Question

What are the limitations of forward-bias, if any, on a semiconductor diode?

What are the limitations of reverse-bias, if any, on a semiconductor diode?

What portion, if any of the volt-ampere characteristic of the diode (when it is forward-biased) is linear?

What can you say about the d-c resistance of the diode over this linear portion?

Answer 1

a) b) In both cases you are limited by power dissipation.
For a forward bias, the current is relatively high because the voltage drop is a fraction of a volt.
In the reverse bias case, it takes very little current times the Zener voltage to equal the power limit. Most diodes make fairly good Zeners, just at very high and unpredictable voltage.
On the low side, some diodes have a leakage current under which their behaviors is unpredictable.

The forward current should not exceed the rated current. Note that the rated current is a function of temperature.

The reverse voltage should not exceed the reverse breakdown rating. Actually, I would want the reverse voltage limited to 70% of rated.

c) The PN-junction diode is connected in forward biased by keeping the key K close and keeping the double throw switch in position one. In forward biasing the p type semiconductor material connects to the positive end of the power supply and the n type semiconductor material is connected to the negative terminal of the supply.

When the voltage is increased by varying the resistor value Rh, the circuit curve increases very slowly and the curve becomes non-linear. The point OA in the curve shown the increasing voltage characteristic.

The current slowly rises in the forward biasing because the applied external voltage is used to cross the potential barrier of the PN-junction diode. But when the potential barrier is completely eliminated, and the external voltage applies to the junction increases, the PN-junction behaves like an ordinary diode and circuit current increases sharply (shown in the region AB).

The increased circuit current is controlled by the resistance Rh and the junction forward resistance Rf. The curve becomes linear. The current exceed from rating value damage the diode.

d) It's fairly constant for the correct operating levels of the diode.
This resistance creates a forward voltage drop across the diode of somewhere around 0.6 to 1 volt depending on the materials and construction of the diode. For example a silicon power diode usually has a 0.6 volt drop. Along with correct voltage and current this must be taken into account when designing circuits using diodes.