What Are Static Induction Transistors – SIT?
The static induction transistor (SIT) is a type of field-effect transistor (FET) and was invented by Japanese engineers Jun-ichi Nishizawa and Y. Watanabe in 1950. Static Induction Devices (SID) are a family of devices that includes; static induction transistor (SIT), static induction thyristor, static induction diode (SID), static induction transistor logic (SITL), space charge limiting load (SCLL) and static induction MOS transistor (SIMOS).
Back in the mid seventies, in Japan, static induction transistors were successfully fabricated and used by two audiophile companies, Yamaha and Sony, in some of their high-end VFET power amplifier designs, but were all eventually discontinued.
Well at least two companies, First Watt, and Digital Do Main are trying to bring the SIT back into fashion. To that end First Watt is about to release their SIT 4 power amplifier. It’s contains two single ended 10 watts/channel Class A amplifiers with a Triode tube-like character, it’s equipped with Tokin SlTs rated at 400 watts and 30 amps having a bandwidth extending to 50 MHz. It is said to produce predominantly 2nd order harmonic distortion with very little 3rd harmonic distortion and drives the speakers directly (via a DC blocking capacitor). The predicted MSRP is $5,000.00
So lets take a look at this older semiconductor technology and see what it is about it that makes it create a sound similar to that of a triode valve. If you need a refresher on semiconductors check out this post.
The SIT
The static induction transistor is a three terminal, vertical current flow, semiconductor device that most closely resembles a solid state version of the triode vacuum tube. It is a voltage controlled device and is one version of a field effect transistor (FET), where the voltage on the gate controls the current flow between the drain and source regions. It’s a special design of a junction field-effect transistor (JFET) and were originally referred to as vertical field effect transistors (VFET). They are capable of high-speed and high-power operation, with low distortion and low noise.
Construction & Symbol
It is a vertical structure device with short multi-channels. The device was originally known as a VFET, with V being short for vertical. Being a vertical device, the SIT structure offers advantages in higher breakdown voltages than a conventional FET. For the SIT, the breakdown voltage is not limited by the surface breakdown between gate and drain, allowing it to operate at a very high current and voltage.
The most obvious differences between the SIT and other FET’s is both its vertical structure and that it uses two internal gate contacts. Most FET’s have a single gate that controls current flow in a channel that is parallel to the gates plane. The SIT is a vertical device, with two gate contacts on opposing sides of its channel. It is this vertical current flow and additional gate control that causes the SIT to exhibit non-saturating current characteristics similar to triode vacuum tubes when under high voltage gate bias conditions. It is this unique characteristic that sets it apart from other FET’s.
Operation
These devices are generally conducting with zero Vgs and an applied Vds. This allows the electrons (majority charge carriers) to flow from the source to the drain creating the drain current Id.
ON. Electron current flow through the channel is controlled by the depletion layer size created by the value of the gate to source voltage Vgs.
As Vgs is increased the depletion layer increases in size, removing majority charge carriers (electrons) from the N type channel material and restricting the flow of the electrons from the source to the drain. Eventually at high gate voltages the channel is almost completely cut off and very little drain current flows.
OFF. High values of the gate to source voltage Vgs causes the depletion layer to remove available charge carriers preventing electron current flow.
Device Characteristics
The typical output characteristic for a pentode, tetrode, BJT and FET are shown below.
As can be clearly seen they are all similar in nature. However, compared to the typical SIT characteristic shown below, they are very different.
Typical SIT Output Characteristic
Comparing the above SIT output characteristic to a couple of common triode tubes shown below, they are seen to be very similar in nature.
The SIT therefore has an output current-voltage characteristic similar to that of a vacuum tube triode and it can, with careful circuitry design, create a similar sound to triode tubes.
SIT Advantages:
- short channel length
- low gate series resistance
- low gate-source capacitance
- small thermal resistance
- low noise
- low distortion
- high audio frequency power capability
- short turn-on and turn-off time, typically 0.25 μs
Part numbers include; Tokin 2SK180, SemiSouth PASS SIT-1, Sony 2SK82 & 2SJ28 and Yamaha 2SJ27.
Uses:
High power high frequency applications such as; microwave amplifiers, induction heaters, AM/FM VHF/UHF transmitters, power supplies and of course high-end audio power amplifiers.
For more information on SIT amplifiers visit the First Watt or Digital Do Main web sites.
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