U. Glaeser

FIGURE 13.11 Unfooted dynamic 4:1 multiplexor.
FIGURE 13.12 SOI noise simulation.
Because of the reduced junction capacitance of SOI devices, the charge redistribution problem presented
earlier is significantly better in SOI than bulk technologies. Partially-depleted SOI has several other
challenges for the circuit designer: bipolar leakage, the “kink effect” in the I-V characteristic, and the
“history effect.” Each of these effects is related to the body of the SOI device. Although the body can be
terminated through a body contact, 21 the capacitive and delay overhead of the contact generally prevents
them from being used for logic transistors. The voltage on the floating body of the device influences the
on-current through the dynamic raising or lowering of the device threshold and the off-current through
the flow of bipolar current and MOS leakage current due to the variation of the device threshold.
For the SOI 4-to-1 unfooted dynamic multiplexor circuit shown in Fig. 13.11 a noise event waveform
is presented in Fig. 13.12. The waveform shows the voltages and the resulting currents through a leg of
the mux which should be off. At location A in the waveform, the body of device N 1 has drifted to a relatively
high steady-state voltage. At B, the device N 2 is made to conduct. This has the effect of coupling the body
of device N 1 down as the source of N 1 is driven low; however, the body of N 1 is still sufficiently high to
allow current to flow through the parasitic bipolar device. In addition, the voltage on the body of the
device lowers the threshold of the MOS device N 1, making it particularly susceptible to noise at the gate
of N 1. At C, a 300 mV noise pulse on the gate of N 1 results in the dynamic node discharging sufficiently
low to produce an approximately 300 mV output noise pulse.
The failure mechanism is a leakage mechanism, therefore, this failure mode can be minimized through
the introduction and proper sizing of keeper devices. In the previous example, the size of the keeper device
helps determine the amplitude and duration of the noise output pulse. In addition, the predischarge
of the internal nodes in the pull-down network can limit the excursion of the body voltages, thereby
limiting the bipolar leakage current and the reduction in MOS threshold voltage due to the floating body.
Limiting the number of potential leakage paths through pull-down network topology changes can also limit
the floating body induced leakage effects.
Because both the current flow and thus the delay of the gate can be modified by the floating body, the
delay of a circuit becomes time and state dependent. This “history effect” has been shown to lead to variations
in delay of approximately 3–8% depending upon the technology, circuit, and time between activity. 22,23
© 2002 by CRC Press LLC
PC
In_top
PC
2.173
-0.004
2.173
-0.108
2.168
In_bot
Body_top
Id_top
Dyn
Out
-0.013
2.022
0.421
0.006
-0.203
2.235
1.345
0.309
-0.009
N1
N2
A B
C
Out