Shot Effect

small random deviations of the anode current in vacuum electrical and semiconductor devices from its mean value; caused by nonuniformity of electron emission from the cathode or by nonuniform diffusion of charge carriers in semiconductors. The effect was predicted theoretically by the German scientist W. Schottky in 1918.

Upon heating of the cathode of a vacuum electrical device the average speed of thermal motion of the conduction electrons is increased. Some of the electrons—those having sufficient kinetic energy—“escape” from the cathode. How-ever, before leaving the cathode, the electron undergoes a very large number of collisions with atoms and other electrons inside the cathode. As a result, the magnitude and direction of the velocity of each electron at the moment of departure from the cathode may be different. Therefore, the departure of individual electrons appears to occur altogether at random, independent of the departure of other electrons. As a result, the number of electrons emitted by the cathode during identical short time periods differs, and in turn the emission current experiences random deviations (fluctuations) from its mean value. The magnitude of the fluctuations of the anode current depends mainly on the conditions under which the device is operating. In electron tubes, if all emitted electrons strike the anode, the emission fluctuations are precisely repeated in the anode current. However, if not all of the electrons strike the anode, then around the cathode a negatively charged cloud is formed that acts as a “damper” and smooths out the shot fluctuations of the anode current.

The shot effect is not characteristic only of thermionic emission; it accompanies any process associated with the formation of flows of charged or neutral particles—for example, the passage of electric current through semiconductors, photoelectric emission, secondary electron emission, and the formation of molecular beams.

The term “shot effect” (and also “shot noise”) originated because, as a result of this phenomenon, an acoustic noise resembling the noise of falling pellets occurs in a loudspeaker connected to the output of an amplifier or radio.