Module 2 : MOSFET
Lecture 8: Short Channel Effects
 
8.4 Threshold Voltage variation with Channel Length
In case of long channel MOSFETs, gate has control over the channel and supports most of the charge. As we go to short channel lengths as seen in the graph above, the threshold voltage begins to decrease as the charge in the depletion region is now supported by the drain and the source also. Thus the gate needs to support less charge in this region and as a result, VT falls down.This phenomenon is known as charge sharing effect.
Now since IDS is propotional to (VGS - VT), therefore as VT begins to fall in case of short channels, IDS starts increasing resulting in larger drain currents. Also when VGS is zero and the MOSFET is in the cut off mode, since VT is small, (VGS - VT) will be a small negative value and will result in leakage current which further multplied by the drain voltage will result in leakage power. In case of long channel MOSFETs, VT is large enough and (VGS - VT) is a comparatively larger negative value, in cut off mode leakage power is very small.
Figure 8.41: Dependence of VT on L for MOSFET  
Transit Time: As seen in previous lecture, the short channel results in velocity saturation over part of the channel. So the argument used to derive the transit time for long channel MOSFET is no longer valid for short channel MOSFETs. We note that the transit time will be larger if electrons were moving at maximum speed all over the channel. Thus,
Figure 8.42 shows that the transit time of a device operating in the 'flat' part of IDS-VGS characteristics curve which concludes that transit time cannot be decreased by increasing further VGS.
Figure 8.42: IDS Vs VGS for short channel  
   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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