Introduction: It’s pointed out already that when source and drain depletion regions form a substantial fraction of channel length, short-channel effects begin to take place. In extreme cases when sum of these depletion widths will approach the channel length (ys +yD = L), effects will be more serious. This condition is commonly known as punch-through. Its net result is a large leakage current between source and drain and this current is a strong function of the drain bias.
Drain-Induced Barrier Lowering (DIBL):
The punch-through originates from the lowering of barrier close to the source, commonly called as DIBL (drain-induced barrier lowering).
When drain is near the source, the drain bias is capable of influencing the barrier at the source end, such that channel carrier concentration at that location does not remain fixed.
This occurrence is demonstrated by energy bands along the surface of the semiconductor, as shown in Fig. below.
A drain bias can alter the effective channel length but the barrier at the source end remains constant for a long channel device. However, for a short-channel device, the same barrier is no longer fixed.
When the source barrier is lowered, it causes an injection of extra carriers that increases the current significantly.
This increase shows up in subthresholdand above-threshold regimes.
It is shown in the Figure given above that punch-through condition is observed at the semiconductor surface. In practically used devices, substrate concentration is reduced below the depth of sourceldrain junction which broadens the depletion widths so that punch-through can also occur via a path in the bulk.
The punch-through drain voltage can be assessedusing depletion approximation as
The space-charge-limited currentwill dominate the drain current:
where A is cross-sectional area of the punch-through path. The space-chargelimited current increases gradually with VD2 and is parallel to inversion-layer current.
The calculated points as shown in the figure are obtained from a 2-dimensional computer calculation that incorporates the field-dependent mobility effect and punch-through effect.