Metal-oxide-semiconductor field-effect transistor (MOSFET)s

Introduction to MOSFETs

  • A type of field-effect transistor (FET) used for amplification and switching.

  • Unlike JFETs, MOSFETs have an insulated gate separated by a silicon dioxide (SiO\(_2\)) layer.

  • Two main types:

    • Enhancement MOSFET (E-MOSFET)

    • Depletion MOSFET (D-MOSFET).

  • Polycrystalline silicon is commonly used for the gate material.

Types of MOSFETs

  • Enhancement MOSFET (E-MOSFET):

    • Operates only in enhancement mode

    • Requires a threshold voltage to induce a channel.

  • Depletion MOSFET (D-MOSFET):

    • Operates in both depletion and enhancement modes

    • Allow bidirectional control of current.

E-MOSFET - Structure & Operation

  • No physical channel present initially.

  • Applying a positive gate-to-source voltage (\(V_{GS}\)) creates an inversion layer, forming a conductive channel.

  • Increasing \(V_{GS}\) enhances conductivity by attracting more electrons (n-channel) or holes (p-channel).

  • Schematic symbols: broken lines represent the absence of a structural channel.

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D-MOSFET - Structure & Operation

  • Channel exists at zero gate voltage.

  • Functions as a capacitor with gate as one plate, channel as another, and SiO\(_2\) as the dielectric.

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  • Operates in Depletion Mode when a negative \(V_{GS}\) repels electrons, reducing conductivity.

  • Operates in Enhancement Mode when a positive \(V_{GS}\) attracts electrons, increasing conductivity.

  • Operation of n-channel D-MOSFET

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Comparison of E-MOSFET and D-MOSFET

Feature BJT FET
Charge Carriers Electrons & Holes (Bipolar) Electrons or Holes (Unipolar)
Control Mechanism Current-Controlled Voltage-Controlled
Input Impedance Low High
Switching Speed Slower Faster
Preferred Applications Amplifiers Switching & High Impedance Circuits

Power MOSFET Structures

  • Conventional E-MOSFETs: Feature a long, thin lateral channel, leading to high drain-to-source resistance, limiting power applications.

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  • Laterally Diffused MOSFET (LDMOSFET): Has a shorter channel and lower resistance, enabling higher current and voltage applications.

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  • VMOSFET: Features a vertical channel structure with a V-groove, allowing higher power dissipation and improved frequency response.

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  • TMOSFET: Uses a vertical structure similar to VMOSFET but with greater packing density for better efficiency.

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Dual-Gate MOSFETs

  • Available in both depletion and enhancement types.

  • Features two gates, reducing input capacitance for high-frequency applications.

  • Commonly used in RF amplifiers and automatic gain control (AGC) applications.

  • Biasing the second gate allows fine-tuning of transconductance.

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E-MOSFET Transfer Characteristics

  • Operates only in enhancement mode, requiring a threshold voltage (\(V_{GS(th)}\)) to induce conduction.

  • No drain current (\(I_D\)) when \(V_{GS} = 0\), unlike JFET and D-MOSFET.

  • \[I_D = K(V_{GS} - V_{GS(th)})^2\]
    equation: Transfer characteristic follows a

  • Constant \(K\) is device-specific and derived from datasheet values.

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D-MOSFET Transfer Characteristics

  • Can operate in both depletion and enhancement modes.

  • \[V_{GS(off)} = -V_p\]
    Transfer characteristic similar to JFET, obeying the square-law equation:

  • Drain current at \(V_{GS} = 0\) corresponds to \(I_{DSS}\).

  • For \(V_{GS} < V_{GS(off)}\), channel is fully depleted, and \(I_D = 0\).

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E-MOSFET Biasing Techniques

  • Zero bias is not possible since V\(_{GS}\) must be greater than V\(_{GS(th)}\).

  • Common biasing methods:

    1. Voltage-divider bias:

      • Provides a stable operating point.

    2. Drain-feedback bias:

      • Provides automatic biasing with feedback stabilization.

      • \(V_{GS}=V_{DS}\) due to negligible gate current.

    • \[\begin{aligned} V_{GS} &= \left( \frac{R_{2}}{R_{1} + R_{2}} \right) V_{DD} \\ V_{DS} &= V_{DD} - I_{D} R_{D} \\ I_{D} &= K (V_{GS} - V_{GS(th)})^2 \end{aligned}\]
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D-MOSFET Biasing Techniques

  • Zero-bias operation is possible (V\(_{GS}\) = 0).

  • Allows AC signal variations at the gate.

  • \[V_{DS} = V_{DD} - I_{DSS} R_{D}\]
    Equation

  • Gate resistor (R\(_{G}\)) isolates the AC signal from ground without affecting biasing.

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Applications of MOSFETs

  • Analog Circuits: Used in amplifiers and voltage regulators.

  • Digital Circuits: Integral in CMOS (Complementary MOS) technology for logic gates.

  • Power Electronics: Used in switching applications such as inverters and DC-DC converters.

  • Radio Frequency (RF) Applications: Found in communication circuits.

Conclusion

  • MOSFETs are crucial in modern electronics, used in digital circuits, amplifiers, and power applications.

  • Different MOSFET types offer flexibility in high-power and high-frequency applications.

  • Power MOSFETs (LDMOSFET, VMOSFET, TMOSFET) provide enhanced efficiency and performance.

  • Biasing techniques ensure stable operation in various applications.