U. Glaeser

FIGURE 16.17 Body effect reduction for low-V t 0.25 µm
device compared to a high-V t 0.25 µm device.
FIGURE 16.18 (a) Increase in V t-roll-off due to V t lowering and body bias. (b) Increase in DIBL (∆V t/∆V ds) due
to body bias, for a 0.25 µm NMOS.
λ b models the V t-roll-off and body effect degradation with channel length reduction, and λ d models
DIBL. This parameter is based on empirical fitting of device parameters and has been verified to be
accurate down to 0.1 µm channel length [23].
Effectiveness of ABB
We know that adaptive body bias requires (1) lower Vt devices and (2) body bias to reduce die-to-die
mean Vt variation. We also know that as technology is scaled, body terminal’s control on the channel
charge diminishes. This is further aggravated if Vt has to be reduced and/or if body bias has to be applied
since both result in increased diode depletions. Figure 16.17 illustrates the shift in threshold voltage of
two 0.25 µm MOS transistors. The two MOS transistors are identical in all aspects except in their threshold
voltage values. The linear threshold voltages of the high-Vt and the low-Vt devices are 400 and 250 mV,
respectively. It is clear from Fig. 16.17 that for 600 mV of body bias, the increase in threshold voltage
for the high-Vt device is significantly more than that of the low-Vt device. The reasons for the reduced
effectiveness of body bias for the low-Vt device are (1) reduced channel doping required for Vt reduction
means these devices will have lower body effect to begin with, (2) low-Vt devices have more diode
depletion charge degrading body effect, and (3) body bias increases diode depletion even more resulting
in added body effect degradation. It has been shown in [24] that with aggressive 30% Vt scaling it will
not be possible to match the mean Vt of all the die samples for 0.13 µm technology.
Impact of ABB on Within-Die Vt Variation
Low-Vt devices that are required for adaptive body bias schemes have worse short channel effects, and
these effects degrade with body bias. As Fig. 16.18(a) illustrates, Vt-roll-off behavior is larger for low-Vt device compared to high-Vt device, and Vt-roll-off increases further with body bias, as expected. Also,
body bias increases DIBL (∆Vt /∆Vds) as expected, and this is depicted in Fig. 16.18(b).
Within-die Vt variation due to within-die variation in the critical dimension (∆L) will depend on
Vt-roll-off (λb) and DIBL (λd). So, increase in Vt-roll-off and DIBL due to adaptive body bias will result
in a larger within-die Vt variation. It has been shown in [24] that this increase in within-die Vt variation
© 2002 by CRC Press LLC
V t (V)
0.75
0.5
high Vt 0.25
0
high V t + bias of 0.6 V
low V t
low V t + bias of 0.6 V
0.2 0.25 0.3 0.35
L g (µm)
V t (V)
Ids (A/ µ
m)
0.75
0.5
0.25
high Vt (Lg = 0.25 µ m) = 0.40 V
low Vt (Lg = 0.25 µ m) = 0.25 V
high V t
low V t
high V t + bias of 0.6 V
low V t + bias of 0.6 V
0
0.2 0.25
Lg ( µ m)
0.3 0.35
1e-3
1e-5
1e-7
Vsb = 0 V
1e-9
1e-11
0 0.5 1 1.5
V gs (V)
(a) (b)
V ds = 1 V
∆Vt1
= 40 mV
Vds = 50 mV
Vsb = 2 V
∆V t2 = 75 mV