Paper 3 Semiconductors and Diodes Answers

Q1 Sensor Circuit Design

1. Put the LDR as the upper resistor and measure output across the lower fixed resistor. As light increases, $R_{LDR}$ decreases, so the fixed resistor gets a larger fraction of supply. [2]
2. $V_{out}=9.0(6.0)/(12.0+6.0)=3.0\mathrm{V}$. [2]
3. Brighter light decreases LDR resistance. With the LDR on top, the total resistance falls and a larger fraction of the supply appears across the fixed lower resistor. [3]

Q2 Semiconductor Control

1. Heating provides energy to create more mobile charge carriers, increasing conductivity and reducing resistance. [2]
2. Light creates more mobile charge carriers in the semiconductor, increasing conductivity and reducing resistance. [2]
3. Sensors use resistance change with conditions to change output voltage; a diode uses direction-dependent conduction to control current direction/rectify. [3]