MOSFETs are widely used in various applications, including computers and smartphones, because they are efficient, fast, and can be miniaturized.
Construction
What Does a MOSFET Do
Mode of Operation
Operating Regions of MOSFET
Applications
- Substrate: The substrate is the foundation of the MOSFET and is usually made of silicon.
- Oxide Layer: Above the substrate is the oxide layer, which acts as an insulator to prevent current leakage.
- Gate: The gate is located atop the oxide layer and controls MOSFET’s conductivity by applying a voltage.
- Source and Drain: These two regions are located on either side of the gate. When the gate voltage activates them, they allow current to flow from the source to the drain.
- Channel: The channel forms underneath the oxide layer between the source and the drain. When the transistor is in the “on” state, the channel acts as a pathway for current flow. The width of the channel is controlled by the voltage applied to the gate. There are two types of channels:
N-channel: Made of N-type semiconductors. Electrons are used as charge carriers to conduct current.P-channel: Made of P-type semiconductors. Holes are used as charge carriers to conduct current.
In its basic operation, when no voltage is applied to the gate terminal, the MOSFET is off, blocking current flow between the source and drain. When a voltage is applied, it generates an electric field that affects the conductivity of the channel between the source and the drain. The electric field either attracts or repels charge carriers, allowing current to flow across the device. The working principle of a MOSFET differs from that of a bipolar junction transistor (BJT) in that the BJT is a current-controlled device.
N-channel Depletion MOSFETP-channel Depletion MOSFETN-channel Enhancement MOSFETP-channel Enhancement MOSFET
- Depletion Mode A depletion mode MOSFET can conduct current without needing a gate voltage input. This means it is “normally on” and allows current to flow from the drain to the source as soon as it is connected. When a voltage is applied to the gate terminal of the MOSFET, the channel between the drain and the source becomes more resistant. As the gate-source voltage increases, the current flow from the drain to the source decreases until it stops completely.
- Enhancement Mode In enhancement mode, the substrate isolates the source and drain terminals from each other. When no voltage is applied to the gate, there is no pathway for current to flow between the two terminals. However, applying voltage to the gate creates a channel for current to be conducted. This is why it is called enhancement mode MOSFET, as the gate voltage “enhances” the channel to conduct current.
- Cut-off Region The MOSFET is turned off, and no current flows between the drain and source terminals. It behaves like an open switch. The gate-source voltage (VGS) is below a threshold value, known as the cut-off voltage (Vt).
- Triode or Ohmic Region The MOSFET operates in the triode region when VGS exceeds Vt. The transistor behaves like a variable resistor, allowing a linear (ohmic) relationship between drain current (ID) and drain-source voltage (VDS), whose value is less than VGS – Vt.
- Saturation Region The MOSFET enters the saturation region as VDS increases beyond VGS – Vt. In this region, the transistor acts as a closed switch with maximum current flow between drain and source terminals. The ID remains relatively constant to changes in VDS in saturation mode. Image Courtesy – Picture.iczhiku.com
