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AlGaN/GaN metal insulator semiconductor heterostructure field effect transistors (MISHFETs) with different thickness of <italic>in-situ</italic> silicon carbon nitride (SiCN) cap layer were investigated. It was found that <italic>in-situ</italic> SiCN layer not only increases the two dimensional electron gas (2DEG) density, but also effectively passivates the surface of the AlGaN/GaN MISHFET. The fabricated device with 2 nm-thick SiCN cap layer exhibits superior device performances, such as larger maximum transconductance (<inline-formula> <tex-math notation="LaTeX">$\text{g}_{\mathrm{ m}}$ </tex-math></inline-formula>) and higher on/off drain-current ratio (<inline-formula> <tex-math notation="LaTeX">$\text{I}_{\mathrm{ ON}}/{\mathrm{ I}}_{\mathrm{ OFF}}$ </tex-math></inline-formula>) compared to those of the device without SiCN cap layer.