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EASA Module 4 study materials
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EASA Module 4 practice questions

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Question 1Semiconductors

Silicon doped with a pentavalent impurity such as phosphorus becomes:

  1. P-type, with holes as the majority carriers
  2. an intrinsic semiconductor with equal carrier numbers
  3. an insulator because the impurity has five valence electrons
  4. N-type, with electrons as the majority carriers Correct answer.

Explanation

A pentavalent atom has five valence electrons, four of which bond with neighbouring silicon atoms while the fifth is only loosely bound and becomes free. Electrons therefore become the majority carriers and the material is N-type.

Question 2Semiconductors

The depletion region at a PN junction is produced by:

  1. thermal generation of electron-hole pairs in the crystal
  2. diffusion of carriers leaving immobile ions Correct answer.
  3. the external bias voltage applied to the junction
  4. the flow of minority carriers across the junction

Explanation

Electrons and holes diffuse across the junction and recombine, leaving behind ionised donor and acceptor atoms that are fixed in the lattice. These immobile charges form the depletion region and its barrier potential.

Question 3Semiconductors

Doping silicon with a trivalent impurity such as boron produces:

  1. an intrinsic material of very high purity
  2. a material with no free charge carriers
  3. an N-type material with donor ions
  4. P-type, with holes as majority carriers Correct answer.

Explanation

A trivalent atom has only three valence electrons, so one covalent bond is left incomplete, creating a hole. Holes are therefore the majority carriers and the material is P-type.

Question 4Semiconductors

Applying forward bias to a PN junction:

  1. narrows the depletion region and lowers the barrier Correct answer.
  2. has no effect on the width of the depletion region
  3. reverses the polarity of the junction barrier
  4. widens the depletion region and increases the barrier

Explanation

Forward bias opposes the internal barrier, so majority carriers are pushed towards the junction and the depletion region narrows. Once the barrier is largely overcome a large forward current flows.

Question 5Semiconductors

A light-emitting diode emits light when it is:

  1. forward biased so carriers recombine Correct answer.
  2. operated beyond its reverse breakdown voltage
  3. reverse biased and carrying leakage current
  4. driven with an alternating current only

Explanation

Under forward bias, electrons and holes are injected into the junction and recombine, releasing energy as photons. The wavelength of the emitted light is set by the band gap of the semiconductor material.

Question 6Semiconductors

A half-wave rectifier supplied from a 50 Hz AC source produces an output ripple at:

  1. 25 Hz
  2. 50 Hz Correct answer.
  3. 100 Hz
  4. 150 Hz

Explanation

A half-wave rectifier passes only one half of each input cycle, so exactly one output pulse appears per cycle. The ripple frequency is therefore equal to the supply frequency.

Question 7Semiconductors

A zener diode is used as a voltage reference because, in reverse breakdown, its voltage:

  1. remains almost constant over a range of current Correct answer.
  2. falls to almost zero as soon as breakdown occurs
  3. increases in proportion to the reverse current
  4. changes in proportion to the frequency of the supply

Explanation

In controlled reverse breakdown the zener voltage varies only slightly over the rated current range, which makes it a useful reference. A series resistor sets the current so that the diode stays within that range.

Question 8Semiconductors

In a PNP transistor, the majority charge carriers that flow into the base from the emitter are:

  1. electrons
  2. negative ions
  3. positive ions
  4. holes Correct answer.

Explanation

The emitter of a PNP transistor is P-type material, so it injects holes into the N-type base. Holes are therefore the majority carriers, while electrons are the minority carriers in the base.

Question 9Semiconductors

A bridge rectifier is supplied with 400 Hz AC. The ripple frequency of its output is:

  1. 400 Hz
  2. 200 Hz
  3. 1600 Hz
  4. 800 Hz Correct answer.

Explanation

Full-wave rectification produces two output pulses for every input cycle, so the ripple frequency is twice the input frequency. Twice 400 Hz is 800 Hz.

Question 10Semiconductors

When a silicon NPN transistor is saturated and used as a closed switch, the collector-emitter voltage is approximately:

  1. VCC
  2. half of VCC
  3. 0.2 V Correct answer.
  4. 0.7 V

Explanation

In saturation both junctions are forward biased and the device behaves as a closed switch. The small residual collector-emitter voltage of a silicon transistor is about 0.2 V.

