Static Induction Thyristors (SITHs) are power semiconductor devices that exhibit characteristics similar to both Silicon Controlled Rectifiers(SCRs) and Gate Turn-Off thyristors(GTO). SITHs have a circuit symbol similar to an SCR and can be turned on by applying a short positive pulse between the gate and cathode, similar to an ordinary thyristor.
One notable feature of SITHs is their low on-state voltage drop, which means that they have a relatively low voltage loss when conducting current in the on-state. This characteristic makes SITHs desirable for high-power applications where minimizing power losses is crucial.
To turn off a SITH, a short negative pulse of large current is applied between the gate and cathode, similar to the turn-off mechanism of a GTO. This negative pulse effectively interrupts the current flow through the device, allowing it to turn off.
SITHs are available with voltage ratings up to about 2500 V and current ratings up to 500 A. These devices can be used in medium power converters that operate at frequencies beyond those typically used for GTOs.
SITh offers advantages such as low noise, low distortion, and high audio frequency power capability. Additionally, it has very short turn-on and turn-off times, typically around 0.25 microseconds.
SITHs offer a combination of low on-state voltage drop and high voltage/current ratings, making them suitable for various power electronic applications where efficient power conversion and control are required.
Circuit symbol
History
The first static induction thyristor (SITh) was invented by Jun-ichi Nishizawa, a Japanese engineer, in 1975. It featured a low forward bias, high current conduction, and a short turn-off time. Tokyo Electric Co. (now Toyo Engineering Corporation) commercially released a self-controlled gate turn-off thyristor based on this design in 1988. The original device comprised a p+nn+ diode and a buried p+ grid.
In 1999, an analytical model of the SITh was developed for the PSPICE circuit simulator. PSPICE is a widely used electronic circuit simulation software, and the development of an analytical model for the SITh would have facilitated its analysis and design in virtual environments.
In 2010, a newer version of the SITh was developed by Zhang Caizhen, Wang Yongshun, Liu Chunjuan, and Wang Zaixing. This newer version of the SITh was notable for its high forward blocking voltage, which indicates an improvement in its ability to withstand high voltages in the forward direction.
Structure
The structural representation of a Static Induction Thyristor (SITH) is similar to a Static Induction Transistor (SIT), with a small variation. The SITH is a four-layer device with PNPN layers, similar to other thyristors. It can be considered as a p+nn+ diode with gate electrodes of p+ configuration buried within the n layer. Additionally, on the anode side of the SITH, there is a p+ layer present.
The basic structure of a SITH consists of the following layers:
- N+ Layer: This is the heavily doped n-type layer acting as the cathode terminal of the device.
- N- Layer: This is the lightly doped n-type layer, also known as the drift region. It provides the main resistance to the flow of current.
- P Layer: This layer is the lightly doped p-type layer acting as the gate region. The gate electrodes of p+ configuration are buried within this layer.
- P+ Layer: This is the heavily doped p-type layer present on the anode side of the device. It provides an additional contact for the anode terminal and helps in controlling the device operation.
In addition to these layers, similar to a Gate Turn-Off Thyristor (GTO), n+ fingers are diffused in the p+ anode layer of the SITH. These fingers help in improving the current spreading and reducing the on-state voltage drop of the device.
The SITH structure combines the characteristics of a thyristor and a static induction transistor, with the addition of the p+ layer on the anode side and the buried gate electrodes in the n-layer.
Working
Turn-On Mechanism
- In the forward biased condition of the anode, the SITH acts as a diode, allowing current to flow from the anode to the cathode.
- Even with a zero voltage between the gate and cathode, the device is in an on-state. This characteristic makes the SITH a normally-on device.
- Applying a small positive gate voltage further improves the on-state performance of the device.
Turn-Off Mechanism
- Under forward biased conditions of the anode, when the gate terminal is reverse biased relative to the cathode, a depletion layer forms at the p+n junction.
- The depletion layer prevents the flow of anode current from the anode to the cathode, effectively turning off the device.
- By varying the magnitude of the negative potential at the gate terminal, the anode current can be controlled.
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Reverse Bias Condition
- When the cathode is made positive with respect to the anode (reverse biased condition), a reverse current flows from the cathode to the anode.
- The reverse current occurs due to the flow of electrons from the anode intermixed n+ layer towards the n layer, passing through the p+ grid, and eventually reaching the cathode via the n+ region.
- It is important to note that the SITH does not have reverse blocking capability, meaning it cannot block reverse current flow when the cathode is made positive with respect to the anode.
Application
The main application of a static induction thyristor (SITh) is in high-power electronic devices and systems that require efficient control of electric power. SITHs are commonly used in applications such as power supplies, motor drives, voltage regulators, and high-frequency inverters. They are especially suitable for high-power and high-frequency applications due to their fast switching speed, high current capability, and low conduction losses.