We present in this paper an ultra-low-power impulse radio ultra-wideband (IR-UWB) transmitter intended for short-range, highly energy-efficient, and compact silicon-area applications, such as implantable brain–computer interfaces (iBCIs). The proposed transmitter features effective spectrum tunability, enabling independent adjustments to both the center frequency and −10 dB bandwidth. Fabricated in TSMC 40 nm CMOS technology, the chip occupies a core area of just 0.001 mm2. Experimental results demonstrate an energy efficiency of 2.45 pJ/b across a data rate range of 10 to 200 Mbps. The peak-to-peak output voltage amplitude is approximately 310 mV when driving a 50 Ω load. Furthermore, in vitro wireless measurements demonstrate reliable through-tissue transmission at an implantation depth of 18 mm and achieve a distance range exceeding 0.8 m.