional positive gate-source voltage increases mobile negative charge density.
• Current typically flows from the drain to the source in n-channel FET applications.
• The channel's mobile negative charge density is controlled by an electric field from the gate to source voltage.
• The gate and channel act as plates, with the oxide as an insulating dielectric, working as a parallel plate capacitor.

Operation of MOSFET

• n-channel Enhancement-type MOSFET is highlighted: Vgs < Vthreshold shows 'capacitor like operation"
• p-channel Enhancement-type MOSFET is highlighted: Vgs = 0V shows Implanted p-channel

Important Terminology

• Threshold Voltage: Minimum voltage to switch on the MOSFET.
• Saturation Region: Area where the drain current maintains a constant value
• Channel & Inversion layer:
  • Electric field near gate attracts electrons near oxide and creates the channel.
  • Electron accumulation near the oxide forms an inversion layer.
  • Electron density near the oxide determines channel current; a larger gate-source voltage means a larger electron density at the gate.

Enhancement Mode vs. Depletion Mode

• Depletion Type MOSFET: A gate voltage must be applied to deplete the channel region in order to turn off the transistor
• Enhancement Type MOSFET: Conduction between source and drain regions is enhanced by applying a gate voltage
• Enhancement Type (normally OFF device): a voltage must be applied to the gate to ON the MOSFET
• Depletion Type (normally ON device): a voltage must be applied to the gate to OFF the MOSFET

Why N-Channel Enhancement Mode is Preferred?

MOSFET Characteristics

• Output Characteristics/Current-Voltage Characteristics/Drain Characteristics: the plot drawn between output voltage (VDS) and output current (ID)
• When VGS < VTH, ID = 0
• When VGS > VTH, if a potential difference is created between drain and source (VDS), current (ID) flows through the output if a potential difference is created between drain and source (VDS)
  • For small VDS, drain current increases linearly
  • For larger VDS, drain current gets saturated
• Three operating regions:
  • Cut-Off Region: MOSFET is OFF with no current flow, behaving like an open switch.
    • VGS < VTH, ID = 0; VDS = VDD
  • Ohmic or Linear Region: Current ID increases with VDS.
    • VGS > VTH, VDS < VDS(sat), MOSFETs are used as resistors.
  • Saturation Region: Current IDs is constant despite increases in VDS, occurring once VDS exceeds the pinch-off voltage.
    • VGS > VTH, VDS > VDS (sat), The device acts like a closed switch.

Current-Voltage Characteristic Equation

• ID = (1/2) * μnCox * (W/L) * (VGS - VTN)^2
• K, is MOSFET transconduction parameter in (AV-2), which is the function of electric and geometric parameters
• constant for a given fabrication technology.
  • Cox is oxide capacitance per unit area
  • µn is the mobility of electrons in the inversion layer
  • Ratio of channel width (W) to channel length (L) - W/L is variable in the design of MOSFETs
  • VGS is the input (Gate-Source) voltage
  • VDs is the output (Drain-Source) voltage
  • VTN (or VTH) is the threshold voltage.