Basic MOS TransistornMOS enhancement mode Transistor
nMOS depletion mode Transistor
MOS transistors when Vds=0V SourceVgs,Vsb=0V
4/9/2014
Action of enhancement mode transistor action(nMOS)
Source and drain regions are formed by doping desired
n-impurityconcentration.
Source and drain are separated by two back to back connected
diodeshence electrons flows from source to drain. Depletion region
surroundsaround source and drain junctions as they are reverse
biased.
Gate oxide (Sio2) is grown above substrate between source and
drainwhile polysilicon/metal is deposited over it.
Channel is not formed when gate voltage is not applied(Vgs=0V)
There are two electric fields in the transistor ,vertical from gate
to
substrate due to Vgs, horizontal from drain to source due to
Vds.
nMOS depletion mode MOSFET
When Vgs=0V, Vd>0 V current flows in the transistor. When
Vgs0V the depletion MOSFET acts as enhancement MOSFET.
Transistor is normally ON. The ntype impurities are buried
in
the channel area.
4/9/2014
When positive gate voltage isapplied the positive voltage onthe
gate plate attracts theminority carriers (electrons) fromthe
substrate.
This also uncovers the positive ionsnear the surface
formingdepletion region .
At a particular gate voltage the number of electrons near the
surfacebecome equal to the doping concentration of the substrate.
The surfaceinverts to n-type. This voltage is known as Threshold
Voltage (Vth).
A uniform channel is formed near the surface at threshold
voltage(Vgs=Vth)
Vgs> 0V ,Vds= 0V
When Vgs>Vth and Vds0V) Current flows from drain to source
as
electrons move through the channelunder the influence of the
horizontalelectric field as well as vertical electricfield.
There is voltage drop in the channel due to VDS varying with
distance,(more near the drain end).
Effective gate voltage is Vg=Vgs-Vth.There will be no voltage
available toinvert the channel at the drain end so long as
Vgs-Vth>=Vds.
4/9/2014
The limiting condition comes when For all voltages Vds
