| 8.4 Threshold Voltage variation with Channel Length (contd...) |
| Quantum Mechanical Increase Effect: Another effect of quantum mechanics that also increases with scaling, is a shift in the surface potential required for strong inversion. This effect arises from so called "energy quantization" of confined particles which preludes electrons and holes from existing at zero energy in the conduction or valence bands. It is a direct consequence of the coupled Poisson-Schrodinger equation solution. This surface potential shift manifests itself as an increase in |VT| which for the long devices is given by - |
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| Above equation tells that |VT| increases as devices are scaled down. |
| 8.5 Drain Induced Barrier Lowering (DIBL) |
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The source and drain depletion regions can intrude into the channel even without bias, as these junctions are brought closer together in short channel devices. This effect is called charge sharing (as mentioned earlier) since the source and drain in effect take part of the channel charge, which would otherwise be controlled by the gate. As the drain depletion region continues to increase with the bias, it can actually interact with the source to channel junction and hence lowers the potential barrier. This problem is known as Drain Induced Barrier Lowering (DIBL). When the source junction barrier is reduced, electrons are easily injected into the channel and the gate voltage has no longer any control over the drain current. In DIBL case,  . |
| For figure 8.5, we can observe that under extreme conditions of encroaching source and drain depletion regions, the two curves can meet. |
| Figure 8.5: Surface potential graph with constant gate voltage (VDS and L are varied) . |
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