We　investigated　the　field　emission　(FE)　enhancement　of　semiconductor　thin　films　on　metal　substrate　by　first-principles　calculations.　For　the　FE　structure　of　GaN　or　AlN　thin　films　on　aluminum　substrate,　the　calculated　results　show　that　by　fine-tuning　the　film　thickness,　the　FE　current　can　be　enhanced　nearly　2　orders.　It　should　be　originated　from　reducing　the　surface　work　function.　When　the　film　thickness　is　less　than　similar　to　10　nm,　the　work　function　can　be　reduced　as　much　as　0.5　eV　by　film　thickness　modulation　of　only　several　nanometers.　The　remarkable　thickness　effects　on　the　work　function　for　the　semiconductor/metal　structure　result　mainly　from　surface/interface　charge　transfer　and　interface　states.