A Short Introduction to Static Induction Transistors
In the mid 1970's a special variety of Jfet invented in Japan called a
Static Induction
Transistor
(SIT) found its way into the “Vfet” power amplifiers produced by Yamaha and Sony.
These amplifiers were produced for several years and then discontinued, but are still highly
regarded in the high end audio community.
SIT devices have a unique characteristic which is of particular value for audio amplifiers.
Quoting inventor Nishizawa's patent abstract, “(The) drain-current to drain-voltage
characteristic simulates the anode-current to anode-voltage characteristic of the triode
vacuum tube very closely.” They have found use in radar and other exotic applications, but
after Sony and Yamaha ceased production, versions suitable for audio power amplification
have been highly prized and difficult to obtain.
Recently there has been renewed interest in SITs, partly because two audio companies have
stepped up to the plate and spent the money required to fabricate new devices suitable for
audio power amplifiers. The first of these is Digital Do Main in Japan, which has produced
two audio amplifiers based on newer versions of original Yamaha Silicon parts. The other is
First Watt, which arranged for a production run of a new SIT device using a newer Silicon
Carbide process by SemiSouth.
To see what's special about SITs, take a look at the curve of a Pentode tube:
Here we see a graphic describing the amount of current which will flow through a Pentode as
a function the Plate to Cathode voltage and eight different values of the Grid control voltage.
