Revealing the operating mode of YFW N-channel enhanced MOSFET

Cutoff State (Vₓ ≤ Vₜₕ)

1. Vₓ = 0V: When the voltage applied to the gate (Vₓ) is 0 volts, no electric field is established across the insulating oxide layer. As a result, the P - type substrate between the drain and the source acts as an effective insulator. The N⁺ - doped drain and source regions are isolated from each other because there is no means to create a conductive path through the P - type material. For example, in a digital circuit where the MOSFET is used as a switch to control the flow of current to a logic gate, when the gate voltage is 0, the MOSFET is in the off - state, and no current can flow from the drain to the source, effectively 切断 the connection between the two nodes.

2. Vₓ < Vₜₕ: Even when a small positive voltage is applied to the gate such that Vₓ is less than the threshold voltage (Vₜₕ), a process called weak inversion occurs. In weak inversion, some electrons are attracted to the surface of the P - type substrate near the gate. However, this accumulation of electrons is not sufficient to form a continuous N - channel that can support significant current flow between the drain and the source. The number of electrons is too low, and they are not organized in a way to create a well - defined conductive path.

3. Key Outcome: In both cases where Vₓ = 0V and Vₓ < Vₜₕ, the drain current (Iₙ) is essentially zero. This is a fundamental property of the cutoff state and is crucial for power - saving in circuits. For instance, in a battery - powered device like a smartwatch, when certain components are not in use, the MOSFETs controlling the power supply to those components can be placed in the cutoff state, minimizing power consumption and extending the battery life.

Linear Region (Vₓ > Vₜₕ, Vₜₛ < Vₓ - Vₜₕ)

1. Pre - Pinch - Off: As the drain - source voltage (Vₜₛ) is increased while keeping Vₓ > Vₜₕ, an interesting phenomenon occurs near the drain end of the channel. The voltage drop along the channel causes the effective gate - to - channel voltage near the drain to decrease. This reduction in the gate - to - channel voltage leads to a narrowing of the N - channel near the drain. As Vₜₛ continues to increase, the channel near the drain becomes increasingly narrow, reducing its cross - sectional area. This narrowing of the channel is a precursor to the channel pinch - off. In a radio - frequency amplifier circuit, the behavior of the channel during pre - pinch - off can affect the signal - handling capabilities of the MOSFET, especially in terms of linearity and gain.

2. Saturation Current: When Vₜₛ reaches a value such that it is equal to or greater than Vₓ - Vₜₕ, the channel near the drain fully pinches off. Despite the channel being pinched off, electrons can still reach the drain due to the strong electric field in the pinched - off region. At this point, the drain current (Iₙ) reaches a maximum value and plateaus. The saturation current is governed by the equation

where μₙ is the electron mobility, which represents how easily electrons can move through the channel material; Cₒₓ is the oxide capacitance, which is related to the insulating oxide layer's ability to store charge and influence the electric field; and W/L is the channel aspect ratio, with W being the width and L being the length of the channel. This ratio plays a crucial role in determining the electrical characteristics of the MOSFET. For example, in a high - speed digital circuit, the saturation - region operation of the MOSFET is utilized to achieve consistent and reliable switching performance, with the saturation current being a key parameter in determining the speed and power consumption of the circuit