To truly understand how a diode behaves, we need to study how the current (I) through it changes as we vary the voltage (V) across it.
This relationship between voltage and current is called the V–I characteristic of the diode.
What are V–I Characteristics?
The V–I characteristic (Voltage–Current characteristic) is a graphical representation that shows how much current flows through the diode for a given applied voltage.
In simple words, if you slowly increase the voltage across a diode and note down the current at each step — then plot those values on a graph — the resulting curve is called the V–I characteristic curve of the diode.
This curve tells us how the diode behaves electrically under:
- Forward bias condition (conducting mode)
- Reverse bias condition (non-conducting mode)
How the V–I Characteristic is Obtained
To obtain the V–I characteristic practically, the diode is connected in a circuit with:
- A variable voltage source,
- A current-measuring device (like an ammeter), and
- A voltage-measuring device (like a voltmeter) across the diode.
We vary the applied voltage gradually — first in the forward direction and then in the reverse direction — and record the corresponding current at each step.
When the voltage and current values are plotted on a graph:
- The x-axis represents the voltage (V) applied across the diode.
- The y-axis represents the current (I) flowing through the diode.
V–I Characteristic in Forward Bias
When the diode is forward biased, the positive terminal of the battery is connected to the P-side and the negative terminal to the N-side.
Initially, as the voltage is applied, the diode does not conduct immediately. This is because the applied voltage must first overcome the potential barrier (V₀) created by the internal electric field of the depletion region.
For silicon diodes, this barrier potential is about 0.7 V, and for germanium, it is about 0.3 V.
At voltages below this barrier potential:
- The external voltage is not sufficient to reduce the potential barrier.
- Only a very small current flows because the applied voltage is not sufficient to overcome the barrier.
- The diode behaves almost like an open circuit.
Once the applied voltage equals the barrier potential:
- The depletion region becomes very narrow.
- The internal electric field is effectively neutralized.
- Majority carriers (electrons from N-side and holes from P-side) can now cross the junction freely.
At this point, the current starts increasing rapidly with even a small increase in voltage. This is because the diode now offers very little resistance in the forward direction.
The forward characteristic curve thus shows:
- A small, almost flat region near zero voltage (negligible current),
- Followed by a sharp exponential rise in current beyond the cut-in or threshold voltage.
V–I Characteristic in Reverse Bias
When a diode is reverse biased, the P-type side is connected to the negative terminal of the battery, and the N-type side is connected to the positive terminal.
So:
- The negative terminal pulls holes away from the junction in the P-side.
- The positive terminal pulls electrons away from the junction in the N-side.
As a result:
- Electrons near the junction on the N-side are pulled back toward the positive terminal.
- Holes near the junction on the P-side are pulled back toward the negative terminal.
This means that majority carriers (electrons in N and holes in P) move away from the junction, causing the depletion region to become wider.
What’s Next?
So far, we have studied how a diode behaves under forward and reverse bias conditions and how its current changes with applied voltage. But what happens when the reverse voltage becomes extremely large?
In the next blog, we will study:
- Reverse breakdown mechanism
- Thermally generated carriers
- Impact ionization
- Avalanche multiplication
- Avalanche breakdown in diodes
👉 Click below to continue to the next part:
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