When is a Gate potential zero?
\$ V_{GS}\$ is what determines if the transistor is on or off. For an NMOS transistor \$ V_{GS}\$ is positive so the transistor turns on. If the transistor were fully off, \$\varphi_s\$ would be -5V and \$ V_{GS}\$ would be 5V which would cause the transistor to turn on.