Thermionic Diode Characteristics

Study Notes

Anode surrounds the cathode in vacuum tubes, facilitating electric current flow.
Anode is larger than the cathode to efficiently dissipate heat, often equipped with cooling fins.
Unidirectional current flow: Current flows from cathode to anode, blocked in the reverse direction.
Non-linear current-voltage relationship: Current is not directly proportional to voltage.
Threshold voltage of 0.5-1.5V is required to initiate current flow.
Saturation current occurs beyond a certain voltage, resulting in constant current.
Temperature influences electron emission from the cathode; higher temperatures increase emission.
High voltage drop (100-200V) is necessary for significant current flow.
Low current density compared to modern semiconductor diodes.
Slow switching times due to the electron emission and transport process.
Higher power consumption to heat the cathode and maintain a vacuum.
Glass or metal construction makes thermionic diodes fragile and bulky.

Describes current density (J) from thermionic emission using the equation:
- J = AT²exp(kT−W)
Variables include:
- J: Current density
- A: Richardson's constant
- T: Absolute temperature
- W: Work function energy
- k: Boltzmann constant
Key in understanding electron behavior from heated surfaces; relevant in vacuum tubes and electron microscopes.

Comprised of an anode and cathode in a vacuum-sealed glass bulb.
Current initiated by heating the cathode, leading to thermionic emission.
If anode is positive relative to the cathode, it repels cathode electrons, halting current flow.
Construction features:
- Anode: Hollow cylinder usually made of nickel or molybdenum.
- Cathode: Nickel cylinder covered with barium oxide and strontium.

Defines maximum space-charge relationship between current density (J), potential difference (Vd), and distance (d):
- J = K*Vd^(2/3)
Assumptions for validity:
- Electrons travel ballistically with negligible scattering.
- Space charge of ions in interelectrode region is negligible.
- Electrons have zero velocity at cathode surface.

Vacuum tubes and electron guns designed for specific electron beam properties.
Plasma sheath formation at solid surface boundaries affects sheath thickness and potential drop.
Methods guide electron beam generation in particle accelerators.

Composed of fast-moving electrons emitted from the cathode.
Travel in straight lines, are negatively charged, and can be deflected by electric and magnetic fields.
Unique properties include causing fluorescence under certain conditions.