08-05-2012, 10:57 AM
Short-Channel Effects in MOSFETs
[attachment=21521]
Short-Channel Devices
A MOSFET device is considered to be short when the channel length is the same order of
magnitude as the depletion-layer widths (xdD, xdS) of the source and drain junction.
As the channel length L is reduced to increase both the operation speed and the number of
components per chip, the so-called short-channel effects arise.
Short-Channel Effects
The short-channel effects are attributed to two physical phenomena:
1. the limitation imposed on electron drift characteristics in the channel,
2. the modification of the threshold voltage due to the shortening channel length.
In particular five different short-channel effects can be distinguished:
1. drain-induced barrier lowering and punchthrough
2. surface scattering
3. velocity saturation
4. impact ionization
5. hot electrons
Velocity saturation
The performance short-channel devices is also affected by velocity saturation, which reduces the
transconductance in the saturation mode. At low ey, the electron drift velocity vde in the channel
varies linearly with the electric field intensity. However, as ey increases above 104 V/cm, the drift
velocity tends to increase more slowly, and approaches a saturation value of vde(sat)=107 cm/s around
ey =105 V/cm at 300 K.
Note that the drain current is limited by velocity saturation instead of pinchoff. This occurs in shortchannel
devices when the dimensions are scaled without lowering the bias voltages.
Using vde(sat), the maximum gain possible for a MOSFET can be defined as
m ox de(sat ) g =WC v
Impact ionization
Another undesirable short-channel effect, especially in NMOS, occurs due to the high velocity of
electrons in presence of high longitudinal fields that can generate electron-hole (e-h) pairs by
impact ionization, that is, by impacting on silicon atoms and ionizing them.
It happens as follow: normally, most of the electrons are attracted by the drain, while the holes enter
the substrate to form part of the parasitic substrate current. Moreover, the region between the source
and the drain can act like the base of an npn transistor, with the source playing the role of the
emitter and the drain that of the collector. If the aforementioned holes are collected by the source,
and the corresponding hole current creates a voltage drop in the substrate material of the order of
.6V, the normally reversed-biased substrate-source pn junction will conduct appreciably. Then
electrons can be injected from the source to the substrate, similar to the injection of electrons from
the emitter to the base. They can gain enough energy as they travel toward the drain to create new eh
pairs. The situation can worsen if some electrons generated due to high fields escape the drain
field to travel into the substrate, thereby affecting other devices on a chip.
Hot electrons
Another problem, related to high electric fields, is caused by so-called hot electrons. This highenergy
electrons can enter the oxide, where they can be trapped, giving rise to oxide charging that
can accumulate with time and degrade the device performance by increasing VT and affect adversely
the gate’s control on the drain current.
[attachment=21521]
Short-Channel Devices
A MOSFET device is considered to be short when the channel length is the same order of
magnitude as the depletion-layer widths (xdD, xdS) of the source and drain junction.
As the channel length L is reduced to increase both the operation speed and the number of
components per chip, the so-called short-channel effects arise.
Short-Channel Effects
The short-channel effects are attributed to two physical phenomena:
1. the limitation imposed on electron drift characteristics in the channel,
2. the modification of the threshold voltage due to the shortening channel length.
In particular five different short-channel effects can be distinguished:
1. drain-induced barrier lowering and punchthrough
2. surface scattering
3. velocity saturation
4. impact ionization
5. hot electrons
Velocity saturation
The performance short-channel devices is also affected by velocity saturation, which reduces the
transconductance in the saturation mode. At low ey, the electron drift velocity vde in the channel
varies linearly with the electric field intensity. However, as ey increases above 104 V/cm, the drift
velocity tends to increase more slowly, and approaches a saturation value of vde(sat)=107 cm/s around
ey =105 V/cm at 300 K.
Note that the drain current is limited by velocity saturation instead of pinchoff. This occurs in shortchannel
devices when the dimensions are scaled without lowering the bias voltages.
Using vde(sat), the maximum gain possible for a MOSFET can be defined as
m ox de(sat ) g =WC v
Impact ionization
Another undesirable short-channel effect, especially in NMOS, occurs due to the high velocity of
electrons in presence of high longitudinal fields that can generate electron-hole (e-h) pairs by
impact ionization, that is, by impacting on silicon atoms and ionizing them.
It happens as follow: normally, most of the electrons are attracted by the drain, while the holes enter
the substrate to form part of the parasitic substrate current. Moreover, the region between the source
and the drain can act like the base of an npn transistor, with the source playing the role of the
emitter and the drain that of the collector. If the aforementioned holes are collected by the source,
and the corresponding hole current creates a voltage drop in the substrate material of the order of
.6V, the normally reversed-biased substrate-source pn junction will conduct appreciably. Then
electrons can be injected from the source to the substrate, similar to the injection of electrons from
the emitter to the base. They can gain enough energy as they travel toward the drain to create new eh
pairs. The situation can worsen if some electrons generated due to high fields escape the drain
field to travel into the substrate, thereby affecting other devices on a chip.
Hot electrons
Another problem, related to high electric fields, is caused by so-called hot electrons. This highenergy
electrons can enter the oxide, where they can be trapped, giving rise to oxide charging that
can accumulate with time and degrade the device performance by increasing VT and affect adversely
the gate’s control on the drain current.