ISCET Semiconductor Devices Questions and Answers — Questions and Answers
Question 1: A technician is building a simple voltage regulator circuit. Which semiconductor device is specifically designed to operate in the reverse breakdown region to maintain a constant voltage across its terminals?
- Signal Diode
- Zener Diode (Correct answer)
- Schottky Diode
- Varactor Diode
Correct answer: Zener Diode
A Zener diode is engineered to have a precise and stable reverse breakdown voltage (Zener voltage). When reverse-biased at this voltage, it conducts current while maintaining a nearly constant voltage drop, making it ideal for voltage regulation applications.
Question 2: A technician needs to control a high-power AC load, such as a motor, using a low-power DC signal. Which three-terminal semiconductor device is most suitable for this application, acting as a latching switch?
- Bipolar Junction Transistor (BJT)
- Zener Diode
- Silicon-Controlled Rectifier (SCR) (Correct answer)
- Field-Effect Transistor (FET)
Correct answer: Silicon-Controlled Rectifier (SCR)
A Silicon-Controlled Rectifier (SCR) is a type of thyristor designed to control large currents and voltages with a small gate current. Once triggered by a pulse to its gate, it 'latches' on and conducts current from anode to cathode until the current falls below a holding threshold, making it perfect for controlling AC power.
Question 3: When comparing a Bipolar Junction Transistor (BJT) to a Field-Effect Transistor (FET), which of the following is a key characteristic of the FET?
- Low input impedance
- Current-controlled device
- High input impedance (Correct answer)
- Constructed with two P-N junctions
Correct answer: High input impedance
Field-Effect Transistors (FETs) are voltage-controlled devices and are characterized by their very high input impedance, typically in the megaohms range. This is because the gate terminal is insulated from the channel, drawing very little to no input current. BJTs, in contrast, are current-controlled and have a much lower input impedance.
Question 4: A technician is testing a standard silicon PN junction diode out of circuit using a digital multimeter's diode test function. What approximate voltage reading should the technician expect when the diode is forward-biased?
- 0.7 V (Correct answer)
- 0.3 V
- 1.2 V
- 0.0 V
Correct answer: 0.7 V
A standard silicon PN junction diode has a forward voltage drop of approximately 0.6V to 0.7V when it is conducting. The diode test function on a multimeter applies a small current and measures this voltage drop. A reading of 0.3V is typical for a germanium diode.
Question 5: In a semiconductor material, the process of intentionally adding impurities to increase its conductivity is known as:
- Refining
- Doping (Correct answer)
- Ionization
- Annealing
Correct answer: Doping
Doping is the process of introducing impurity atoms into an intrinsic (pure) semiconductor crystal to change its electrical properties. Adding pentavalent impurities creates N-type material (excess electrons), while adding trivalent impurities creates P-type material (excess holes), both of which significantly increase conductivity.
Question 6: A technician is troubleshooting a circuit and suspects a transistor has failed. A visual inspection of the NPN BJT reveals a small crack in its casing and a slight burnt smell. This type of damage is most often caused by:
- Electrostatic discharge (ESD)
- Reverse polarity on the base-emitter junction
- Insufficient base current
- Excessive collector current or overheating (Correct answer)
Correct answer: Excessive collector current or overheating
Physical damage like a cracked casing, burn marks, or a burnt smell on a transistor is a classic sign of thermal failure. This occurs when the power dissipated (primarily Vce * Ic) exceeds the device's rating, leading to excessive heat that damages the semiconductor material and its packaging.
ISCET Semiconductor Devices Questions and Answers
A technician is building a simple voltage regulator circuit.
Which semiconductor device is specifically designed to operate in the reverse breakdown region to maintain a constant voltage across its terminals?