Thyristors, MOSFETs, and IGBTs: Construction and Advantages

SCR Construction

A thyristor, or SCR, consists of four P-N-P-N layers and has three PN junctions (J1, J2, J3) in series. It has three terminals: Anode, Cathode, and Gate. The Gate terminal is attached to the P-type layer near the Cathode terminal. The two-transistor model shows that an SCR is a combination of one PNP transistor and one NPN transistor.

The emitter of the PNP transistor is taken as the Anode terminal, the emitter of the NPN transistor is taken as the Cathode, and the base of the PNP is taken as the Gate terminal. The base of the PNP is connected to the collector of the NPN, and the collector of the PNP is connected to the base of the NPN transistor.

Advantages of SCRs:

• Easy to turn ON
• Can handle large voltage, current, and power
• Possible to control AC power
• Can be protected with a fuse
• Low cost

Disadvantages of SCRs:

• Conducts in only one direction, controlling only during the half cycle
• Cannot be used at high frequencies
• Gate current should be positive
• Very difficult to turn OFF

MOSFET Construction

A power MOSFET (metal-oxide-semiconductor field-effect transistor) is a device that controls current flow between two terminals by changing the gate terminal voltage. The structure of a power MOSFET is vertically oriented and has four layers of alternating P and N-type layers. The P-type middle layer is called the body of the MOSFET. The N-layer is called the drift region, which determines the device's breakdown voltage.

Steps to make a MOSFET:

1. Make the body: The body is typically made from a P-type semiconductor.
2. Form the source and drain terminals: These are made from heavily doped N-type regions on either side of the body.
3. Deposit silicon dioxide: This provides electrical isolation.
4. Deposit a thin metallic layer: This forms a capacitor-like structure.
5. Pattern and etch the metallic layer: This defines the gate terminal.

IGBT Characteristics and Merits

V-I characteristics of an N-channel IGBT. Sometimes the collector is also called the drain, and the emitter is also called the source. The above characteristics are plotted for drain (collector) current I_d with respect to drain-source (collector-emitter) voltage V_DS. The characteristics are plotted for different values of gate-to-source (V_GS) voltages. When the gate-to-source voltage is greater than the threshold voltage V_th, the IGBT turns ON. The IGBT is OFF when V_CC is less than V_GS. The figure shows the 'ON' and 'OFF' regions of the IGBT. The BV_DSS is the breakdown-to-source voltage when the gate is open-circuited. The IGBT is a popular device nowadays. It has the simplest drive circuit and low ON-state losses.

Merits of IGBTs:

1. Voltage-controlled device. Hence, the drive circuit is very simple.
2. ON-state losses are reduced.
3. Switching frequencies are higher than thyristors.
4. No commutation circuits are required.
5. The gate has full control over the operation of the IGBT.
6. IGBTs have approximately flat temperature coefficients.