A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.
Control grid 
The grid nearest the cathode is the "control grid"; the voltage applied to it causes the anode current to vary. In normal operation, with a resistive load, this varying current will result in varying (AC) voltage measured at the anode. With proper biasing, this voltage will be an amplified (but inverted) version of the AC voltage applied to the control grid, thus the tetrode can provide voltage gain.
Screen grid 
The second grid, called "screen grid" or sometimes "shield grid", provides a screening effect, isolating the control grid from the anode, reducing the parasitic capacitance between the two. This helps to suppress unwanted oscillation, and to reduce an undesirable effect in triodes called the "Miller effect", where the gain of the tube causes a feedback effect which increases the apparent capacitance of the tube's grid, limiting the tube's high-frequency gain. In normal operation the screen grid is connected to a positive voltage, and bypassed to the cathode with a capacitor. This shields the grid from the anode, reducing Miller capacitance between those two electrodes to a very low level and improving the tube's gain at high frequencies. When the tetrode was introduced, a typical triode had an input capacitance of about 5 pF, but the screen grid reduced this capacitance to about 0.01 pF. 
As the screen grid is positively charged, it collects electrons, which causes current to flow in the screen grid circuit. This uses power and heats the screen grid; if the screen heats up enough it can melt and destroy the tube. There are two sources of electrons collected by the screen grid—in addition to the electrons emitted by the cathode, the screen grid can also collect secondary electrons ejected from the anode by the impact of the energetic primary electrons. Secondary emission can increase enough to decrease the anode current, since a single primary electron can eject more than one secondary electron. The reduction in anode current is because the external anode current (through the connection pin) is due to the cathode-to-anode current minus the secondary emission current. This can give the tetrode valve a distinctive negative resistance characteristic, sometimes called "tetrode kink". This is usually undesirable, although it can be exploited as in the dynatron oscillator. The secondary emission can be suppressed by adding a suppressor grid, making a pentode, or beam plates to make a beam tetrode/kinkless tetrode.
The positive influence of the screen grid in the vicinity of the control grid allows a designer to shift the control grid operating voltage range entirely into the negative region (a triode of similar geometry would likely require positive grid drive to attain the same maximum anode current). When any grid is driven positive relative to the cathode it can intercept electrons from the cathode, loading the drive circuitry. If the input signal causes the control grid to become positive (where current flow begins), nonlinearity is to be expected. (The control grid draws no current while negative—high impedance—but draws current while positive—low impedance.) With the control grid operating entirely in the negative region, and with the RF shielding afforded by the screen grid, tetrode input impedance is quite high even at high frequencies. Gain can be nearly flat from DC to full frequency. Linearity is good. Power gain in excess of 10,000 is possible.
The triode vacuum tube also develops a "space charge" between the cathode and control grid, which reduces its gain, especially at low anode voltages. The screen grid of a tetrode neutralizes the space charge and increases the tube's gain.
Power tetrodes are commonly used in radio transmitting equipment, because the need for neutralization is less than with triodes (see Radio transmitter design and Valve amplifier for more details). Screen current does represent loss. Some tube designers attempt to minimize screen current by placing each wire in the screen mesh directly behind a corresponding wire in the control grid mesh. Propagating electrons emerge from the control grid as a projected image of openings in the grid. By placing the screen in the shadow of the control grid, interception of electrons by the screen is minimized in normal operation. Screen current is negligible in many designs. Shadow grids are used in a variety of forms for a number of applications.
- Circuit design considerations
Under certain operating conditions, the tetrode exhibits negative resistance due to secondary emission of electrons from the anode (to the screen). The shape of the characteristic curve of a tetrode operated in this region led to the term "tetrode kink".  In general, if the anode voltage exceeds the screen voltage, this region is avoided, and good performance can be expected. But this lower limit on total tube voltage drop prevents widespread adoption of tetrodes for consumer amplification applications. Secondary emissions from a screen have the effect of pulling the screen upward, toward the anode voltage. This implies the need for both source and sink current capability in the ideal screen power supply. A bleeder resistor can usually be selected to prevent the screen voltage from getting out of control. Arcs from the anode generally hit the screen. As such, special care is required in design of the socket wiring, to provide a direct discharge path for arc current. The undesirable nature of the tetrode kink led tube designers to add a third grid, called the suppressor grid; the resulting vacuum tube is called a pentode. More modern tubes have anodes treated to minimise secondary emission.
The tetrode tube was developed by Dr. Walter H. Schottky of Siemens & Halske GmbH in Germany in 1919. Thousands of variations of the tetrode design, as well as its later development the pentode, have been manufactured since then, although vacuum tubes in low-power equipment have been almost totally superseded by solid-state semiconductor devices.
See also 
- * L.W. Turner, (ed), Electronics Engineer's Reference Book, 4th ed. Newnes-Butterworth, London 1976 ISBN 0 408 00168 page 7-19
- Turner 1976, page 7-19
- Turner 1976, page 7-20
- http://www.ece.umd.edu/~taylor/Electrons3.htm A thumbnail history of electronics, retrieved 2009 Feb 5