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