VLSI DEsign MethodologyThe Devices:
MOS Transistor
[Adapted from Rabaey’s Digital Integrated Circuits, ©2002, J. Rabaey et al.]
EE415 VLSI Design
The MOS Transistor
EE415 VLSI Design
The MOS Transistor
The MOS transistor, or MOSFET is a very simple device to manufacture. It also lends itself to high scale integration. Several thousand devices can be manufactured on a single chip without the devices interacting with one another.
Heavily doped n-type source and drain regions are implanted (diffused) into a lightly doped p-type substrate (body). A thin layer of SiO2 (gate oxide) is grown over the region between the source and drain and is covered by a polysilicon gate.
Neighboring devices are shielded with a thick layer of SiO2 (field oxide) and a reverse-biased np-diode formed by adding a an extra P+ region (channel-stop implant or field implant)
When a voltage larger than the threshold voltage, VT is applied to the gate, a conducting channel is formed between drain and source. Current can then flow from drain to source through the channel if there exists a potential difference between them.
Current is carried by electrons in an NMOS transistor. This is unlike a diode where both electrons and holes carry the current though different types of material.
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Open (off) (Gate = ‘0’)
Closed (on) (Gate = ‘1’)
Fourth terminal, body (bulk on previous slide)- substrate, not shown.
Assumed connected to the appropriate supply rail, GND for NMOS, VDD for PMOS
Electrons flow from source to drain – so current is referenced drain to source (IDS)
Performs very well as a switch, little parasitic effects
Today: STATIC (steady-state view) and later DYNAMIC (transient view)
VGS < 0.43 V for off
VGS > 0.43 V for on
EE415 VLSI Design
Open (off) (Gate = ‘1’)
Closed (on) (Gate = ‘0’)
| VGS | > | VDD – | VT | |
| VGS | < | VDD – |VT| |
holds flow source to drain – so current is referenced source to drain (ISD)
VGS > 2.5 - .4 = 2.1 V for off
and Vgs < 2.1 V for on
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MOS transistors Symbols
Channel
MOS transistors can be either enhancement (no channel at VGS = 0) or depletion (finite channel at VGS = 0) types. Notice the thick line on the symbol that represents the channel.
All MOSFET transistors actually have 4 pins (including the base [substrate] pin). Since the substrates are connected to the supply lines in digital circuits, they are typically not drawn.
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MOSFET Static Behavior
The gate and substrate form the plates of a capacitor.
Negative charges accumulate on the substrate side (repels mobile holes)
A depletion region is formed under the gate (like pn junction diode)
When a positive VGS is applied, the capacitor under the gate is charged with the gate having positive charges and the substrate (under the gate) having negative charges.
The negative charges repel the mobile holes to form a depletion region under the gate.
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Assume VGS > VT
A voltage difference VDS will cause ID to flow from drain to source
At a point x along the channel, the voltage is V(x), and the gate-to-channel voltage is VGS - V(x)
For channel to be present from drain to source, VGS - V(x) > VT, i.e. VGS - VDS > VT for channel to exist from drain to source
Now that a channel is formed from source to drain, a potential difference between them will cause current, ID to flow.
However the difference of potential between source and drain, also affects the depth of the channel. So the difference between the gate and drain voltage must always be larger than the threshold voltage to maintain a channel from source to drain.
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Linear (triode) Region
When VGS - VDS > VT , the channel exists from drain to source
Transistor behaves like voltage controlled resistor
So long as the the difference between the gate and drain voltage is larger than the threshold voltage, the channel will conduct current. It will have a finite resistance and behave like a voltage controlled resistor. Higher the gate voltage, lower the channel resistance.
148.bmp
When VGS - VDS VT , the channel is pinched off
Electrons are injected into depletion region and accelerated towards drain by electric field
Transistor behaves like voltage-controlled current source
Pinch-off
When the difference between the gate and drain voltage is less than the threshold voltage, the condition for the channels existence is no longer true near the drain region. The channel begins to pinch off, leaving a narrow depletion region near the drain.
The charges will inject through the narrow depletion region and find its way to the drain since there is a large enough electric field to accelerate them. Therefore a small current will flow (saturation current).
Under this (saturation) condition, the MOSFET behaves like a voltage-controlled current source.
149.bmp
Long Channel transistor
NMOS transistor, 0.25um, Ld = 10um, W/L = 1.5, VDD = 2.5V, VT = 0.4V
Quadratic
Relationship
Resistive
Saturation
n+
n+
p-substrate
Field-Oxyde
(SiO
2