Characteristics of Semiconducting Diodes
By Sultan LeMarc
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
Context• Electrical component
• First developed around 1906
• Asymmetric transfer characteristic Current flow in one direction
• Most common type - semiconductor
• Applications LEDs Photodiode Rectifiers
[en.wikipedia.org]
[www.environ.org.uk]
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)
Doping
N-type
• Negative
• Donor atoms
• Charge carriers: Electrons
• Addition of impurities
• Adjusts conductivity of semiconductor
• Modifies band structure and resistivity
P-type
• Positive
• Acceptor atoms
• Charge carriers: Positive ‘holes’
Diodes
Germanium (Ge)
• Similar to Silicon diode
• High reverse resistance
• Permanent damage at high reverse current
• Conducts in only one direction
• p-n junction diode: - p junction - depletion region
- n junction
Zener (Ze)
• Flow in forward and reverse direction
• Avalanche breakdown at sufficiently high reverse voltage
(allaboutcircuits.com)
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
Leakage Current
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]
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
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
• 10µm turn potentiometer used to adjust diode voltage
• Dependent variable – current across diode
[KCL 2nd Year Physics Laboratory Manual 2012/13]
Experiment 2: Results
Further Extensions
• Different semiconducting materials
• Resistance level characteristics
• Metal-junction diode: Schottky
• Devices: - LED - Photodiode (solar cell) - Thermal diode Photodiode
[rapidonline.com]