- To be able to use a diode in a circuit, you must know how it would behave under different voltages and currents. Or in other words, you must know the characteristics of diode. Here we discuss the forward bias VI characteristics of diode. In the next page, we discuss reverse bias VI characteristics of diode.
- We also discuss why the forward current increases drastically after threshold voltage is reached. We discuss Shockley’s equation to explain this effect.
- Finally we discuss how the characteristics of commercially available diode differs from that of ideal diode. So if you want to use the diode practically, make sure you understand the characteristics of diode in detail.
We discussed in the previous section what is forward biasing and how to forward bias a diode. Just to have a quick revision, lets look at the basic definition of forward bias. When the positive terminal of the battery is connected to the p-type semiconductor and the negative terminal of the battery is connected to the n-type semiconductor, the diode is said to be forward biased.
When the diode is forward biased, there is a reduction in the net electric field in the depletion region. As a result, the electrons can now pass from n-type semiconductor to p-type semiconductor. As the magnitude of forward bias is increased, there is an increase in the number of electrons flowing from n-type to p-type semiconductor, thus increasing the value of current. When the voltage is increased above threshold voltage, there is a sudden rise in the magnitude of current. (Threshold voltage is defined as the voltage above which the current increases very rapidly with respect to the voltage). The graph showing the variation of diode current ID with respect to diode voltage VD and is shown below. Lets us observe how the current through the diode changes with respect to voltage. (Refer the graph above)
- Initially when the forward bias voltage is zero, the current through the diode is also zero. The reason is quite obvious. The current through the diode is zero because there is no voltage applied across its terminals which could establish current.
- When the forward bias voltage is gradually increased from zero to threshold voltage( shown in the above graph). there is a gradual increase in the value of current.
- When the forward bias voltage is increased above threshold voltage, the current increases very rapidly with respect to voltage. A small change in the value of forward bias voltage would result in large changes in the value of forward bias current. As it can be seen from the above graph, a small change in forward bias voltage ΔV results in drastic change in the value of forward current ΔI.
You might be curious to know why there is sudden rise in the value of current even though the change in voltage is very small. Well, if you are not curious to know the reason, then you should be, because this is one of the characteristics of diode due to which it is used in enormous number of applications. The sudden rise in the current can be explained through the use of solid-state physics. The equation governing the behavior of (forward) current and voltage is given by
ID = IS(eVD/ηVT – 1)
This equation is called Shockley’s equation. The terms used in the equation are as follows.
ID = Current through the diode
IS = Reverse saturation current
η = Ideality factor
VT is called thermal voltage and is given by VT = kT/q
k = Boltzmann’s constant = 1.38 x 10-23 J/K
T= Temperature in Kelvin and
q = magnitude of charge on an electron = 1.6 x 10-19 C
Shockley’s equation, which is derived under ideal conditions, suggest that the forward current rises exponentially with respect to forward voltage. If we expand the R.H.S of Shockley’s equation, we get
ID = ISeVD/ηVT – IS
The R.H.S of Shockley’s equation is now split in two terms. The first term is ISeVD/ηVT and the second term is IS. For positive values of VD, the first term will grow very quickly and it will totally dominate the second term. The second term IS the reverse saturation current, whose typical value is in nano ampere range- a very small number. Hence the relationship between forward current and voltage can be written as
ID ≈ ISeVD/ηVT
This equation shows exponential relation between forward current and voltage, which is compatible with the graph shown above.
Ideal vs Practical (Non ideal) forward bias characteristics
Shockley’s equation given above is derived under certain assumption and it represents the characteristics of ideal diode. But you might have heard something like – “nothing is ideal”. (yes, this site is not about philosophy). Having said that, the diodes are no exceptions. Characteristics of commercially available diodes deviates from ideal diode characteristics. The graph given below shows how the characteristics of commercially available diodes deviates from that of ideal diode. There are two characteristic curves shown in the above graph. One curve shows the characteristics of an ideal diode and the other shows the characteristics of commercially available diodes. As indicated the above graph, we take an arbitrary value of diode current Ix. For this value of Ix, there corresponds two different value of voltage V1 and V2. V1 is the corresponding voltage for ideal diode whereas V2 is the corresponding voltage for commercial diode. Clearly V2 > V1, which indicates that there occurs additional voltage drop in the commercial diode as compared to that of ideal diode. In general, the characteristics of commercially available diodes shift to the right indicating additional voltage drop across the diode. The additional voltage drop across the commercial (non-ideal) diode is due to internal body resistance and the external contact resistance.