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Discrete semiconductor devices questions
Four components with four jobs. A transistor whose gate takes no current at all switches amps from a logic pin, a diode run backwards pins a supply rail steady, and two more turn light and magnetic field into numbers a circuit can read.
17 original questions · 51 marks · the discrete semiconductor devices notes · Electronics
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Explain why the gate of a MOSFET draws no steady current, and state what a logic output must supply in order to switch one.
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The gate sits on a thin insulating oxide layer, out of contact with the silicon beneath, so it behaves as a small capacitor rather than as a conducting terminal and no steady current can enter it (1). The driver supplies only the brief charging current of that gate capacitance when the state changes (1). The standard error is to treat the gate like a bipolar transistor's base and quote a steady input current.State what happens to the drain current of a MOSFET below its threshold voltage and above it.
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Below the threshold no proper channel forms, so the drain current is negligible at any drain voltage within the device's rating (1). Above it the channel opens and widens, and provided the drain voltage is large enough to keep the channel saturated, the current climbs approximately with the square of the excess gate voltage above the threshold (1). Saying the current 'rises steadily' misses both points: a complete answer names the channel, and the shape is a square law, not a straight line. Both halves are the idealised model: below threshold a small leakage still flows, and the square law is the saturated behaviour, not the behaviour at every drain voltage.A zener diode is used to hold a supply rail steady. State how the diode is connected and what the series resistor does.
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The zener goes across the load, reverse biased, so that it sits in breakdown at its rated voltage (1). The series resistor sits between the raw supply and the diode and drops whatever the supply has left over, which is what sets the current and lets the output stay put when the supply wanders (1). Drawing the zener forward biased, or leaving the series resistor out and shorting the supply through the diode, are the two standard errors.State how a photodiode is biased in a light meter, and state how its photocurrent depends on the illumination.
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It is used reverse biased, so that almost nothing flows in the dark and every absorbed photon adds to the reverse current (1). The photocurrent is proportional to the illumination, so the response is linear and one constant converts current to illumination across the range, where a curve would be needed for a device that is not (1). That constant still comes from a calibration against a known source: linearity saves the shape of the graph, not the fixing of its scale. Writing 'forward biased', or claiming the current depends on the applied pd, are the marks most often thrown away here: the characteristics are flat, so the light sets the current.State what a Hall probe measures, and explain why the probe is rotated before a reading is taken.
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It measures magnetic flux density, the Hall voltage across the slab being proportional to B when the current through the slab is held constant (1). The Hall voltage is largest when the field is perpendicular to the face of the slab, so the probe is turned until the reading is a maximum and that value is taken (1). Reading the probe at any old angle, and quoting the number as though it were B, is the standard error.Give two reasons a photodiode is preferred to a light-dependent resistor at the receiving end of an optical fibre.
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It responds in nanoseconds, while carriers in an LDR take milliseconds to build up and longer to disperse, so an LDR cannot follow anything flickering faster than a few hundred hertz (1). Its photocurrent is proportional to the illumination, whereas an LDR's resistance is not, so the photodiode reproduces the pulse shape faithfully (1). Answering on cost alone earns nothing: the comparison is won on response time and linearity.A MOSFET has a threshold voltage of 2.0 V and passes a drain current of 0.10 A when the gate-source voltage is 3.0 V. Its drain current follows ID = k(VGS − VT)2. Calculate k, and the drain current when the gate-source voltage is 4.5 V.
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The excess above threshold is 3.0 − 2.0 = 1.0 V, so k = 0.10/1.02 = 0.10 A V−2 (1). At 4.5 V the excess is 4.5 − 2.0 = 2.5 V (1), so ID = 0.10 × 2.52 = 0.625 A (1). Squaring the gate voltage itself instead of the excess above the threshold is the standard error, and it gives 2.02 A, three times too big.A MOSFET switches a 6.0 Ω heater across a 9.0 V supply. With the gate at 5 V the channel resistance is 0.12 Ω. Calculate the current, the pd across the heater and the power dissipated in the MOSFET.
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Channel and heater are in series, so R = 6.0 + 0.12 = 6.12 Ω and I = 9.0/6.12 = 1.47 A (1). Heater pd = 1.47 × 6.0 = 8.82 V (1). MOSFET power = I2R = 1.472 × 0.12 = 0.26 W, against 13.0 W in the heater (1). Using the full 9.0 V across the channel instead of I2R is the standard error; the channel only ever has 0.18 V across it.A 5.1 V zener diode is fed through a 390 Ω series resistor from a supply of 15 V, and the load draws 12 mA. Calculate the current in the resistor, the current in the zener and the power dissipated in the zener.
A reverse-biased photodiode delivers a photocurrent of 24 μA at a certain illumination, and its current is passed through a 75 kΩ load resistor. Calculate the pd across the resistor, and the pd when the illumination is trebled.
A Hall effect sensor carries a constant current and gives an output of 2.4 mV when the flux density through its face is 0.20 T. Calculate its sensitivity in mV T−1 and its output in a field of 0.55 T.
A 5.1 V zener diode is fed through a 390 Ω series resistor from a supply that can fall as low as 9.0 V. Calculate the resistor current at 9.0 V and hence state the largest load current the regulator can supply.
A MOSFET with a threshold voltage of 2.0 V passes 0.10 A when the gate-source voltage is 3.0 V. Without calculating k, deduce the drain current at a gate-source voltage of 6.0 V.
A 5.1 V zener regulator must supply a load drawing up to 15 mA from a supply that wanders between 9.0 V and 15.0 V. The zener needs at least 5.0 mA to stay in breakdown and is rated at 400 mW. Calculate the largest series resistance that meets the specification, choose the preferred value 180 Ω, and check the design at both ends of the supply range, including the power rating needed for the resistor.
A logic output that can supply 4.0 mA drives the gate of a MOSFET. The gate behaves as a 1.5 nF capacitor and must be taken to 5.0 V to switch the transistor on. Estimate the switching time, and state the current the logic output supplies once the MOSFET is on.
A Hall sensor giving 2.4 mV in a flux density of 0.20 T is mounted beside a rotating shaft carrying four small magnets, each bringing a peak flux density of 0.45 T past the slab. A counter registers 120 pulses in 2.0 s. Calculate the peak output voltage and the rotation rate of the shaft in revolutions per minute.
A photodiode gives a photocurrent of 20 μA per unit of illumination. It is to produce 2.5 V across a load resistor at an illumination of 3 units. Calculate the resistance required, state the pd obtained with the nearest preferred value of 43 kΩ, and explain why a light-dependent resistor could not replace the photodiode in a fibre-optic receiver working at 1 MHz.
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