Question 11Semiconductors

A transistor has a common-base current gain (alpha) of 0.98. Its common-emitter current gain (beta) is approximately:

  1. 49 Correct answer.
  2. 0.98
  3. 98
  4. 50

Explanation

The two gains are related by beta = alpha / (1 - alpha). Substituting alpha = 0.98 gives 0.98 / 0.02, which is 49.

Question 12Semiconductors

In a common-emitter amplifier stage, the output signal is:

  1. in phase with the input signal at all frequencies
  2. 180 degrees out of phase with the input Correct answer.
  3. leading the input signal by 90 degrees
  4. lagging the input signal by 90 degrees

Explanation

In the common-emitter connection an increase in base current increases collector current, which increases the drop across the collector load and lowers the collector voltage. The output is therefore inverted, that is 180 degrees out of phase.

Question 13Semiconductors

In a simple zener shunt regulator, the resistor in series with the zener diode is there to:

  1. increase the value of the regulated output voltage
  2. limit the current through the zener diode Correct answer.
  3. smooth the ripple on the output voltage
  4. rectify the alternating input voltage

Explanation

The series resistor drops the difference between the input voltage and the zener voltage, thereby setting the current. Its value must keep the zener current between the minimum needed for regulation and the maximum rating.

Question 14Semiconductors

A photodiode used as a light sensor is normally connected:

  1. in forward bias, with the current limited by a resistor
  2. without any bias, relying on photovoltaic action only
  3. in forward breakdown, so light triggers conduction
  4. in reverse bias, so light increases leakage current Correct answer.

Explanation

In reverse bias only a small leakage current flows in darkness. Incident light creates additional electron-hole pairs, and the resulting photocurrent is proportional to illumination and easy to measure.

Question 15Semiconductors

The main advantage of a field-effect transistor over a bipolar transistor is its:

  1. much lower input resistance
  2. much higher current gain
  3. ability to rectify alternating current
  4. very high input resistance Correct answer.

Explanation

The gate of a FET is either a reverse-biased junction or an insulated oxide layer, so it draws virtually no current. This very high input resistance allows the device to be driven from high-impedance sources.

Question 16Semiconductors

An SCR that has been triggered into conduction continues to conduct until:

  1. the gate-cathode junction is forward biased once more
  2. the anode current falls below the holding current Correct answer.
  3. the anode-to-cathode voltage is increased further
  4. the triggering pulse is removed from the gate terminal

Explanation

Once triggered, internal regenerative action keeps the SCR on and the gate loses control. The device turns off only when the anode current falls below the holding current, for example when the supply is interrupted.

Question 17Semiconductors

In a half-wave rectifier with a reservoir capacitor, between the peaks of the input the capacitor mainly discharges through the:

  1. load resistance Correct answer.
  2. supply source
  3. rectifier diode
  4. transformer secondary winding

Explanation

During the interval when the input falls, the rectifier diode is reverse biased and blocks. The capacitor can then discharge only through the load, and the RC time constant determines the ripple amplitude.

Question 18Semiconductors

When an NPN transistor is cut off, its collector-emitter voltage is approximately:

  1. half of VCC
  2. 0.2 V
  3. zero
  4. equal to VCC Correct answer.

Explanation

Cut-off means there is no base current, and consequently no collector current and no drop across the collector load. The collector therefore sits at the full supply voltage.

Question 19Semiconductors

An analogue ohmmeter is used to test a silicon diode. A serviceable diode shows:

  1. a low resistance reading in both directions of test
  2. low resistance one way and high resistance the other Correct answer.
  3. the same moderate resistance in both directions
  4. an infinite resistance reading in both directions

Explanation

The ohmmeter applies its own small voltage, which forward biases the diode in one direction and reverse biases it in the other. A good diode therefore reads low resistance one way and very high resistance the other.

Question 20Semiconductors

In an insulated-gate MOSFET, the gate electrode is separated from the channel by:

  1. a reverse-biased PN junction
  2. a thick layer of metal
  3. a very thin layer of oxide Correct answer.
  4. a forward-biased PN junction

Explanation

The gate is a metal or polysilicon electrode separated from the channel by an extremely thin silicon dioxide layer. This insulation is what gives the MOSFET its very high input resistance.

