We　report　a　tunneling　diode　enabling　efficient　and　dense　electron　emission　from　SiO2　with　low　poisoning　sensitivity.　Benefiting　from　the　shallow　SiO2　channel　exposed　to　vacuum　and　the　low　electron　affinity　of　SiO2　(0.9　eV),　hot　electrons　tunneling　into　the　SiO2　channel　from　the　cathode　of　the　diode　are　efficiently　emitted　into　vacuum　with　much　less　restriction　in　both　space　and　energy　than　those　in　previous　tunneling　electron　sources.　Monte　Carlo　simulations　on　the　device　performance　show　an　emission　efficiency　as　high　as　87.0%　and　an　emission　density　up　to　3.0　X　10(5)　A/cm(2).　By　construction　of　a　tunneling　diode　based　on　Si　conducting　filaments　in　electroformed　SiO2,　an　emission　efficiency　up　to　83.7%　and　an　emission　density　up　to　4.4　X　10(5)　A/cm(2)　are　experimentally　realized.　Electron　emission　from　the　devices　is　demonstrated　to　be　independent　of　vacuum　pressure　from　10(-4)　to　10(-1)　Pa　without　poisoning.