This presentation discusses about the need for Lightly Doped Drain. Also, why are LDD implants required in nMOS but not in pMOS

1. Lightly Doped Drain
Sudhanshu Janwadkar
TA, ECED, SVNIT
22nd August 2017

2. Consider a nMOS Transistor operating in
Saturation region. The relation between the
node voltages in this region is given by:
Pinch-off in n-channel MOSFET

3. • For nMOS, the drain is positive while the source is
negative.
• Since the drain is positive, when VDS is large
(specifically, when VDS > (VG-VT)) all the electrons in
the channel region near the drain will be pulled by the
drain.
• Thus the channel practically disappears near the drain
region. The onset of this phenomenon is called pinch
off.
Pinch-off in n-channel MOSFET

5. Therefore, the channel (i.e.) the inversion layer is
pinched-off in Saturation region (i.e.) when VDS > (VG-
VT)
The Drain-source current equation in Saturation
region is given by:
The current becomes saturated and doesn’t increase
with increase in VDS
Pinch-off in n-channel MOSFET

6. • When the VDS is very large, the electrons from
the source are accelerated in the channel and
arrive at the drain with high velocities and hence
high kinetic energies.
• These can cause degradation of the transistor or
instability. This phenomenon is called hot carrier
effect.
• One of the problems that can arise due to hot
carrier effect is that the electrons may go
through the gate oxide into the gate. Thus, the
gate oxide will be degraded and the threshold
voltage will change.
Hot-electron Effect in N-Channel MOSFET

7. To overcome this hot-electron effect, the
electric field at the drain-channel junction
must be reduced. How?
• The doping concentration at the reverse-
biased drain-channel junction is reduced.
• This increases the depletion width at this
junction.
• Hence, the electric field can be reduced.
Lightly Doped Drain

8. To minimize the hot carrier effect, the
drain near the channel is doped less
compared to the main drain area. This is
called Lightly Doped Drain (LDD).
To minimize the process complications,
both source and drain are doped lightly
near the channel region.
Lightly Doped Drain

9. Lightly Doped Drain

10. • Hot-carrier effect is less problematic in p-channel
MOSFETS. This is because:
• The channel mobility of holes is about 2.5 times less
than that of electrons. Hence under same electric
field, the kinetic energy gained will be equally less.
• Hence, while LDD is mandatory for the n-channel
MOSFET, it is not often used for p-channel devices
What about pMOS then?