Low-Pressure Gas Thermionic Diodes

In the gas or mercury-vapor diode, the space charge around the cathode is neutralized by positive ions produced by ionization by collision in the following way. If the voltage between the cathode and the anode approximately equals the ionizing potential of the gas (about 10 to 12 volts for mercury vapor), then some of the negative electrons being drawn over to the positive anode attain velocities sufficient to knock negative electrons out of the otherwise neutral gas atoms if a collision occurs.

As a result of such collisions, positive ions are formed. They consist of the relatively massive atoms that have lost a negative electron. Because of their large mass, the positive ions drift slowly toward the negative cathode and hence remain in the space between the electrodes for a relatively long time. This accumulation of positive ions neutralizes the negative space charge. For this reason, the voltage drop across a gas or mercury-vapor tube equals approximately the ionizing potential of the gas or vapor. Accordingly, the voltage drop is much lower than that across a high-vacuum tube. Furthermore, over the operating range, the voltage drop is essentially independent of the magnitude of the current being rectified (Fig. 5).