MOSFET Notes

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)

• Also known as Metal-Insulator Semiconductor FET.
• Widely used in electronic devices, especially digital circuits, due to their small size, allowing thousands to be fabricated on a single IC.
• The gate terminal is insulated from the channel.
• Also referred to as Insulated Gate FET (IGFET).
• Typically made using silicon (semiconductor), $SiO_2$ (insulator), and metal or heavily doped polysilicon (gate electrode).

JFET vs. MOSFET

• A primary drawback of JFETs is that the gate must be reverse-biased for proper operation, which means:
  • Negative gate operation for n-channel JFETs.
  • Positive gate operation for p-channel JFETs.
• This limits operation to decreasing channel width (decreasing conductivity) from its zero-bias size, known as depletion-mode operation.
• MOSFETs have advantages over JFETs, including:
  • High input impedance.
  • Low cost of production.

Types of MOSFETs

• Two basic types:
  • Depletion-type MOSFET (D-MOSFET): Operates in both depletion and enhancement modes.
  • Enhancement-type MOSFET (E-MOSFET): Operates only in enhancement mode.
• The construction determines whether a MOSFET is D-MOSFET or E-MOSFET.

D-MOSFET Construction

• n-channel D-MOSFET construction:
  • A piece of n-type material with a p-type region (substrate) on one side and an insulated gate on the other.
  • Free electrons flow from source to drain through the narrow channel between the gate and substrate.
  • A thin layer of metal oxide (usually silicon dioxide, $SiO_2$) is deposited over a portion of the channel.
  • A metallic gate is deposited over the oxide layer, insulated from the channel, forming a capacitor.
    • One plate is the gate, the other is the channel, with $SiO_2$ as the dielectric.
  • The substrate is usually connected to the source (S) internally, resulting in three terminals: source (S), gate (G), and drain (D).
  • Either negative or positive voltage can be applied to the gate due to its insulation from the channel.

D-MOSFET Symbols

• Two types of D-MOSFETs:
  • n-channel D-MOSFET.
  • p-channel D-MOSFET.
• The p-type substrate constricts the channel between the source and drain.

Circuit Operation of D-MOSFET

• The gate forms a capacitor.
  • One plate: gate.
  • Other plate: channel with metal oxide layer as the dielectric.
• Changing gate voltage alters the electric field of the capacitor, changing the resistance of the n-channel.
• Either negative or positive voltage can be applied to the gate because the gate is insulated.

Depletion Mode

• Negative gate operation.
• Electrons on the gate repel free electrons in the n-channel, creating a layer of positive ions.
• The n-channel is depleted of free electrons, reducing current conduction.
• Increasing the negative gate voltage decreases the current from source to drain, effectively increasing channel resistance.

Enhancement Mode

• Positive gate operation.
• Positive gate induces negative charges (free electrons) in the n-channel.
• Free electrons are added to those already in the channel, increasing conductivity.
• Increasing the positive gate voltage increases conduction from source to drain.

Key Points of D-MOSFET Operation

• Source-to-drain current is controlled by the electric field of the capacitor formed at the gate.
• The gate of a JFET behaves as a reverse-biased diode, while the gate of a D-MOSFET acts as a capacitor.
• D-MOSFETs can be operated with positive or negative gate voltage.
• Negligible gate current flows due to the capacitor formation, resulting in very high input impedance (10,000 M$Ω$ to 10,000,00 M$Ω$).
• Extremely small dimensions of the oxide layer under the gate result in very low input capacitance, making D-MOSFETs useful in high-frequency applications.

D-MOSFET Transfer Characteristic

• The transfer characteristic curve (transconductance curve) explains the device's behavior.
  • When $V<em>{GS} = 0$, $I</em>D = I<em>{DSS}$. This is expected since $I</em>{DSS}$ is the drain current when the gate and source are shorted ($V_{GS} = 0$).
  • As $V<em>{GS}$ goes negative, $I</em>D$ decreases below $I<em>{DSS}$ until $I</em>D$ reaches zero when $V<em>{GS} = V</em>{GS(off)}$, similar to JFETs.
  • When $V<em>{GS}$ is positive, $I</em>D$ increases above $I_{DSS}$.