B. Pixel Design and Layout
As discussed in the previous section, the 1/ f noise originating
from the SF transistor can be reduced using a SF
with a thinner oxide and lower gate width. From a designer
perspective, this can be achieved by using a thin oxide (1.8 V)
transistor instead of a thick oxide (3.3 V) in the 180 nm CMOS
process used for this work. The sense node needs to be reset
at a voltage higher than 2 V for a good dynamic range and
also to make sure that the sense node attracts efficiently the
charges from the PPD when the potential barrier under the
transfer gate is removed. Yet, the voltage difference between
the gate and the bulk of the thin oxide SF must remain smaller
than 1.8 V. Hence, the bulk voltage of the thin oxide SF must
be shifted. To this purpose a PMOS thin oxide transistor is
chosen. Indeed, the bulk voltage of a PMOS transistor can be
controlled through the n-well connection. An NMOS transistor
with a separated p-well could also be used, but at the cost of
a larger area. Also, a PMOS thin oxide SF usually shows
a lower 1/ f noise parameter K than NMOS in the target
technology nodes. Consequently, the K parameter in (1) is
expected to be lower for the PMOS thin oxide SF. A thick
oxide PMOS row select (RS) transistor is chosen in order to
put it in the same n-well as the SF and optimize the layout
footprint of the in-pixel readout transistors. Fig. 5(a) shows
the schematic of the proposed pixel. The bulk voltage of the
thin oxide PMOS SF is set to VDD at 3.3 V to insure that the
voltage difference between its bulk and the gate (sense node)
is positive and below 1.8 V. The drain voltage of the SF is
connected to (Vlow) which is set to 1.5 V in order to shift
the ground of the SF stage and make sure that the voltage
differences between all the thin oxide SF terminals remain
below 1.8 V. The SF gate is sized to minimize its input-referred
TRN according to 1, which is plotted in Fig. 6 versus the
gate width W and length L. It clearly shows that W should
be chosen as minimum (W = 0.22μm) whereas L should