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U. Glaeser

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E C<br />

FIGURE 2.88 SOI device operation modes.<br />

Drain Current (A)<br />

FIGURE 2.89 Id-Vd characteristics in FD and PD modes.<br />

than PD devices. For example, the thickness of the body region of a PD device is about 100 nm, but that<br />

of an FD device is about 50 nm. In the PD device, on the other hand, the body region is only partly<br />

depleted and electrically neutral region exists. The presence of the region, focusing attention on the<br />

change in potential in the depth direction of the body region from the gate oxide layer, limits the gate<br />

field effect to within the body region, and the neutral region in which there is no potential gradient exists<br />

in the lower part of the body. Accordingly, the difference in potential between the surface of the body<br />

region and the bottom of the region is greater in a PD device than in an FD device, and the potential<br />

barrier corresponding to the holes between the source and body near the bottom of the body region is<br />

higher in the PD structure than in the FD structure. This difference in potential barrier height corresponding<br />

to the holes creates a difference in the number of holes that can exist within the body region,<br />

as shown in Fig. 2.88. These holes are created by impact ionization when the channel electrons pass<br />

through the high electric field region near the drain during n-MOSFET operation. The holes flow to the<br />

source via the body region. At that time, more holes accumulate in the body region of the PD structure,<br />

which has a higher potential barrier than the FD structure. This fact brings about a large difference in<br />

the floating body effect of the FD device and the PD device, determines whether or not a kink appears<br />

in the drain current-voltage characteristic and creates a difference in the subthreshold characteristic, as<br />

shown in Fig. 2.89.<br />

© 2002 by CRC Press LLC<br />

n +<br />

SiO 2<br />

Si Substrate<br />

E V<br />

Source<br />

Fully-Depleted NMOS Partially-Depleted NMOS<br />

n +<br />

n +<br />

SiO 2<br />

Si Substrate<br />

E C<br />

E V<br />

Source<br />

n +<br />

n + n +<br />

p<br />

Neutral p region<br />

Drain Drain<br />

HOLES HOLES<br />

6.0E-03<br />

5.0E-03<br />

4.5E-03<br />

5.0E-03<br />

4.0E-03<br />

4.0E-03<br />

3.5E-03<br />

3.0E-03<br />

3.0E-03<br />

2.5E-03<br />

2.0E-03<br />

2.0E-03<br />

1.5E-03<br />

1.0E-03<br />

1.0E-03<br />

0.0E+00<br />

0 0.5 1 1.5<br />

Drain Voltage (V)<br />

2<br />

5.0E-04<br />

0.0E+00<br />

0 0.5 1 1.5<br />

Drain Voltage (V)<br />

2<br />

FD Mode (V SUB = 0 V)<br />

Drain Current (A)<br />

PD Mode (V SUB = -15 V)

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