Field Controlled Thyristor (FCT)

Field Controlled Thyristor (FCT)

It is a new developed thyristor which is also called as field controlled diode (FCD) or bipolar static induction thyristor (BSITH).
Similar to the conventional SCR, FCT also can block the RU current flow for both polarities of anode voltage and allows only forward current to flow.

Basic structure of FCT

Figure (a) shows the basic structure of FCT and Figure (b) shows its circuit symbol.
FCT is a three terminal device. The terminals are anode, cathode and gate. The gate is control terminal.

Operation of FCT

This device can be operated by grounding the cathode and applying bias voltages to the anode and gate with respect to cathode.
It is necessary to forward bias s the device for its successful operation. With negative voltages applied to anode, the device blocks anode current because junction J₁is reverse biased.
With positive voltage applied to anode, the junction J₁ is forward biased the device will allows the anode current to pass easily through it.

Figure (a)

Figure (b)

If we want the FCT to remain OFF even after the application of positive anode voltage then we have to apply a negative voltage to the gate with respect to cathode. This happens because the junction J₂ is reverse biased due to the negative V_GS.
But if we increase the positive anode voltage, then FCT will turn ON in spite of application of negative gate voltage.
In short when the positive anode voltage is increased above a particular value for each applied VGs the FCT will be turn ON.

Blocking gain

The ratio of anode voltage (at which FCT turns on) to the corresponding gate voltage is called as the blocking gain of FCT.

Output (anode) characteristics of FCT

Figure (c) shows the I-V characteristics or output characteristics of FCT.
It shows that the reverse characteristics is same as that of an SCR.
The forward breakdown voltage of FCT increases as we increase the negative gate voltage.
Thus an FCT has a normally ON characteristics and requires application of a negative V_GS to turn it OFF.

Figure (c)

Why is it called FCT ?

The turn on process is controlled by the negative gate to cathode voltage. Hence this device is called as field controlled thyristor.

How to turn off FCT ?

A conducting FCT can be turned off by applying a large negative gate to cathode voltage.
A conducting FCT can turn off very quickly. Its turn off time is less than one microsecond.

Features of FCT