BEE Bachelor of Electrical Engineering Semiconductor Devices and Electronics Questions and Answers — Questions and Answers
Question 1: An N-channel enhancement-mode MOSFET (E-MOSFET) is operating with a gate-to-source voltage (VGS) equal to zero. Which of the following statements correctly describes the state of the device?
- The device is in the 'OFF' state, and no significant drain current (ID) flows because no conductive channel exists between the drain and source. (Correct answer)
- The device is in the 'ON' state, and a significant drain current (ID) flows due to the pre-existing doped channel.
- The device operates in the saturation region, acting as a constant current source.
- The device is in a depletion mode, and a negative VGS is required to turn it off.
Correct answer: The device is in the 'OFF' state, and no significant drain current (ID) flows because no conductive channel exists between the drain and source.
Enhancement-mode MOSFETs are 'normally-off' devices. This means that without a positive gate-to-source voltage (for an N-channel device), there is no conductive channel between the drain and source. A channel must be 'enhanced' or created by applying a sufficient VGS (greater than the threshold voltage) to attract electrons and form an inversion layer.
Question 2: Which of the following is a key characteristic of an ideal operational amplifier (op-amp)?
- Finite open-loop voltage gain
- Non-zero output impedance
- Infinite input impedance (Correct answer)
- Finite bandwidth
Correct answer: Infinite input impedance
An ideal op-amp is a theoretical model with perfect characteristics. One of its defining features is infinite input impedance, which means it draws no current from the input source, preventing any loading effects on the preceding circuit stage. Practical op-amps have very high, but finite, input impedance.
Question 3: A technician is building a simple DC voltage regulator circuit. To provide a stable 5.1V output from a fluctuating 9V input source, a specific type of diode is used. How must this diode be connected in the circuit to achieve voltage regulation?
- A Zener diode connected in series with the load and in forward-bias.
- A standard rectifier diode connected in parallel with the load and in forward-bias.
- A Zener diode connected in parallel with the load and in reverse-bias. (Correct answer)
- A Schottky diode connected in parallel with the load and in reverse-bias.
Correct answer: A Zener diode connected in parallel with the load and in reverse-bias.
Zener diodes are specifically designed to operate in the reverse breakdown region. When reverse-biased at a voltage exceeding their Zener voltage (Vz), they maintain a nearly constant voltage across their terminals. By placing it in parallel with the load and in reverse-bias, it can maintain a stable output voltage (equal to Vz) even when the input voltage or load current changes.
Question 4: When a P-N junction diode is forward-biased, what is the primary effect on the depletion region and the potential barrier?
- The depletion region widens, and the potential barrier increases.
- The depletion region narrows, and the potential barrier is reduced. (Correct answer)
- The depletion region and potential barrier remain unchanged.
- The depletion region disappears completely, and the potential barrier inverts.
Correct answer: The depletion region narrows, and the potential barrier is reduced.
Applying a forward-bias voltage (positive to P-type, negative to N-type) opposes the built-in potential barrier of the P-N junction. This opposition reduces the height of the potential barrier and causes the depletion region to become narrower, allowing majority charge carriers to cross the junction and current to flow easily.
Question 5: Which of the following accurately describes a key characteristic of a Bipolar Junction Transistor (BJT) amplifier in a common-emitter (CE) configuration?
- Low voltage gain and high input impedance.
- A 180-degree phase shift between the input and output signals. (Correct answer)
- Low current gain and low output impedance.
- No phase shift between the input and output signals.
Correct answer: A 180-degree phase shift between the input and output signals.
A fundamental characteristic of the common-emitter amplifier is that it provides an inverted output signal. This means the output voltage waveform is 180 degrees out of phase with the input voltage waveform. This configuration is known for providing high voltage and high current gain.
Question 6: An engineer compares a practical (real-world) op-amp to its ideal model. Which parameter, defined as the ratio of differential-mode gain to common-mode gain, is infinite in the ideal model but finite and typically measured in decibels (dB) in a practical op-amp?
- Slew Rate
- Gain-Bandwidth Product
- Output Impedance
- Common-Mode Rejection Ratio (CMRR) (Correct answer)
Correct answer: Common-Mode Rejection Ratio (CMRR)
The Common-Mode Rejection Ratio (CMRR) is a measure of an op-amp's ability to reject signals that are common to both inputs. An ideal op-amp would have zero common-mode gain, resulting in an infinite CMRR. Practical op-amps have a finite, though very high, CMRR, which is often expressed in dB.
BEE Bachelor of Electrical Engineering Semiconductor Devices and Electronics Questions and Answers
An N-channel enhancement-mode MOSFET (E-MOSFET) is operating with a gate-to-source voltage (VGS) equal to zero.
Which of the following statements correctly describes the state of the device?