Electronic Design

First Field-Effect Transistor Available

A semiconductor amplifying device with up to 100-meg input impedance is now available from an American manufacturer. (Some French firms already have announced field-effect devices.) Crystalonics, Cambridge, Mass., is selling field-effect transistors for $35 to $72 each. However, the price should eventually come down below that for conventional transistors, since the new devices are simpler to make, the firm said.

As a circuit element, the field-effect transistor is similar to a vacuum tube. (The manufacturer has adopted "anode," "cathode," and "grid" terminology for the electrodes.) The main conduction path (see sketch) is through a bar of n-type silicon with ohmic "anode" and "cathode" contacts at either end. Control is through depletion layers projected by p-n junctions formed on the sides of the main n-type bar. The high input impedance comes from the reverse bias applied to the p-n control "grid" junctions.

The depth of the depletion layers is in turn controlled by the amount of reverse bias on the junctions. In effect, the depletion layers "choke" off the current flow through the main bar, the maximum effect being achieved when they meet in the center.

Crystalonics has listed six amplifier types (designations (C610-C615) and four switching types (C650-C653). All are supplied in TO-5 packages. Maximum anode currents are 50 mA and maximum power dissipations are 250 mW. The transconductances vary from 100 to 1200 at normal temperatures. But, unlike transistors, the transconductance increases at lower temperature. At the temperature of liquid nitrogen, a device which has a gm of 500 at room temperature was reported to have a gm of 5000. Interelectrode capacitances are high, 35 to 50 pf, and do present design problems at frequencies as low as 1 kc. However, the transconductance itself remains constant up to 250 mc, so the device can be used in tuned amplifiers up to the kmc region. (Electronic Design, March 29, 1961, p. 66)

I can't help wondering if the junction FET had been introduced earlier, when engineers were struggling to understand bipolar transistors, whether its similarities with the vacuum tube would have greatly expanded its use. As it was, it did find applications. By then, though, conventional transistors were well understood, and the JFET's impact was limited. Crystalonics, which was founded in 1958, was acquired by Teledyne in 1963 and was sold to its current owner in 1992. Today, it continues to supply discrete devices, including many transistors that were previously discontinued by Motorola Semiconductor.

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