ECE 3254 PreLab 4 notes
Edited 01-19-07
Rectifier Diode – may also be called a switching diode for high frequency use.
The ideal rectifier diode operates as a one-way check valve or switch. Conduction occurs only when the
diode is forward biased. The ideal diode has no forward voltage drop and does not conduct at all when
reverse biased.
A real diode has a typical forward voltage of 0.6V to 0.8V, and a reverse breakdown voltage that is
greater than the diode’s peak inverse voltage (PIV) rating. Conduction in both directions follows an
exponential V-I curve. The diode current I is given by Equation 1 where IS is a scale factor called the
saturation current, q is the charge on an electron , k is Boltzmann's constant, T is the absolute
temperature of the p-n junction and VD is the voltage across the diode. The term kT/q is the thermal
voltage, sometimes written VT, and is approximately 26 mV at room temperature. n is the emission
coefficient, which is typically 1 for most devices.
Equation 1: Silicone saturation current IS ≈ 10-13A
Germanium saturation current IS ≈ 10-8A
Rectifier diode ratings include:
•Forward current
•PIV
•Forward voltage drop
•Reverse recovery time
•Junction Capacitance
Rectifier diodes are normally only operated in forward condition, forward cutoff, or reverse cutoff. The
PIV rating is not an indication of where reverse conduction actually occurs; it only guarantees that the
diode will not break down before the PIV rating is reached. It is not a good idea to design a circuit that
depends on reverse conduction in a rectifier diode because the reverse conduction voltage can not be
predicted. Because uncontrolled reverse breakdown quickly leads to diode failure, (which can also
damage other circuit components) you should never attempt to operate a rectifier diode in reverse
conduction.
The diode Cathode is identified by a band
on the cathode end.
Prelab 4 Notes - Page 1 of 3
