Field-effect transistor versus analog transistor (static induction transistor)

The reason why the usual FET shows the saturated characteristics has been shown that with increasing drain voltage, the effect of the negative feedback action, increases through a marked increase of the Series channel resistance in the neighborhood of the pinch-off voltage, under which condition the apparent transfercon-ductance
G_{m'}= G_{m}/(1 + r_{s}.G_{m})
becomes
G_{m'} \simeq r_{s}^{-1}
. It is also pointed out that a transistor in analogy to the vacuum type proposed by Watanabe and Nishizawa in 1950, exhibits the nonsaturated build-up character only When the internal negative feedback action is as little as
G_{m'} \simeq G_{m}
. In this case, when the channel has not yet pinched off, the characteristics are ohmic and then the transistor can operate as a good variable resistor; on the other hand, when the channel has already pinched, the transistor shows the build-up characteristics similar to those of a vacuum tube triode as a result of the static induction from the drain. The transistor similar to that of the vacuum tube triode type is named "Static Induction Transistor," because its output character is based on the static induction as well as input characteristics. The SIT has the exponential characteristics in contrast with the "Analog Transistor" which is expected by Shockley to follow the space-charge conduction law. The SIT has already been ascertained to have low noise, low distortion, and high-power capability, and its fabrication has been already realized in the form of a high-power transistor (2 kW, 8 MHz), a high-frequency transistor (a few watts, UHF), and a high-speed thyristor. Microwave transistors and very high-speed integrated circuits are being constructed, as well as variable resistors.