Just like every electrical element, we need to understand the characteristics curve of diode. This curve will help us greatly when determining whether we should use a diode or not. Furthermore, we can determine its rating and type to make it work properly in our circuit.
Characteristics of Diode
Before going straight to the characteristics curve of diode, we should learn what variables we need to read, observe, and understand. We will find what we call:
- Forward biased,
- Forward voltage,
- Forward current,
- Reverse biased,
- Reverse voltage,
- Reverse current,
- Peak reverse voltage or peak inverse voltage,
- Breakdown voltage, and
- Avalanche current.
The diode characteristic curve is a plot between voltage and current, where the horizontal axis is the voltage while the vertical axis is the current.
We will learn them one by one.
The forward bias is when the direction of current enters the diode through the Anode terminal. The current will increase significantly until its maximum value when the voltage applied to the diode is bigger than its voltage requirement.
The current in forward biased is what we know with Forward Current.
The voltage requirement of a diode before it passes the current is what we call Forward Voltage.
Thus, we can rewrite the first paragraph above into:
The forward bias is when the direction of current enters the diode through the Anode terminal (Forward Current). The current will increase significantly until its maximum value when the voltage applied to the diode is bigger than its voltage requirement (Forward Voltage).
This explains that in forward biased, the resistance against the current is relatively small.
Keep in mind that the forward voltage of a diode depends on its material. The most common diode is silicon diodes which have forward voltage of 0.7 V and 0.3 V for Germanium diodes. It means as long as the voltage applied to the diode is bigger than 0.7 V (Silicon) or 0.3 V (Germanium), it will pass the current.
As opposed to forward biased, the reverse bias is when the direction of current enters the diode through the Cathode terminal. The diode can withstand the voltage up to a certain limit before it breaks and the current leaks in the opposite direction with maximum value.
The current in reverse biased is what we know with Reverse Current.
The voltage in reverse biased is what we know with Reverse Voltage.
The reverse voltage requirement for a diode before it breaks and the current leaks is what we call Breakdown Voltage.
Thus, we can rewrite the first paragraph above into:
As opposed to forward biased, the reverse bias is when the direction of current enters the diode through the Cathode terminal (Reverse Current). The diode can withstand the voltage (Reverse Voltage) up to a certain limit (Breakdown Voltage) before it breaks and the current leak (Avalanche Current) in the opposite direction with maximum value.
After the diode breaks, it will not block any reverse current.
There is a difference between breaking down and destroying. If the reverse voltage surpasses the breakdown voltage, the reverse current will flow. It will still work fine as a diode as long as the reverse voltage doesn’t exceed its maximum reverse voltage, Peak Reverse Voltage or Peak Inverse Voltage (PRV or PIV).
As soon as the reverse voltage exceeds the peak reverse voltage, the diode is destroyed and we have to replace it with a new one. A diode should have peak inverse voltage much higher than the applied voltage to it.
Characteristics Curve of Diode
After we learned about what variables we need to look at when dealing with diode characteristic curve, we will put them all in a single graph.
We will divide the diode graph into forward and reverse regions.
The main characteristics of diodes we should pay attention to the most are the forward voltage VF, forward current IF, reverse voltage VR, and reverse current IR. The regions consisting of these four represent the forward and reverse operation regions.
The figure below is the IV characteristics of the diode. The curve shows the characteristic of a diode in voltage and current relationship. The Y-axis represents the current where its left is the reverse operation region and its right is the forward operation region. The X-axis represents the voltage where its top is forward current and its bottom is reverse current.
The half left is the reverse operation where the half right is the forward reverse operation. The horizontal axis represents the voltage and the vertical axis represents the current axis.
The diode IV curve is different from the resistor IV curve. The IV curve of a resistor has a linear relationship where the IV curve of a diode depends on the voltage applied to the diode.
On the forward region, there is a little slope of current and significantly increases. When the forward voltage (VF) hasn’t reached the 0.7 V (for silicon diode), the current will not pass. As soon as the forward voltage reaches 0.7 V, the current will flow with maximum value.
On the reverse region, there is a little slope of reverse current (IR) and significantly increases with negative value. This negative value indicates the current is flowing in reverse direction. As long as the reverse voltage (VR) is less than breakdown voltage (VB), the diode won’t break thus the reverse current won’t leak.
As soon as the reverse voltage surpasses the breakdown voltage, the diode breaks and the reverse current (IR) leaks and flows with maximum value. This leaked current is what we call avalanche current (IA).
Keep in mind that we should consider the Peak Reverse Voltage or Peak Inverse Voltage since the diode will be destroyed if we apply voltage more than it.
From this characteristic, a diode can be used as a switch.