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]