Power Semiconductor Diodes and Transistors MCQ Quiz - Objective Question with Answer for Power Semiconductor Diodes and Transistors - Download Free PDF

Explanation:

Conductively-Modulated Field Effect Transistor

Definition: A Conductively-Modulated Field Effect Transistor (C-MOSFET) is a type of power semiconductor device that combines the high input impedance of a MOSFET and the high current-carrying capacity of a bipolar junction transistor (BJT). This combination is achieved through conductivity modulation, which enhances its performance in power switching applications. The most common device that operates on this principle is the Insulated Gate Bipolar Transistor (IGBT).

Working Principle:

The IGBT operates by leveraging both the voltage-controlled characteristics of a MOSFET and the conductivity modulation of a BJT. Here’s how it works:

• Voltage-Controlled Gate: The IGBT has an insulated gate structure similar to a MOSFET. By applying a voltage to the gate, the device is turned on or off. This high input impedance ensures low gate drive power requirements.
• Conductivity Modulation: When the IGBT is on, it allows a large current to flow through the collector and emitter. The conductivity modulation effect occurs in the drift region of the device, where the injected carriers (electrons and holes) significantly reduce the on-state resistance. This results in lower conduction losses compared to a standard MOSFET.

Advantages of IGBT:

• High efficiency due to low conduction and switching losses.
• High input impedance, reducing the gate drive power requirements.
• Capability to handle high voltages and currents, making it ideal for power electronics applications.
• Compact size and lightweight compared to traditional power devices.

Disadvantages of IGBT:

• Relatively slower switching speed compared to MOSFETs, which may limit its use in very high-frequency applications.
• Possibility of thermal runaway if not properly managed, requiring robust thermal management systems.

Applications:

• Inverters for renewable energy systems, such as solar and wind power.
• Motor drives in industrial and automotive applications.
• Switch-mode power supplies (SMPS).
• HVDC (High Voltage Direct Current) transmission systems.

Correct Option Analysis:

The correct option is:

Option 1: Insulated Gate Bipolar Transistor (IGBT)

The IGBT is the most widely recognized implementation of a Conductively-Modulated Field Effect Transistor. It combines the advantages of MOSFETs and BJTs to deliver high efficiency and performance in power electronics applications. The conductivity modulation mechanism in the IGBT’s drift region is what sets it apart, enabling it to handle high voltages and currents efficiently.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

MOSFETs are voltage-controlled devices that are widely used in power electronics. While they share some characteristics with IGBTs, such as high input impedance and fast switching speeds, MOSFETs do not utilize conductivity modulation. Instead, their operation relies solely on the movement of majority carriers (either electrons or holes). As a result, MOSFETs typically have higher on-state resistance than IGBTs when handling high currents, making them less efficient in high-power applications.

Option 3: Bipolar Junction Transistor (BJT)

BJTs are current-controlled devices that rely on the movement of both majority and minority carriers. While they offer high current-carrying capacity, they have lower input impedance and require more complex drive circuits compared to IGBTs. BJTs do not feature an insulated gate structure or conductivity modulation, and their slower switching speeds make them less suitable for modern high-frequency power electronics.

Option 4: MOS-Controlled Thyristor (MCT)

The MOS-Controlled Thyristor is a type of thyristor that combines the characteristics of a MOSFET and a thyristor. While it offers high current and voltage handling capabilities, it operates differently from IGBTs and does not rely on conductivity modulation in the same way. MCTs are primarily used in specific niche applications and are less common in general-purpose power electronics compared to IGBTs.

Conclusion:

The Insulated Gate Bipolar Transistor (IGBT) is the correct answer as it represents the most common implementation of a Conductively-Modulated Field Effect Transistor. By combining the advantages of MOSFETs and BJTs, IGBTs have become indispensable components in power electronics, enabling efficient and reliable operation in a wide range of high-power applications.

