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The theory and operating characteristics of a new semiconductor tetrode is discussed in this paper. This semiconductor junction device has the usual geometry of the planar transistor, but with an additional metal electrode placed on the oxide which covers the surface of the emitter-base junction. This electrode serves as a grid. The grid-base voltage controls the surface potential, surface recombination rate and the size of the surface channel, and thereby the current gain of the transistor. In the common emitter connection the grid serves as a second input in addition to the base input. Input impedance of the grid is typically in the range of 1 to 100 pf and 1015 ohms. Transconductances dIC/dVGE of several thousand micromhos have been achieved in the common emitter connection. The cutoff frequency of the transconductance is approximately the same as that of the hfe.