A-LevelPhysicsElectricity9702/42 · Oct/Nov 2019 · Q1011 Marks
10 (a) The upper electron energy bands in an intrinsic semiconductor material are illustrated in Fig. 10.1. [Figure 10.1] Use band theory to explain why the resistance of an intrinsic semiconductor material decreases as its temperature increases.
- (a)The upper electron energy bands in an intrinsic semiconductor material are illustrated in Fig. 10.1. [Figure 10.1] Use band theory to explain why the resistance of an intrinsic semiconductor material decreases as its temperature increases.[4]
- (b)(i)Determine the temperature at which the light-emitting diode (LED) in Fig. 10.2 switches on or off. temperature = ..................................................... °C [4][4]
- (b)(ii)State and explain whether the thermistor is above or below the temperature calculated in (i) for the LED to emit light.[3]
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Mark scheme
Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.
- [B1] (as temperature rises) electrons in valence band gain energy
- [B1] electrons jump to conduction band
- [B1] holes are left in the valence band
- [B1] increased number (density) of charge carriers causes lower resistance
- [C1] V⁻ = V⁺
- [C1] 1.50 / 1.20 = R_T / 1.76
- [C1] R_T = 2.2 (kΩ)
- [A1] temperature = 14 °C
- [B1] (For LED to conduct,) V_OUT must be negative
- [B1] V⁻ > V⁺
- [B1] R_T must be lower so temperature must be above (b)(i) value
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