SIT MCT FCT Static Induction Thyristors (SIT) • Construction and output characteristics • A SIT or SI-thyristor, is a self controlled GTO like ON-OFF device. It is a three terminal device. The three terminals are Anode, Cathode and Gate. • The characteristics of SIT are very similar to those of a power MOSFET. It is turned on by applying a positive gate voltage and turned off by applying a negative gate voltage. • It is minority carrier device. Its important characteristics are fast switching speeds, high dv/dt, di/dt capacity and low on state resistance. • Figure (a) shows the basic structure of SIT and Figure (b) shows its circuit symbol. • This device does not have a reverse blocking capability that it will start conducting means immediately if a reverse voltage is applied between its anode and cathodes. • The turn off characteristics of SIT are similar to these of a GTO i.e. to turn off SIT, it is necessary to apply a large negative gate current. • SIT is a normally ON device i.e. if its anode is positive and gate voltage is zero, then the device will behave like a forward biased diode and the current will flow freely. • The forward biasing of the P+ N junction will cause a hole injection into the Nregion and its conductivity will change. • When the gate is reverse biased, with respect to cathode, a depletion layer will block the anode current and the device gets OFF. Features of SIT 1.SIT is a normally ON device. 2.The ON state voltage drop is high. 3.Low turn off gain i.e. large negative IG is required for turning off SIT. 4.High switching frequency. 5.High dv/dt and di/dt ratings. MCT • Out of many semiconductor controlled devices, MCT is considered to be the latest. The device is basically a thyristor with two MOSFET’s built into the gate structure. A MOSFET is used for turning ON the MCT and another one is used for turning it OFF. The device is mostly used for switching applications and has other characteristics like high frequency, high power, and low conduction drop and so on. An MCT combines the feature of both conventional four layer thyristor having regenerative action and MOS- gate structure. In this device, all the gate signals are applied with respect to anode, which is kept as the reference. In a normally used SCR, cathode is kept as the reference terminal for gate signals. MCT The equivalent circuit of the MCT is shown in the figure below. • It consists of an ON-FET, an OFF-FET and two transistors. The MOS structure of the MCT is represented in the equivalent circuit. It consists of one ON-FET, a p-channel MOSFET, and an OFF-FET. Both n-p-n and p-n-p transistors are joined together to represent the np-n-p structure of MCT. An n-channel MOSFET is represented by drawing the arrow towards the gate terminal. A p-channel MOSFET is indicated by drawing the arrow away from the gate terminal. The two transistors in the equivalent circuit indicate that there is regenerative feedback in the MCT just as it is an ordinary thyristor. Turning ON Process • The device is turned ON by a negative voltage pulse at the gate with respect to the anode. • For turning ON MCT, gate is made negative with respect to anode by the voltage pulse between gate and anode. So, MCT must be initially forward biased, and then only a negative voltage be applied. • With the application of this negative voltage pulse, ON-FET gets turned ON whereas OFF-FET is already OFF. With ON-FET ON, current begins to flow from anode A, through ON-FET and then as the base current and emitter of n-p-n transistor and then to cathode K. This turns on n-p-n transistor. • This causes the collector current to flow in n-p-n transistor. As OFF FET is OFF, this collector current of npn transistor acts as the base current of p-n-p transistor. Subsequently, p-n-p transistor is also turned ON. If both the transistors are ON, regenerative action of the connection scheme takes place and the MCT is turned ON. Turning OFF process • The device is turned OFF by applying a positive voltage pulse at the gate. The positive voltage pulse causes the OFF-FET to turn ON and ON-FET to turn OFF. After OFF-FET is turned ON, emitter based terminals of p-n-p transistor are short circuited by OFF-FET. So, now anode current begins to flow through OFF-FET and thus base current of p-n-p transistor begins to decrease. The device has the disadvantage of reverse voltage blocking capability. Field Controlled Thyristor (FCT) • t 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 Reverse 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 J1 is 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. • 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 VGS. • 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. Output (anode) characteristics of FCT • Figure 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 VGS to turn it OFF.
