Paper 4 Semiconductors and Diodes Answers

Q1 Measuring Diode I-V Characteristics

1. Connect diode in series with variable supply/rheostat and protective resistor; measure current with ammeter in series and p.d. across diode with voltmeter; vary p.d. and record readings. [3]
2. It limits current and prevents diode damage after the threshold region. [1]
3. Take repeated readings, use small voltage steps near threshold, avoid heating, or plot enough points. [1]

Q2 Thermistor Calibration

1. Axes correctly oriented and labelled [1]; sensible scales [1]; six points plotted accurately [2]; smooth decreasing best-fit curve [1].

2. Interpolation gives approximately $4.7\mathrm{k\Omega }$; accept a value consistent with the candidate’s curve. [1]

3. With the output measured across the lower thermistor,

   $$V_{\mathrm{out}}=6.0\frac{4.7}{5.0+4.7}\approx 2.9\mathrm{V}.$$

   Accept a value consistent with part 2. [2]

4. The thermometer measures the bath temperature, whereas $R$ depends on the thermistor’s own temperature. Equilibrium is needed so these temperatures are the same and no longer changing appreciably. [2]

5. Any two: stir the bath; use small temperature intervals; repeat and average readings; keep measuring current low to reduce self-heating; use calibrated digital meters; wait for stable readings; place thermometer close to the thermistor without contact. [2]