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A field effect transistor1 is a unipolar device, distinguishable from a junction transistor in the following two ways: 1. the current flow is carried predominantly by one type of carrier alone; and 2. the electric field intensity is relatively high, thus the current flow is caused by field drift instead of by diffusion. Fig. 1(A) shows the construction of a field effect transistor. It consists of a piece of n-type germanium or silicon slab with a p-type junction formed around the center of the slab. Two ohmic contacts are placed in the vicinity of the junction. Three electrodes are connected to the transistor. The electrode connected to the junction is called the Â¿gate.Â¿ The electrodes connected to the ohmic contacts are called the Â¿sourceÂ¿ and the Â¿drain.Â¿ Five papers have already appeared on the field-effect transistors1Â¿5 in which the design theory, their experimental verification, the d-c equivalent circuit, and the estimate of the maximum cutoff frequency were presented. This paper describes an a-c equivalent circuit of the field effect transistor; the measured parameters of a type of germanium field effect transistor, and some of its circuit applications, with emphasis on oscillators and switching circuits. Special features of a field effect transistor with cutoff frequency near 5 mc (megacycles) are also presented.