Power Electronics Quiz: Semiconductor Devices!

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled device because its operation is controlled by the voltage applied to the gate terminal. The gate voltage determines the conductivity of the channel between the source and drain terminals, allowing or blocking the flow of current. This characteristic makes the MOSFET suitable for various applications, such as amplification and switching, where precise control of the current flow is required. Unlike a BJT (Bipolar Junction Transistor) or a diode, which are current-controlled devices, the MOSFET's behavior is primarily determined by the applied voltage.

The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) can operate at high switching frequencies due to its ability to switch on and off quickly. This is because of its structure, which consists of a metal gate separated from the semiconductor channel by an insulating oxide layer. This allows for efficient control of the current flow, making it suitable for high-frequency applications such as power amplifiers, motor drives, and switching regulators. The other options, BJT (Bipolar Junction Transistor), TRIAC (Triode for Alternating Current), and IGBT (Insulated Gate Bipolar Transistor), may not be as efficient in high-frequency operations.

The correct answer is "All of the above". In a UJT triggered SCR half-wave rectifier, the same supply is used for both the main circuit and the firing circuit. The firing circuit requires a DC supply, and the SCR is used in the main circuit while the UJT is used in the firing circuit. Therefore, all of the given statements are true for a UJT triggered SCR in a half-wave rectifier.

In a Bipolar Junction Transistor (BJT), there are inherent power losses due to the continuous flow of current through the device, even when it's in the "on" state. These losses, known as "I^2R losses" (where I is the current and R is the resistance), can make BJTs less efficient than Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) in certain applications.

MOSFETs, on the other hand, have very low power dissipation when in the "on" state because they act like a resistor with very low resistance (channel resistance) when fully conducting. This means they have minimal I^2R losses compared to BJTs, making them more efficient in many situations.

The given options refer to different types of commutation. Class B commutation is a type of commutation where the current in the load is transferred from one device to another through a commutation circuit. This type of commutation is commonly used in power electronic circuits to ensure smooth and efficient current transfer. Class C, D, and E commutation are different types of commutation methods used in various applications, but they are not the correct answer in this case.

The maximum possible firing angle using an R firing circuit is 90 degrees. This is because an R firing circuit is used to control the firing angle of a thyristor in a phase-controlled rectifier. The firing angle determines the point at which the thyristor is triggered to conduct current. In an R firing circuit, the firing angle can be varied from 0 to 90 degrees. Therefore, the correct answer is 90 degrees.

A Triac is a fully controlled ac switch because it can conduct current in both directions, allowing it to control the flow of alternating current. It is commonly used in applications that require dimming or speed control, such as in lighting or motor control systems. The other options, SCR, Diac, and IGBT, are also semiconductor devices, but they have different characteristics and are not fully controlled ac switches.

The presence of a parasitic diode in a MOSFET depends on its construction. MOSFETs are composed of a source, drain, and gate, and the presence of a diode is determined by the way these components are connected. In some MOSFET designs, the construction may result in a parasitic diode being formed either parallel or anti-parallel to the main MOSFET structure. Therefore, whether a MOSFET has a parasitic diode or not is contingent upon its specific construction.

Parallel operation is possible in BJT but it is not advisable due to negative on state thermal resistance

The reverse recovery is more in a general diode compared to a Schottky diode and a Zener diode. Reverse recovery refers to the time it takes for the diode to switch from the conducting state to the non-conducting state when the voltage across it changes from forward bias to reverse bias. General diodes have a higher reverse recovery time compared to Schottky diodes and Zener diodes, which have faster switching characteristics. Therefore, the reverse recovery is more pronounced in a general diode.