Step Recovery Diode presumably. They are hard to make with CMOS. An alternative is an NLTL. This dissertation shows how to make those but they use 6 layers of metal with a 180nm process: https://curve.carleton.ca/system/files/etd/4eb6a4fe-5ff0-4810-8f0c-6115dec33083/etd_pdf/59d1a6506b9a824a2ff1541d9f34d87c/li-allsiliconcmospulsecompressionnonlineartransmission.pdf