Vacuum level

1. No electrical connection between emitter (cathode) and collector (anode) . The vacuum levels are aligned Notation: EF Fermi energy EFE ,EFC Fermi energy of emitter and collector respectively WE ,WC Work function of emitter and collector respectively Photon energy hn KEE Vacuum level Vertical axis is energy Emitter WE Collector Kinetic energy of the electron when leaving the emitter Is: WC EFE EFC Emitter and connector are connected (V=0). Note that now the Fermi energies in the two metals are aligned Kinetic energy of the electron when reaching the collector. Is: KEC KEE Collector WC -WE WC Emitter WE

2. Retarding voltage Apply a retarding voltage V between collector and emitter. The two Fermi energies are now offset by eV where e is the electron charge (note e is negative) KEC Now: KEE Collector eV+WC -WE WC Emitter WE Potential difference between collector and emitter as seen by the electron: eV Condition of zero current corresponds to Zero Kinetic energy of the electron when reaching the collector: PROVIDED that the photoelectric effect happened ie:

3. Accelerating voltage Now consider Forward bias - accelerating voltage KEE KEC Potential difference between collector and emitter as seen by the electron: Emitter WE Collector WC eV Now it looks like the electron velocity and therefore the current increases with V. But the current cannot increase indefinitely it will saturate when the electron flux equals to the incoming photon flux where P is the light power impinging on the emitter.

4. Saturation voltage electron is neither accelerated nor retarded Now consider Forward bias - accelerating voltage KEC KEE Emitter WE Collector WC eV Now it looks like the electron velocity and therefore the current increases with V. But the current cannot increase indefinitely it will saturate when the electron flux equals to the incoming photon flux where P is the light power impinging on the emitter.