  On-chip electron sources have wide potential applications in miniature vacuum
electronic devices and emission efficiency is one of their performance
benchmarks. A cascade electron source based on series metal-insulator-metal
horizontal tunneling junctions is proposed, where free electrons are additively
extracted from each tunneling junction. A cascade electron source with n
horizontal tunneling junctions shows a theoretical emission efficiency of
approximately {\eta}(n)=1-(1-{\eta}_0 )^n, with {\eta}_0 being the efficiency
of a single tunneling junction. Experimentally, a cascade electron source with
three Si-SiOx-Si tunneling junctions is demonstrated, achieving an emission
efficiency as high as 47.6%. This work provides a new way of realizing highly
efficient on-chip electron sources.