Question 21Semiconductors

Which bipolar transistor configuration has a current gain of less than unity?

  1. common source
  2. common base Correct answer.
  3. common emitter
  4. common collector

Explanation

In the common-base connection the input is the emitter current and the output is the collector current. Because the collector current is slightly smaller than the emitter current, the current gain alpha is just below one, although the voltage gain can be high.

Question 22Semiconductors

A zener diode whose breakdown voltage is below about 5 V has a temperature coefficient that is:

  1. the same as that of a silicon rectifier diode
  2. positive, so the voltage rises as temperature rises
  3. negative, so the voltage falls as temperature rises Correct answer.
  4. zero, so the voltage is independent of temperature

Explanation

Below about 5 V the zener (tunnelling) mechanism dominates, and its breakdown voltage falls as temperature rises, giving a negative coefficient. Above about 6 V avalanche breakdown dominates and the coefficient becomes positive.

Question 23Semiconductors

Compared with an SCR, a triac has the property that it:

  1. needs a much larger gate triggering current
  2. cannot be used on alternating current
  3. has a higher holding current
  4. can conduct current in both directions Correct answer.

Explanation

A triac is a bidirectional device: it can be triggered into conduction in either direction, which makes it suitable for AC power control. An SCR conducts in one direction only.

Question 24Semiconductors

A heat sink fitted to a power transistor reduces:

  1. the current gain of the transistor
  2. the maximum collector-emitter voltage
  3. the input capacitance of the device
  4. the rise in junction temperature Correct answer.

Explanation

The heat sink provides a low thermal resistance path from the case to the surrounding air. For a given power dissipation this lowers the junction temperature and keeps it within the rated maximum.

Question 25Semiconductors

An N-channel JFET has IDSS = 8 mA and a pinch-off voltage of -4 V. With a gate-source voltage of -2 V, the drain current is approximately:

  1. 8 mA
  2. 6 mA
  3. 2 mA Correct answer.
  4. 4 mA

Explanation

The JFET follows the square law ID = IDSS(1 - VGS/VP)^2. Here VGS/VP is 0.5, so ID = 8 mA multiplied by 0.25, which is 2 mA.

Question 26Semiconductors

In an intrinsic semiconductor at room temperature, the concentration of free electrons is:

  1. zero throughout the material
  2. equal to the concentration of holes Correct answer.
  3. greater than the concentration of holes
  4. less than the concentration of holes

Explanation

Thermal energy breaks covalent bonds and creates electrons and holes in pairs, so in a pure crystal the two concentrations are identical. This value is the intrinsic carrier concentration and depends strongly on temperature.

Question 27Semiconductors

Adding a small amount of phosphorus (a pentavalent element) to pure silicon produces:

  1. an n-type material in which electrons are the majority carriers Correct answer.
  2. a p-type material with acceptor ions
  3. an intrinsic material with equal carrier densities
  4. a material with no free carriers at room temperature

Explanation

A pentavalent atom has five outer electrons; four form bonds with silicon and the fifth is only loosely held, so it becomes free. The impurity is a donor and the material becomes n-type with electrons as majority carriers.

Question 28Semiconductors

The depletion region of an unbiased PN junction contains:

  1. equal densities of free electrons and holes
  2. immobile ions and no mobile carriers Correct answer.
  3. a high density of free electrons
  4. a high density of free holes

Explanation

Diffusion of carriers across the junction leaves behind fixed ionised donor and acceptor atoms and sweeps out free carriers. The resulting space-charge layer is therefore free of mobile charge.

Question 29Semiconductors

An LED emits light when:

  1. electrons and holes recombine across the junction Correct answer.
  2. it is reverse biased close to breakdown
  3. carriers cross the junction without recombining
  4. the junction is heated by the forward current

Explanation

In forward bias, electrons and holes are injected into the junction region where they recombine and release energy as photons. The emitted wavelength is set by the semiconductor band gap.

Question 30Semiconductors

In a bipolar junction transistor, the most heavily doped region is the:

  1. base
  2. substrate
  3. emitter Correct answer.
  4. collector

Explanation

The emitter is doped heavily so that it can inject a large number of majority carriers into the base. The base is thin and lightly doped, and the collector is moderately doped with a large area.

