Free BEE Bachelor of Electrical Engineering Semiconductor Devices and Circuits Questions and Answers — Questions and Answers
Question 1: In a Bipolar Junction Transistor (BJT) operating in the active region for amplification, which of the following describes the biasing of the emitter-base (EB) and collector-base (CB) junctions?
- EB junction is forward-biased, and CB junction is forward-biased.
- EB junction is reverse-biased, and CB junction is reverse-biased.
- EB junction is forward-biased, and CB junction is reverse-biased. (Correct answer)
- EB junction is reverse-biased, and CB junction is forward-biased.
Correct answer: EB junction is forward-biased, and CB junction is reverse-biased.
For a BJT to function as an amplifier, it must operate in the active region. This requires the emitter-base junction to be forward-biased, allowing current to flow from the emitter into the base, and the collector-base junction to be reverse-biased, which allows the collector to gather the majority of the charge carriers from the emitter.
Question 2: A technician is designing a simple voltage regulator circuit to provide a stable 5V output from a fluctuating 9V input. Which semiconductor device is most suitable for this application, and how should it be connected in the circuit?
- A Zener diode, connected in forward bias in parallel with the load.
- A standard PN junction diode, connected in reverse bias in series with the load.
- A Schottky diode, connected in forward bias in series with the load.
- A Zener diode, connected in reverse bias in parallel with the load. (Correct answer)
Correct answer: A Zener diode, connected in reverse bias in parallel with the load.
Zener diodes are specifically designed to operate in the reverse breakdown region, where they maintain a constant voltage (the Zener voltage) across them over a wide range of currents. To function as a voltage regulator, the Zener diode is placed in reverse bias and in parallel with the load, with a series resistor to limit the current.
Question 3: Which of the following statements best differentiates a MOSFET from a BJT in terms of their fundamental control mechanism?
- A MOSFET is a current-controlled device, while a BJT is a voltage-controlled device.
- A MOSFET is a voltage-controlled device, while a BJT is a current-controlled device. (Correct answer)
- Both are voltage-controlled devices, but MOSFETs have a lower input impedance.
- Both are current-controlled devices, but BJTs have a faster switching speed.
Correct answer: A MOSFET is a voltage-controlled device, while a BJT is a current-controlled device.
The fundamental difference lies in how their output current is controlled. A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) uses an electric field generated by a voltage on the gate terminal to control the conductivity of a channel, making it a voltage-controlled device. A BJT (Bipolar Junction Transistor) uses a small current into the base terminal to control a much larger current flow between the collector and emitter, making it a current-controlled device.
Question 4: When applying a reverse bias voltage to a standard PN junction diode that is below the breakdown voltage, what is the primary reason for the very low current flow?
- The depletion region narrows, allowing majority carriers to flow easily.
- The electric field across the depletion region is neutralized.
- The depletion region widens, creating a large potential barrier for majority carriers. (Correct answer)
- The generation of electron-hole pairs ceases completely.
Correct answer: The depletion region widens, creating a large potential barrier for majority carriers.
In reverse bias, the applied voltage opposes the natural direction of current flow. This causes the positive and negative charge carriers (majority carriers) to be pulled away from the junction, which widens the depletion region. This widened region acts as a large potential barrier, making it very difficult for the majority carriers to cross, resulting in only a very small leakage current carried by minority carriers.
Question 5: To create an N-type semiconductor from intrinsic silicon, the silicon crystal must be doped with an element that has:
- Four valence electrons, such as Germanium.
- Three valence electrons, such as Boron.
- Five valence electrons, such as Phosphorus. (Correct answer)
- Six valence electrons, such as Sulfur.
Correct answer: Five valence electrons, such as Phosphorus.
Intrinsic silicon has four valence electrons. To create an N-type semiconductor, dopant atoms with five valence electrons (pentavalent), like Phosphorus or Arsenic, are introduced. Four of the dopant's valence electrons form covalent bonds with the surrounding silicon atoms, leaving the fifth electron free to move within the crystal lattice as a negative charge carrier (electron). These extra electrons make the material N-type.
Question 6: In an ideal operational amplifier (Op-Amp), which two characteristics are assumed to be infinite?
- Output Impedance and Bandwidth
- Common-Mode Rejection Ratio (CMRR) and Slew Rate
- Input Impedance and Open-Loop Voltage Gain (Correct answer)
- Power Consumption and Input Offset Voltage
Correct answer: Input Impedance and Open-Loop Voltage Gain
An ideal operational amplifier is a theoretical model with perfect characteristics. Among these, it is assumed to have infinite input impedance (so it draws no current from the source) and infinite open-loop voltage gain (so it can amplify any signal without limit in an open-loop configuration). Real op-amps have very high, but finite, values for these parameters.
Free BEE Bachelor of Electrical Engineering Semiconductor Devices and Circuits Questions and Answers
In a Bipolar Junction Transistor (BJT) operating in the active region for amplification, which of the following describes the biasing of the emitter-base (EB) and collector-base (CB) junctions?