  Internet-of-Things (IoT) applications require nW-power current references
that are robust to process, voltage and temperature (PVT) variations, to
maintain the performance of IoT sensor nodes in a wide range of operating
conditions. However, nA-range current references are rarely area-efficient due
to the use of large gate-leakage transistors or resistors, which occupy a
significant silicon area at this current level. In this paper, we introduce a
nA-range constant-with-temperature (CWT) current reference, relying on a
self-cascode MOSFET (SCM) biased by a four-transistor ultra-low-power voltage
reference through a single-transistor buffer. The proposed reference includes a
temperature coefficient (TC) calibration mechanism to maintain performance
across process corners. In addition, as the proposed design relies on the body
effect, it has been fabricated and measured in 0.11-$\mu$m bulk and 22-nm
fully-depleted silicon-on-insulator (FD-SOI) to demonstrate feasibility in both
technology types. On the one hand, the 0.11-$\mu$m design consumes a power of
16.8 nW at 1.2 V and achieves a 2.3-nA current with a line sensitivity (LS) of
2.23 %/V at 25{\deg}C and a TC of 176 ppm/{\deg}C at 1.2 V from -40 to
85{\deg}C. On the other hand, the 22-nm design consumes a power of 16.3 nW at
1.5 V and achieves a 2.5-nA current with a 1.53-%/V LS at 25{\deg}C and an
82-ppm/{\deg}C TC at 1.5 V from -40 to 85{\deg}C. Thanks to their simple
architecture, the proposed references achieve a silicon area of 0.0106 mm$^2$
in 0.11 $\mu$m and 0.0026 mm$^2$ in 22 nm without compromising other figures of
merit, and are thus competitive with state-of-the-art CWT references operating
in the same current range.