Silicon-controlled rectifier (SCR)

• It is a 4-layer, 3-junction semiconductor device. Additionally, it has three terminals: anode, cathode, and gate.
• An SCR is considered to be a semi-controlled device because it can be turned on but not off with a gate pulse. During the positive half cycle, SCR is in forwarding blocking mode.
• By applying a gate pulse, the SCR can be turned ON. Once turned ON, the gate signal can be removed.
• ​SCR (Silicon-Controlled Rectifier) is a unidirectional device because it allows current to flow only in one direction from anode to cathode when it is in conduction mode.
• SCR (Silicon Controlled Rectifier) is a bipolar device. 
• SCRs can withstand (not conduct) both positive and negative voltages when they are in the OFF state.
• In the forward blocking mode, it blocks forward voltage until triggered. In the reverse blocking mode, it blocks reverse voltage like a diode.
• SCRs are commonly used in AC applications (which have bipolar voltage swings) because they can withstand both polarities before being triggered.

IGBT (Insulated Gate Bipolar Transistor)

IGBT is a semiconductor device that combines the characteristics of MOSFET and BJT (Bipolar Junction Transistor) to provide high efficiency and fast switching. Over time, several alternative names have been used to describe IGBTs, which are as follows:

• IGT (Insulated Gate Transistor): This is a generic term sometimes used to refer to IGBTs since they utilize an insulated gate structure like MOSFETs.
• COMFET (Conductively Modulated Field Effect Transistor): Another alternative name that describes the working principle of IGBTs, where conductivity modulation improves efficiency.
• GEMFET (Gain-Enhanced MOSFET): This term highlights the gain-enhanced feature of IGBTs due to their combination of MOSFET and BJT properties.

IGCT (Integrated Gate-Commutated Thyristor)

• This is not an alternative name for IGBT. IGCT is a separate semiconductor device, different from IGBT, used in high-power applications.
• IGCTs are enhanced GTO (Gate Turn-Off Thyristors) that operate with high efficiency in power electronics applications.

Explanation:

Silicon Controlled Rectifier (SCR)

Definition: A Silicon Controlled Rectifier (SCR) is a semiconductor device that functions as a switch to control high power. It is a type of thyristor, which is a four-layered, three-junction semiconductor device. SCRs are widely used in applications requiring control of high voltage and current.

Structure: An SCR consists of four layers of alternating P and N-type semiconductor materials, forming a structure represented as PNPN. This structure creates three junctions, labeled as J1, J2, and J3. The layers and junctions are as follows:

• P (Anode)
• N
• P
• N (Cathode)

The three junctions are:

• J1: Between the first P and N layers
• J2: Between the middle N and P layers
• J3: Between the last P and N layers

Operation: An SCR operates in three modes: forward blocking, forward conduction, and reverse blocking.

• Forward Blocking Mode: When a positive voltage is applied to the anode relative to the cathode, junctions J1 and J3 are forward biased, while J2 is reverse biased. In this state, the SCR blocks current flow, and only a small leakage current flows through the device.
• Forward Conduction Mode: When a sufficient gate current is applied to the gate terminal, the reverse bias of junction J2 is overcome, and the SCR turns on, allowing a large current to flow from the anode to the cathode. The SCR remains in the conducting state even if the gate current is removed, as long as the anode-to-cathode current remains above a certain threshold called the holding current.
• Reverse Blocking Mode: When a negative voltage is applied to the anode relative to the cathode, junctions J1 and J3 are reverse biased, and J2 is forward biased. In this state, the SCR blocks current flow, similar to a reverse-biased diode.

Applications: SCRs are used in various applications, including:

• Power control in AC and DC circuits
• Rectifiers in power supplies
• Motor speed controls
• Light dimmers
• Overvoltage protection circuits

Advantages:

• High efficiency in controlling large amounts of power
• Ability to handle high voltage and current
• Long life and reliability

Disadvantages:

• Requires a triggering circuit to turn on
• Once turned on, it cannot be turned off by the gate signal alone

Correct Option Analysis:

The correct option is:

Option 2: 4 layers and 3 junctions.

