This document summarizes experiments on the I-V characteristics of semiconducting diodes. It describes the forward and reverse bias characteristics of germanium and silicon diodes, and how the reverse saturation current varies with temperature, allowing determination of the band gap. Experiments are presented that measure the exponential forward I-V relationship and higher conductivity of silicon diodes compared to germanium, as well as the small reverse current of silicon that prevents its measurement with available equipment. The temperature dependence of carrier mobility and concentration are also discussed.

1. Characteristics of Semiconducting Diodes
By Sultan LeMarc

2. Aims
• I-V characteristics of superconducting diodes
▫ Forward bias (Germanium and Silicon)
▫ Reverse bias (Germanium)
• Variation of reverse current as function of temperature
▫ Reverse saturation current
▫ Band gap determination

4. What is a semiconductor?
• Properties between conductors and insulators
• Two types:
▫ Intrinsic
▫ Extrinsic
• Smaller energy gap between bands than insulators
• Negative and positive charge carriers
(Serway, 2010)

5. Doping
• Addition of impurities
• Adjusts conductivity of semiconductor
• Modifies band structure and resistivity
N-type
P-type
• Negative
• Positive
• Donor atoms
• Acceptor atoms
• Charge carriers: Electrons
• Charge carriers: Positive ‘holes’

6. Diodes
• Conducts in only one direction
• p-n junction diode: - p junction
- depletion region
- n junction
Germanium (Ge)
• Similar to Silicon diode
(allaboutcircuits.com)
Zener (Ze)
• Flow in forward and reverse
direction
• High reverse resistance
• Permanent damage at high
reverse current
• Avalanche breakdown at
sufficiently high reverse
voltage

7. Temperature dependence
• 0K - all electrons in valance band
• Temperature dependence of
▫ Carrier mobility
▫ Carrier concentration
• Conductivity increases with temperature
▫ Excitation of electrons across band gap

8. Leakage Current

9. Experiment 1: Forward Characteristics
• p-n junction transistors: Si & Ge
▫ Transistors used as diodes – base emitter junction
• Independent variable – supply voltage
• Dependent variable – current across diode
• Restraints
▫
▫
▫
▫
Current < 10mA
Voltage < 25V
2.2kΩ resistance to limit current
Room temperature (24.7oC)
[KCL 2nd Year Physics Laboratory
Manual 2012/13]

10. Experiment 1: Results
• Disobedience of Ohm’s law
• Exponential relationship
• Si shows greater
conductivity
• Forward bias voltage:
▫ Si: 0.7V
▫ Ge: 0.3V
• Ge used in low power
circuits – more efficient
than Si

11. Experiment 2: Reverse Characteristics
• Only for Ge
▫ Si reverse current too small to measure with apparatus
• Sensitive measuring instruments
▫ Levell multimeter
▫ Digital voltmeter
• Supply voltage fixed to ~0.5V
[KCL 2nd Year Physics Laboratory
Manual 2012/13]
• 10µm turn potentiometer used to adjust diode voltage
• Dependent variable – current across diode

12. Experiment 2: Results

13. Further Extensions
• Different semiconducting materials
• Resistance level characteristics
• Metal-junction diode: Schottky
• Devices:
- LED
- Photodiode (solar cell)
- Thermal diode
Photodiode
[rapidonline.com]