476 CHAPTER 9. FIELD EFFECT TRANSISTORS: MOSFET
IDS
Vth
(short channel)Vth
(long channel)VGS
S D
N+ N+
- -
-
-
- - --- -
-
-
-
---
VG
S D
N+ N+
- -
-
-
- -----
-
-
- -
VG
Figure 9.28: Schematic of long and short channel MOSFETs (above) with corresponding
current-voltage characteristics below.
Three-Dimensional Transport
In our simple model for the MOSFET current, we assumed that the current flow was one-
dimensional and we could use the gradual channel approximation. For a very short-channel
device, the current flow is not just parallel to the gate, but one has to consider the current flow
from the source and drain side, which is highly two-dimensional. Also, if the gate widthZis
small, the transport becomes truly three-dimensional, requiring enormous computations to do a
proper device simulation.
Charge Sharing and VT lowering (Drain Induced Barrier Lowering)
It is observed that the threshold voltage of a MOSFET becomes increasingly negative as the
gate length of the device shrinks with all other parameters remaining the same. In conventional
analysis there is no dependence ofVTon the gate length or channel length. This is because
conventional analysis assumes that the band bending in the semiconductor and hence the onset