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| 8.6 Drain Punch Through |
| When the drain is at high enough voltage with respect to the source, the depletion region arround the drain may extend to the source, causing current to flow irrespective of gate voltage (i.e. even if gate voltage is zero). This is known as Drain Punch Through condition and the punch through voltage VPT given by : |
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| So when channel length L decreases (i.e. short channel length case), punch through voltage rapidly decreases. |
| 8.7 Hot Carrier Effect |
| Electric fields tend to be increased at smaller geometries, since device voltages are difficult to scale to arbitrarily small values. As a result, various hot carrier effects appear in short channel devices. The field in the reversed biased drain junction can lead to impact ionization and carrier multiplication. The resulting holes contribute to substrate current and some may move to the source, where they lower source barrier and result in electron injected from source into p-region. In fact n-p-n transistor can result within source channel drain configuration and prevent gate control of the current. |
| Another hot electron effect is the transport of the energetic electrons over (or tunneling through) the barrier into the oxide. Such electrons become trapped in the oxide, where they change the threshold voltage and I-V characteristics of the device. Hot electron effects can be reduced by reducing the doping in the source and drain regions, so that the junction fields are smaller. However lightly doped source and drain regions are incompatible with small geometry devices because of contact resistances and other similar problems. A compromise design of MOSFET, called Lightly Doped Drain(LDD), using two doping levels with heavy doping over most of the source and drain areas with light doping in a region adjecent to the channel. The LDD structure decreases the field between drain and channel regions, thereby reduces injection into the oxide, impact ionization and other hot electron effects. |
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