Current vs. voltage properties of a diode
A diode is a two-terminal electronic component that conducts current primarily in one direction A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and In , Thomas Edison observed unidirectional current between heated and unheated elements in a bulb. As such, you can not simply look up the resistance of a diode and use "Ohm's Law" to determine the relationship between voltage and current. A forward-biased diode conducts current and drops a small voltage across it, of the P-N junction's inherent exponential current/voltage relationship and thus.
A diode's I—V characteristic can be approximated by four regions of operation: At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs that causes a large increase in current i. The avalanche diode is deliberately designed for use in that manner. In the Zener diodethe concept of PIV is not applicable. A Zener diode contains a heavily doped p—n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material, such that the reverse voltage is "clamped" to a known value called the Zener voltageand avalanche does not occur.
Both devices, however, do have a limit to the maximum current and power they can withstand in the clamped reverse-voltage region.V-I characteristics of p-n junction diode
Also, following the end of forward conduction in any diode, there is reverse current for a short time. The device does not attain its full blocking capability until the reverse current ceases. For a bias less than the PIV, the reverse current is very small. However, this is temperature dependent, and at sufficiently high temperatures, a substantial amount of reverse current can be observed mA or more.
There is also a tiny surface leakage current caused by electrons simply going around the diode as though it were an imperfect insulator. With a small forward bias, where only a small forward current is conducted, the current—voltage curve is exponential in accordance with the ideal diode equation. There is a definite forward voltage at which the diode starts to conduct significantly.
This is called the knee voltage or cut-in voltage and is equal to the barrier potential of the p-n junction. This is a feature of the exponential curve, and appears sharper on a current scale more compressed than in the diagram shown here.
At larger forward currents the current-voltage curve starts to be dominated by the ohmic resistance of the bulk semiconductor. The truth is, nothing has a fixed resistance. Even your humble quarter watt resistor will change resistance when it warms up and as it ages. If you think this is just he opinion of one man, you would be right, his name is Georg Simon Ohm Chances are you have never actually read his workor if you read German, the original version.
If you ever do, and, at pages or antiquated English and electrical terminology, I warn you, it is a very hard thing to read, you will discover that he indeed covered non-linear devices and, as such, they should be included in Ohm's Law.
In fact there is a whole Appendix, some 35 pages, devoted entirely to the subject.
Diode - Wikipedia
He even acknowledges there were things to still be discovered there and leaves it open for further investigation. Further, after any change in the excitation of the circuit as a whole, a rebalance must occur before the formula is effective. Maxwell, on the other hand qualified it as, R must not change with V or I. That may not be what your were taught in school, or even what you have heard quoted or read from many reputable sources, but it is from Ohm himself. The real issue is many people perceive or understand only a very simplified interpretation of Ohm's thesis, penned by Maxwell, that has been, possibly mistakenly, propagated over the decades since the great man actually performed his work as "Ohm's Law".
Figure below Depletion region expands with reverse bias. Conversely, if a forward-biasing voltage is applied across the P-N junction, the depletion region collapses becoming thinner. The diode becomes less resistive to current through it.
In order for a sustained current to go through the diode; though, the depletion region must be fully collapsed by the applied voltage. This takes a certain minimum voltage to accomplish, called the forward voltage as illustrated in Figure below.
Inceasing forward bias from a to b decreases depletion region thickness. For silicon diodes, the typical forward voltage is 0.
For germanium diodes, the forward voltage is only 0. The chemical constituency of the P-N junction comprising the diode accounts for its nominal forward voltage figure, which is why silicon and germanium diodes have such different forward voltages.
Forward voltage drop remains approximately constant for a wide range of diode currents, meaning that diode voltage drop is not like that of a resistor or even a normal closed switch.
For most simplified circuit analysis, the voltage drop across a conducting diode may be considered constant at the nominal figure and not related to the amount of current. Diode Equation Actually, forward voltage drop is more complex.
An equation describes the exact current through a diode, given the voltage dropped across the junction, the temperature of the junction, and several physical constants. It is commonly known as the diode equation: At room temperature, this is about 26 millivolts.
Just understand that the voltage dropped across a current-conducting diode does change with the amount of current going through it, but that this change is fairly small over a wide range of currents.
Introduction to Diodes And Rectifiers
This is why many textbooks simply say the voltage drop across a conducting, semiconductor diode remains constant at 0. Also, since temperature is a factor in the diode equation, a forward-biased P-N junction may also be used as a temperature-sensing device, and thus can only be understood if one has a conceptual grasp on this mathematical relationship.
A reverse-biased diode prevents current from going through it, due to the expanded depletion region.