This work presents an implantable impulse-radio ultrawideband transmitter (IR-UWB TX) optimized for high-density neural recording. To address the tradeoffs between data rate, transmission range, implant size, and power consumption, we propose a novel co-designed power amplifier (PA) and an antenna interface. This approach eliminates the need for a bulky and lossy matching network (MN), thereby simplifying implementation and enhancing the energy efficiency of the TX. Moreover, unlike other implantable TXs that utilize conventional free-space communication antennas, our TX features an antenna specifically optimized in size and structure for neural implants. A compact and power-efficient ring oscillator (RO)-based wide-band phase-locked loop (PLL) is proposed to generate an RF carrier within the 3-6-GHz UWB band. Additionally, we introduce a 4-bit asymmetric pulse shaping technique that reduces intersymbol interference (ISI), enhancing data transmission rates. The proposed TX was fabricated in a 65-nm TSMC process. The wireless module, including a custom-designed printed antenna, has a compact form factor of only 49.8 mm 2. The efficiency of the wireless link was demonstrated in vitro at implantation depths of 3 and 15 mm, achieving a record-long transmission range of over 1 m and a distance-normalized energy efficiency below 27 pJ/b/m at data rates up to 500 Mb/s.