Drain-induced barrier lowering
Drain-induced barrier lowering (DIBL) is a short-channel effect in MOSFETs referring originally to a reduction of threshold voltage of the transistor at higher drain voltages. In a classic planar field-effect transistor with a long channel, the bottleneck in channel formation occurs far enough from the drain contact that it is electrostatically shielded from the drain by the combination of the substrate and gate, and so classically the threshold voltage was independent of drain voltage. In short-channel devices this is no longer true: The drain is close enough to gate the channel, and so a high drain voltage can open the bottleneck and turn on the transistor prematurely.
The origin of the threshold decrease can be understood as a consequence of charge neutrality: the Yau charge-sharing model.
In effect, the channel becomes more attractive for electrons. In other words, the potential energy barrier for electrons in the channel is lowered. Hence the term "barrier lowering" is used to describe these phenomena. Unfortunately, it is not easy to come up with accurate analytical results using the barrier lowering concept.
Barrier lowering increases as channel length is reduced, even at zero applied drain bias, because the source and drain form
The term DIBL has expanded beyond the notion of simple threshold adjustment, however, and refers to a number of drain-voltage effects upon MOSFET I-V curves that go beyond description in terms of simple threshold voltage changes, as described below.
As channel length is reduced, the effects of DIBL in the
DIBL also affects the current vs. drain bias curve in the active mode, causing the current to increase with drain bias, lowering the MOSFET output resistance. This increase is additional to the normal channel length modulation effect on output resistance, and cannot always be modeled as a threshold adjustment.
In practice, the DIBL can be calculated as follows:
where or Vtsat is the threshold voltage measured at a supply voltage (the high drain voltage), and or Vtlin is the threshold voltage measured at a very low drain voltage, typically 0.05 V or 0.1 V. is the supply voltage (the high drain voltage) and is the low drain voltage (for a linear part of device I-V characteristics). The minus in the front of the formula ensures a positive DIBL value. This is because the high drain threshold voltage, , is always smaller than the low drain threshold voltage, . Typical units of DIBL are mV/V.
DIBL can reduce the device operating frequency as well, as described by the following equation:
where is the supply voltage and is the threshold voltage.
References
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Narain Arora (2007). Mosfet Modeling for VLSI Simulation: Theory And Practice. World Scientific. p. 197, Fig. 5.14. ISBN 978-981-256-862-5.
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Yannis Tsividis (2003). Operation and Modeling of the MOS Transistor (Second ed.). New York: Oxford University Press. p. 268; Fig. 6.11. ISBN 0195170148.