Charge Sharing Model
The Charge Sharing Model observes that the depletion layer formed in the channel of a MOSFET is influenced by both the gate and the drain-source PN junctions. In short-channel devices, the depletion regions formed by the drain and source become significant. The depletion zone under the gate is affected by both gate and source, leading to a shared control. This model helps explain the impact of DIBL, where the drain’s potential affects the gate’s control over the channel, leading to a lowering of the threshold voltage.
DIBL (Drain-Induced Barrier Lowering)
- DIBL is a phenomenon observed in short-channel MOSFET devices.
- It occurs due to the interaction between the drain voltage (Vds) and the gate voltage (Vgs).
- In traditional MOSFET models, threshold voltage (Vth) is assumed to be independent of channel length (L). However, in short-channel devices, there is a strong dependence of Vth on L.
- DIBL is driven by the competition between the gate-source capacitance (Cox) and the drain-channel capacitance (Cd).
- As channel length decreases, DIBL becomes more pronounced, leading to increased leakage current and a lower effective threshold voltage.
Three Models to Understand DIBL
- Charge Sharing Model: In this model, it’s observed that the depletion layer formed in the channel is partially contributed by the reverse-biased PN junctions at the source and drain. In short-channel devices, the depletion regions at the drain become significant, affecting the barrier at the source and reducing the threshold voltage.
- Capacitive Coupling Model: It considers that the drain is capacitive-coupled to the channel through a stray capacitance in the depletion region. In short-channel devices, the reduced distance between the drain and channel causes the stray capacitance to be significant, leading to a lower threshold voltage.
- Barrier Lowering Model: This model focuses on the change in the conduction band energy profile within the channel. A lowered barrier at the source due to drain potential results in electrons flowing into the channel, even with no gate voltage applied, contributing to a lower threshold voltage.
Impact of DIBL
- DIBL results in a lower threshold voltage, making it easier for the transistor to turn on.
- It causes subthreshold conduction to increase, which is equivalent to a reduction in off-state current (Ioff).
- A thinner gate oxide is favored to combat DIBL, as it strengthens gate control over the channel and mitigates the impact of drain-induced effects.
Charge Sharing Factor (f)
- The charge sharing factor, denoted as f, represents the ratio of the depletion charge contributed by the gate to the total depletion charge in the channel.
- In long-channel devices, f is typically assumed to be unity, meaning the gate controls all the charges at the surface.
- In short-channel devices, f becomes a more significant factor, reflecting the competition between the gate, source, and drain in controlling the channel charges.
DIBL is a crucial consideration in modern semiconductor device design, especially as channel lengths continue to shrink, and it affects the performance and leakage characteristics of MOSFET transistors.
Why is the charge-sharing model not applicable in long-channel MOSFET?
Charge Sharing is related to Short channel devices, in which the channel length is small, while in long-channel MOSFET, the channel length is more, so charge-sharing model not applicable in long-channel MOSFET