Question 31Semiconductors

A silicon diode with a forward voltage of 0.7 V carries a current of 20 mA. The power dissipated in the diode is:

  1. 7 mW
  2. 35 mW
  3. 14 mW Correct answer.
  4. 0.7 mW

Explanation

Power equals the product of the voltage across the device and the current through it. Here 0.7 V × 0.020 A = 0.014 W, which is 14 mW.

Question 32Semiconductors

A bridge rectifier is fed from a secondary winding whose peak voltage is 100 V. The maximum reverse voltage appearing across any one diode is about:

  1. 50 V
  2. 141 V
  3. 100 V Correct answer.
  4. 200 V

Explanation

In a bridge circuit two diodes conduct in series with the load while the other two are reverse biased across the full secondary voltage. Each diode therefore sees a peak reverse voltage equal to the peak secondary voltage.

Question 33Semiconductors

An unfiltered full-wave rectifier is supplied with a sinusoidal voltage of 50 V peak. Its mean DC output voltage is approximately:

  1. 15.9 V
  2. 50.0 V
  3. 31.8 V Correct answer.
  4. 35.4 V

Explanation

For a full-wave rectified sine wave the average value is 2/π times the peak, which is 0.637 × 50 V ≈ 31.8 V. A half-wave circuit would give half of this.

Question 34Semiconductors

A 12 V zener diode is fed from a 20 V supply through a 400 Ω series resistor, with no load connected. The current in the zener is:

  1. 20 mA Correct answer.
  2. 50 mA
  3. 30 mA
  4. 8 mA

Explanation

The series resistor drops the difference between the supply and the zener voltage, 20 V − 12 V = 8 V. The current is then 8 V / 400 Ω = 20 mA, and with no load all of it flows in the zener.

Question 35Semiconductors

A transistor with a current gain of 100 is required to pass a collector current of 200 mA. The base current needed is:

  1. 0.5 mA
  2. 200 µA
  3. 2 mA Correct answer.
  4. 20 mA

Explanation

Collector current equals base current multiplied by the current gain. Dividing 200 mA by 100 gives 2 mA.

Question 36Semiconductors

Thermal runaway in a power transistor occurs because, as the junction temperature rises, the collector current:

  1. depends only on the collector supply voltage
  2. rises, which increases the dissipation further Correct answer.
  3. stays constant, so the dissipation is fixed
  4. falls, which reduces the dissipation

Explanation

Collector current tends to increase with temperature because leakage rises and the base-emitter voltage needed falls. The extra current dissipates more heat, raising the temperature again in a self-reinforcing loop.

Question 37Semiconductors

Two transistors with current gains of 50 and 60 are connected as a Darlington pair. The overall current gain is about:

  1. 300
  2. 110
  3. 3000 Correct answer.
  4. 55

Explanation

In a Darlington arrangement the emitter current of the first device becomes the base current of the second, so the two gains multiply. Taking 50 × 60 gives approximately 3000.

Question 38Semiconductors

In normal operation, the gate-channel junction of an N-channel JFET is:

  1. left unbiased
  2. forward biased
  3. biased into breakdown
  4. reverse biased Correct answer.

Explanation

The JFET is controlled by the width of the depletion layer around a reverse-biased gate junction. Keeping the gate reverse biased ensures the very high input resistance that makes the device useful.

Question 39Semiconductors

An enhancement-mode N-channel MOSFET remains cut off until:

  1. the gate is left floating
  2. the drain-source voltage falls to zero
  3. the gate-source voltage exceeds the threshold voltage Correct answer.
  4. the gate-source voltage is taken negative

Explanation

No channel exists at zero gate voltage in an enhancement device; a positive gate-source voltage above the threshold inverts the surface and forms the channel. Below the threshold the drain current is essentially zero.

Question 40Semiconductors

Compared with an ordinary silicon PN junction diode, a Schottky diode has:

  1. a lower forward drop and faster recovery Correct answer.
  2. a higher forward drop and slower recovery
  3. a negative resistance region in forward bias
  4. a much higher reverse breakdown voltage

Explanation

A Schottky diode uses a metal-semiconductor junction, which conducts at around 0.3 V and stores no minority-carrier charge. It therefore switches much faster and dissipates less in conduction.

