Gate-Induced Drain Leakage in MOSFET
we find that as the gate voltage is reduced below VT, the subthreshold current drops and then bottoms out at a level determined by the source/drain diode leakage. However, for even more negative gate biases we find that the off-state leakage current actually goes up as we try to turn off the MOSFET more for high
VD. This is known as gate-induced drain leakage (GIDL).
GIDL & its band diagram:
- The same effect is seen at a fixed gate bias of near zero, for increasing drain bias. The reason for GIDL can be understood from Fig. given below, where we show the band diagram as a function of depth in the region where the gate overlaps the drain junction.
- As the gate is made more negative (or alternatively, for a fixed gate bias, the drain is made more positive), a depletion region forms in the n-type drain.
- Since the drain doping is high, the depletion widths tend to be narrow. If the band-bending is more than the band gap Eg across a narrow depletion region,the conditions are conducive to band-to-band tunneling in this region, thereby creating electron-hole pairs. The electrons then go to the drain as GIDL.
- It must be emphasized that this tunneling is not through the gate oxide, but entirely in the Si drain region.
- For GIDL to occur, the drain doping level should be moderate (~1018 cm-3).
- If it is much lower than this, the depletion widths and tunneling barriers are too wide.
- On the other hand, if the doping in the drain is very high, most of the voltage drops in the gate oxide, and the band-bending in the Si drain region drops below the value Eg.
- GIDL is an important factor in limiting the off-state leakage current in state of-the-art-MOSFETs.