- You have studied how the diode behaves in forward bias configuration. In this section,we discuss how the diode behaves in reverse bias configuration. In other words, we study the characteristics of diode when it reverse biased.
- We also study how the characteristics of ideal diode differs from that of practical diode.
Before discussing about the V-I characteristics of a diode, let us have a brief look at definition of reverse bias. The diode is said to be reverse biased when the positive terminal of the battery is connected to the n-type semiconductor and the negative terminal of the battery is connected to the p-type terminal. The figure below shows a p-n junction diode in reverse bias configuration.
When the diode is reverse biased, there is a very little flow of current due to minority carriers.(The reason is explained in detail in the section on diode biasing). This current is of the order of nano-ampere to micro-ampere. This current does not change significantly with bias voltage. The current is called reverse saturation current and it remains almost constant with the change in reverse bias voltage. When the bias voltage is increased above certain voltage called reverse breakdown voltage, the current increases very rapidly. There is a drastic change in the value of current even for a small change in voltage. The graph below shows a plot of reverse bias voltage vs current.
To understand the graph shown above, we divide it into two sections and try to interpret what each section suggest.
- Graph from zero reverse bias voltage to breakdown voltage : Consider the graph shown above from zero reverse bias voltage to breakdown voltage. In this voltage range, the current remains almost constant. This current is called reverse saturation current.
- Graph from breakdown voltage and above : If the reverse bias voltage is increased above the breakdown voltage, the diode is said to be in breakdown region and the current in the circuit increases drastically. It can be seen from the graph that even though the current increases drastically, the voltage remains almost constant. This property of diode is useful in many applications which we shall see in later sections.
While discussing about the V-I characteristics of forward biased diode, we used Shockley’s to explain its characteristics. Let us apply Shockley’s equation in case of reverse biased diode. Shockley’s equation is as follows.
ID = IS(eVD/ηVT – 1)
The diode voltage VD is negative when reverse bias is applied. Hence the magnitude of the term eVD/ηVT will quickly drop to zero with increase in magnitude of reverse bias voltage. Hence Shockley’s equation will quickly reduce to ID ≈ -IS , which is reverse saturation current.
Ideal vs practical reverse diode characteristics
Shockley’s equation suggests that the reverse saturation current flowing through the diode is IS. However the current flowing through the actual commercial diode is more than IS. The graph shown below differentiates the two curves – one observed practically and the other suggested by Shockley’s equation.
The practical observation does not seem to be compatible with the theory. Does that mean that Shockley’s equation is incorrect? The answer is NO. If you remember well, while discussing Shockley’s equation for the first time in the section on forward bias V-I characteristics, we saw that Shockley’s equation is derived under certain assumptions. The actual reverse saturation current of commercial diode is more than that of IS in Shockley’s equation because Shockley’s equation does not include the effects of surface leakage current and generation of carriers in the depletion region. These effects, which occur in actual diode, are not taken into consideration while deriving Shockley’s equation. Hence the reverse saturation current of actual diode is more than that if IS.