This option correctly describes the structure of an SCR. An SCR consists of four alternating layers of P and N-type semiconductor materials, creating three junctions (J1, J2, J3).

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: 3 layers and 2 junctions.

This option is incorrect as it describes the structure of a different type of semiconductor device, such as a transistor, which typically has three layers and two junctions.

Option 3: 3 layers and 4 junctions.

This option is incorrect because a structure with three layers cannot have four junctions. Junctions are formed between adjacent layers, so the number of junctions is always one less than the number of layers.

Option 4: 2 layers and 2 junctions.

This option is incorrect as it describes a diode, which typically has two layers (P and N) and one junction. A device with two layers cannot have two junctions.

Conclusion:

Understanding the structure and operation of SCRs is crucial for their application in controlling high power in various electronic circuits. The correct structure of an SCR involves four layers and three junctions, making it a versatile and efficient device for power control applications. The incorrect options highlight the importance of recognizing the specific configurations and characteristics of different semiconductor devices.

IGBT (Insulated Gate Bipolar Transistor)

• IGBT is a three-terminal device. The three terminals are Gate (G), Emitter (E), and Collector (C).
• An Insulated Gate Bipolar Transistor (IGBT) is a power semiconductor device that combines the advantages of MOSFETs and BJTs.
• Like a MOSFET, it has a high input impedance, which means it requires very little gate current to turn on.
• Like a BJT, it has low conduction (on-state) power loss due to its low saturation voltage.
• It has superior current conduction capability compared with the bipolar transistor.
• It also has excellent forward and reverse blocking capabilities.

The main drawbacks are:

• Switching speed is inferior to that of a Power MOSFET and superior to that of BJT.
• The collector current tails due to the minority carrier causes the turnoff speed to be slow.
• At the highest temperature, the maximum current rating goes down to 2/3 of the value.
• There is a possibility of latch-up due to the internal PNPN thyristor structure.

In majority carrier devices conduction is only because of majority carriers whereas in minority carrier devices conduction is due to both majority and minority carriers.

1. MOSFET is a majority carrier device.

2. Diode is both majority and minority carrier device.

3. Thyristor is minority carrier device

Concept:

MOSFET

• MOSFET stands for metal oxide semiconductor field effect transistor.
• MOSFET is a four-terminal device.
• The four terminals are the Source, Gate, Drain, and Body.
• The gate is separated from the channel by a thin insulating layer of silicon dioxide.
• When a voltage is applied between the gate and body terminals, the electric field generated penetrates through the oxide and creates an inversion layer or channel at the semiconductor-insulator interface.
• The inversion layer provides a channel through which current can pass between the source and drain terminals.
• Varying the voltage between the gate and body modulates the conductivity of this layer and thereby controls the current flow between the drain and the source.

Concept:

Shockley's equation is given as:

\({I_D} = {I_{DSS}}{\left( {1-\frac{{{V_{GS}}}}{{{V_P}}}} \right)^2}\)

Where,

VGS = Gate to source voltage

IDSS = Drain to source saturation current

VP = Pinch-off voltage

I_D = Drain current

Calculation:

Given,

VGS = -4 V

IDSS = 10 mA 

VGS(off) = V_p = -8 V

Drain current \({I_D} = {I_{DSS}}{\left( {1-\frac{{{V_{GS}}}}{{{V_P}}}} \right)^2} = {10\times10^{-3}}{\left( {1-\frac{{{-4}}}{{{-8}}}} \right)^2}=2.5\times10^{-3}\; A\)

I_D = 2.5 mA

Uni Junction Transistor (UJT):

• The uni-junction transistor is a solid-state three-terminal device that can be used in gate pulse, timing circuits, and trigger generator applications to switch and control both thyristors and triacs for AC power control type applications.
• When a UJT is used for triggering an SCR, the wave shape of the voltage obtained from the UJT circuit is a saw-tooth wave.

