We extract an AlGaN surface state spectrum with a density ( \$\{D\}\_\{\textbackslash{}text\{SS\}\}\$ ) ranging between \$1.4\textbackslash{}times10\$ 12 and \$4.7\textbackslash{}times10\$ 12 eV-1cm-2. The low \$\{D\}\_\{\textbackslash{}text\{SS\}\}\$ is achieved with in situ SiN passivation. \$\{D\}\_\{\textbackslash{}text\{SS\}\}\$ is extracted from ungated AlGaN/AlN/GaN heterostructures with varied AlN thicknesses between 0 and 2 nm: increased AlN thicknesses in heterostructures monotonously increase two-dimensional electron gas (2DEG) densities \$\{N\}\_\{\textbackslash{}text\{sh\}\}\$ and AlGaN surface potential energies \$\{q\} \{\textbackslash{}varphi \} \_\{\textbackslash{}text\{s\}\}\$ , and \$\{D\}\_\{\textbackslash{}text\{SS\}\}\$ is extracted with \$\textbackslash{}boldsymbol \{\textbackslash{}Delta \} \{N\}\_\{\textbackslash{}text\{sh\}\}\$ - \$\textbackslash{}boldsymbol \{\textbackslash{}Delta \} \{\textbackslash{}varphi \}\_\{\textbackslash{}text\{s\}\}\$ correlations. The \$\{D\}\_\{\textbackslash{}text\{SS\}\}\$ extraction methodology in this work features calibrated polarization charge values, Hartree approximation-based 2DEG profile calculation, and incorporated C-doped GaN (C-GaN) substrate impact on 2DEG. We also extract the charge neutrality level at the AlGaN surface, which is 0.95 eV below conduction band minimum.