1. (N-Channel Metal Oxide Semiconductor)
NMOS
(N-Channel Metal Oxide Semiconductor)
Transistor
gate
source
drain
metal
metal
oxide insulator
metal
n-type
n-type e e h e h e e e
p-type h h h h
metal

2. NMOS Transistor in Equilibrium
gate
source
drain
metal
metal
oxide insulator
metal
n-type _
n-type _ h + + + + + + h _ _ _ _
p-type h h
metal
When the transistor is left alone, some electrons from the n-type wells diffuse into the p-type material to fill holes.
This creates negative ions in the p-type material and positive ions are left behind in the n-type material.

3. NMOS Transistor in Cutoff
VGS > 0
gate - +
source
drain
metal
metal
oxide insulator
metal
n-type _ _ _ _
n-type _ + + + + + + _ _ _ _
p-type h h h h h
metal
When a small, positive VGS is applied, holes “move away” from the gate.
Electrons from complete atoms elsewhere in the p-type material move to fill holes near the gate instead.

4. NMOS Transistor Channel
VGS > VTH(N)
gate - +
source
drain
metal
metal
oxide insulator
metal
n-type _ _ _ e e e _ e e
n-type + + + _ + + + _ _ _ _
p-type h h h h h h h h h h
metal
When VGS is larger than a threshold voltage VTH(N), the attraction to the gate is so great that free electrons collect there.
Thus the applied VGS creates an induced n-type channel under the gate (an area with free electrons).

5. NMOS Transistor Drain Current- +
VGS > VTH(N)
VDS > 0
gate - +
source
drain
metal
metal
oxide insulator
metal
n-type _ _ _ e e e _ e e
n-type + + + + + + _ _ _ _ _
p-type h h h h h h h h h h
metal
When a positive VDS is applied, the free electrons flow from the source to the drain. (Positive current flows from drain to source).
The amount of current depends on VDS, as well as the number of electrons in the channel, channel dimensions, and material.

6. NMOS Transistor VDS VGS ID IG + IG VGS _ ID - VDS + gate source drain
metal
metal
oxide insulator
metal
n-type
n-type
p-type
metal G +
VGS _ IG ID S D
- VDS +

7. NMOS I-V CHARACTERISTICG +
VGS _ IG ID S D
- VDS +
Since the transistor is a 3-terminal device, there is no single I-V characteristic.
Note that because of the insulator, IG = 0 A.
We typically define the MOS I-V characteristic as
ID vs. VDS for a fixed VGS.

8. MODES OF OPERATION
For small values of VGS, VGS ≤ VTH(N), the n-type channel is not formed. No current flows. This is cutoff mode.
When VGS > VTH(N), current ID may flow from drain to source, and the following modes of current flow are possible.
The mode of current flow depends on the propelling voltage, VDS, and the channel-inducing voltage, VGS – VTH(N).
When VDS < VGS – VTH(N), current is starting to flow. ID increases rapidly with increased VDS. This is triode mode.
When VDS ≥ VGS – VTH(N), current is reaching its maximum value. ID does not increase much with increased VDS. This is called saturation mode.

9. NMOS I-V CHARACTERISTIC
Cutoff Mode
Occurs when VGS ≤ VTH(N)
ID = 0
Triode Mode
Occurs when VGS > VTH(N) and VDS < VGS - VTH(N)
Saturation Mode
Occurs when VGS > VTH(N) and VDS ≥ VGS - VTH(N)

10. NMOS I-V CHARACTERISTICSID
triode mode
saturation mode
VGS = 3 V
VDS = VGS - VTH(N)
VGS = 2 V
VGS = 1 V
VDS
cutoff mode

11. PMOS (P-Channel Metal Oxide Semiconductor) Transistor
gate
source
drain
metal
metal
oxide insulator
metal
p-type
p-type
n-type
metal
Same as NMOS, only p-type and n-type switched

12. PMOS Transistor Channel
VGS < VTH(P) < 0
gate - +
source
drain
metal
metal
oxide insulator
metal
p-type h + h + h + h + h +
p-type _ _ _ _ _ _ + + + + + +
n-type e e e e e e e e e e
metal
When VGS is more negative than a threshold voltage VTH(P), the gate attracts many positive ions and holes (repels electrons)
Thus the applied VGS creates an induced p-type channel under the gate (an area with positive ions).

13. PMOS Transistor Drain Current- +
VGS < VTH(P) < 0
VDS < 0
gate - +
source
drain
metal
metal
oxide insulator
metal
p-type h h + h + h + + h +
p-type _ _ _ _ _ _ + + + + + +
n-type e e e e e e e e e e
metal
When a negative VDS is applied, the positive ions flow from the source to the drain. (Positive current flows from source to drain).
The amount of current depends on VDS, as well as the number of ions in the channel, channel dimensions, and material.

14. Symbol has “dot” at gate. NMOS does not.
PMOS TRANSISTOR G IG +
VGS _ ID S D
- VDS +
Symbol has “dot” at gate. NMOS does not.
ID, VGS, VDS, and VTH(P) are all negative.
These values are positive for NMOS.
Channel formed when VGS < VTH(P). Opposite for NMOS.
Saturation occurs when VDS ≤ VGS – VTH(P). Opposite for NMOS.

15. PMOS I-V CHARACTERISTIC
Cutoff Mode
Occurs when VGS ≥ VTH(P)
ID = 0
Triode Mode
Occurs when VGS < VTH(P) and VDS > VGS - VTH(P)
Saturation Mode
Occurs when VGS < VTH(P) and VDS ≤ VGS - VTH(P)

16. PMOS I-V CHARACTERISTICS
cutoff mode
VDS
VGS = -1 V
VGS = -2 V
VDS = VGS - VTH(P)
VGS = -3 V ID
saturation mode
triode mode