Question 41Semiconductors

A varactor diode is used as a:

  1. stable voltage reference
  2. current-limiting device
  3. voltage-controlled capacitor Correct answer.
  4. light-sensitive detector

Explanation

The varactor is operated in reverse bias, where the depletion layer acts as the dielectric of a capacitor. Changing the reverse voltage alters the layer width and hence the capacitance.

Question 42Semiconductors

An optocoupler gives galvanic isolation between its input and output because:

  1. both sides are formed on one shared chip
  2. its input and output share a common earth
  3. its output is driven by a magnetic field
  4. the signal crosses the gap as light Correct answer.

Explanation

The input drives a light source and a photo-detector on the output side receives it across a transparent insulating gap. No electrical conductor joins the two sides, so no fault current can pass between them.

Question 43Semiconductors

A diac is most often used to:

  1. trigger a triac in a lamp dimmer
  2. stabilise a DC supply voltage
  3. rectify a high-frequency AC supply Correct answer.
  4. amplify small AC signals

Explanation

A diac breaks down symmetrically in either polarity and delivers a sharp pulse, which is ideal for firing a triac in AC phase-control circuits. Its snap action gives clean, repeatable triggering.

Question 44Semiconductors

A full-wave rectifier supplies a 0.5 A load through a 1000 µF reservoir capacitor from a 50 Hz supply. The peak-to-peak ripple is about:

  1. 10 V
  2. 0.5 V
  3. 5 V Correct answer.
  4. 2.5 V

Explanation

A full-wave rectifier fed at 50 Hz recharges the capacitor at 100 Hz, so the ripple is approximately I / (f × C). That gives 0.5 A / (100 Hz × 1000 µF) = 5 V peak to peak.

Question 45Semiconductors

Compared with germanium, silicon has:

  1. a lower forward voltage drop
  2. a higher permissible junction temperature Correct answer.
  3. a lower melting point
  4. a higher reverse leakage current

Explanation

Silicon has a wider band gap, which raises its forward drop to about 0.7 V but greatly reduces leakage and allows junctions to run at higher temperatures. This is why silicon dominates in aircraft and general electronics.

Question 46Semiconductors

A transistor dissipates 20 W with a junction-to-case resistance of 1.5 °C/W, a case-to-heatsink resistance of 0.5 °C/W and a heatsink-to-air resistance of 3 °C/W. At an ambient of 25 °C, the junction temperature is:

  1. 145 °C
  2. 65 °C
  3. 85 °C
  4. 125 °C Correct answer.

Explanation

The thermal resistances add in series to give 5 °C/W, so the rise is 20 W × 5 °C/W = 100 °C. Adding that to the 25 °C ambient gives a junction temperature of 125 °C.

Question 47Semiconductors

The three terminals of a thyristor (SCR) are:

  1. anode, cathode and base
  2. anode, cathode and gate Correct answer.
  3. base, emitter and collector
  4. source, drain and gate

Explanation

A thyristor is a four-layer PNPN device with an anode and cathode in the main current path and a gate for triggering. It behaves as a latching switch once fired.

Question 48Semiconductors

The latching current of a thyristor is the minimum anode current that:

  1. causes it to turn off again
  2. is needed to fire the gate electrode
  3. keeps it conducting as the gate pulse ends Correct answer.
  4. can flow without damaging the gate

Explanation

While the gate pulse is applied the anode current must rise above the latching value for the device to stay on after the pulse is removed. Turn-off is governed by the lower holding current, not the latching current.

Question 49Semiconductors

Compared with a photodiode, a phototransistor provides:

  1. the same sensitivity but lower noise
  2. a faster response with no current gain
  3. greater sensitivity but a slower response Correct answer.
  4. a linear output over a smaller range

Explanation

The phototransistor multiplies the photodiode current by its transistor action, so it is far more sensitive to low light levels. The stored charge in the base region makes it respond more slowly than a simple photodiode.

Question 50Semiconductors

A 12 V zener is fed from a 20 V supply through a 200 Ω resistor and supplies a 600 Ω load. The current in the zener is:

  1. 10 mA
  2. 20 mA Correct answer.
  3. 60 mA
  4. 40 mA

Explanation

The series resistor carries (20 V − 12 V) / 200 Ω = 40 mA, and the load takes 12 V / 600 Ω = 20 mA. The difference, 20 mA, must flow through the zener.