What happens is short channeled MOSFET

In a short channel MOSFET, the channel length is comparable to the depletion layer widths of the source and drain junctions. This leads to several effects:

1. Velocity Saturation: As the channel length decreases, the electric field strength increases. This can cause the carriers to reach their maximum velocity, leading to velocity saturation. This can limit the current flow and affect the device's performance.

2. Drain Induced Barrier Lowering (DIBL): The depletion region of the drain can interact with the source, reducing the barrier for carriers to flow from the source to the drain. This can cause the threshold voltage to decrease with increasing drain voltage, leading to a higher off-state current.

3. Hot Carrier Injection: The high electric field can give the carriers enough energy to overcome the silicon dioxide barrier and get injected into the gate oxide. This can lead to a permanent change in the threshold voltage, degrading the device's performance over time.

4. Short Channel Effects: As the channel length becomes comparable to the carrier's mean free path, traditional drift-diffusion models of current flow become less accurate. Quantum mechanical effects can become significant, leading to phenomena like tunneling.

5. Punchthrough: If the depletion regions of the source and drain overlap, carriers can flow directly from source to drain without being controlled by the gate. This leads to a high off-state current and can cause the device to fail.

These effects can limit the scalability of MOSFETs and are a major concern in modern integrated circuit design.