• The UJT is a three-terminal, semiconductor device which exhibits negative resistance and switching characteristics for use as a relaxation oscillator in phase control applications.

Applications of UJT:

• Uses negative oscillator resistance property for the generation of the sawtooth waveform
• Used as a Relaxation oscillator.
• Used as a Voltage regulator.
• Used as a switching circuit.
• Widely used as a triggering device for silicon control rectifiers (SCR).
• Used as a phase control circuit.
• Used as a timing circuit.
• Used in a sawtooth generator.
• Used to generate magnetic flux.

The correct answer is option 3):(SCR)

Concept:

•  The power semiconductor device is a semiconductor device used as a switch or rectifier in power electronics (for example in a switch-mode power supply). Such a device is also called a power device or, when used in an integrated circuit, a power 
• SCR is used where high current and voltage ratings are involved. SCR is used as the load side for control of ac because the negative polarity of the voltage across SCR switched it off easily. Otherwise, a commutation circuit is needed to switch off the device. The typical switching frequency of SCR is 500 Hz

Power semiconductor device features

•  Highest switching speed- MOSFET
•  Highest voltage / current ratings- SCR
•  Easy drive features -MOSFET
• Can be most effectively paralleled- MOSFET
• Can be protected against over-currents with a fuse- SCR
• Gate-turn off capability with regenerative features- GTO
• Easy drive and High power handling capability- SCR

MOSFET:

• MOSFET is a voltage-driven/controlled device.
• The current through the two terminals is controlled by a voltage at the third terminal (gate)
• It is a unipolar device (current conduction is only due to one type of majority carrier either electron or hole)
• It has a high input impedance.

BJT:

• BJT's are current-driven devices.
• The current through the two terminals is controlled by a current at the third terminal (base).
• It is a bipolar device (current conduction by both types of carriers, i.e. majority and minority electrons and holes)
• It has a low input impedance.

The correct answer is option 3.

Concept:

• Concept:

  MOSFET - Majority carrier device

  Power BJT, SCR - Minority carrier devices.

  IGBT - Exhibits the properties of both minority carrier and majority carrier devices.

  Minority carrier devices will have stored charge whereas the majority carrier devices don't.

  Explanation:

  Since minority carrier devices like Power BJT and SCR consist of stored charge, these devices take more time to charge and discharge thereby the speed of operation reduces.

  Since power MOSFET is a majority carrier device, it takes less time to charge and discharge, thereby the speed of operation is highest.

  Since IGBT consists of both minority and majority carriers, its speed is less than power MOSFET and more than the power BJT and SCR.

  Increasing speed of operation is SCR, Power BJT, IGBT, power MOSFET

The P-N-P-N devices with zero, one or two gates constitute the basic thyristor. But with the advancement in technology thyristor family includes other similar multilayer devices also.

Devices in Thyristor family:

The following symbols represent DIAC, SCR, TRIAC, and GTO respectively.

• The most important member is SCR (silicon-controlled rectifier); It is a four-layer (P-N-P-N), three junction semiconductor devices with three terminals, namely, the anode, the cathode, and the gate; It is a unidirectional device
• DIAC and TRIAC are bidirectional devices, The DIAC is a two-terminal, three-layer device and is commonly used for triggering TRIACs; The TRIAC is a 3-terminal semiconductor device and may be considered equivalent to two SCRs connected in antiparallel
• A gate turn-off thyristor (GTO) is a special type of thyristor, which is a high-power semiconductor device; Normal thyristors (silicon-controlled rectifiers) are not fully controllable switches but GTO can be Turned ON as well as Turned OFF using Gate signal
• A bipolar junction transistor (bipolar transistor or BJT) is a type of transistor that uses both electrons and holes as charge carriers; BJTs can be used as amplifiers or switches