Class 12 > Unit # 22:Solid State of Electronics > PN Junction Diode

P-N Junction Diode, Depletion Layer, Potential Barrier, Biasing, and I-V Characteristics - Talha's Physics Academy

Talha's Physics Academy

P-N Junction Diode, Biasing, and I-V Characteristics

Video Lecture

Watch the complete video lecture below to understand P-N junction diodes, depletion layers, biasing methods, and I-V characteristics.

P-N Junction, Depletion Layer, and Potential Barrier

"The P-N junction is formed when p-type and n-type semiconductors are joined within a single crystal of material rather than just simply joining or fusing together two separate pieces."

A diode is a semiconductor device formed through a P-N junction that is used to allow the flow of electric current in one direction while blocking it in the opposite direction.

Properties of a Diode

  • A diode has the ability to rectify electric current.
  • It can create a potential barrier and make use of its capacitance properties.
  • A diode creates various nonlinear current-voltage characteristics.

Depletion Layer or Region

When the P-N junction is formed, a few electrons from the n-type diffuse through the junction and combine with the holes on the p-side to form negative ions, leaving behind positive ions on the n-side. This results in the formation of the depletion layer, which acts as a barrier and does not allow any further flow of electrons from the n-region to the p-region.

Potential Barrier

With the formation of an electric field, the depletion area acts as a barrier in that specific region. The quantity of potential difference needed for the movement of electrons through the barrier is called the potential barrier.

Fig: Depletion region and Potential Barrier.

Biasing: Forward Biasing and Reverse Biasing

Biasing: Biasing is the act of applying a steady, predetermined voltage or current to an electronic component like a transistor or diode to establish its baseline operating conditions. There are two types of biasing: (i) Forward Biasing and (ii) Reverse Biasing.

1. Forward Bias

When the P-type region is connected to the positive terminal of the battery and the N-type region is connected to the negative terminal, it is called forward bias.

Explanation: In this condition, the applied electric field and the built-in electric field at the P-N junction are in opposing directions. Combining both gives a resultant electric field that is smaller than the built-in electric field, causing the depletion region to become thinner and less resistant. When the applied voltage is high, the resistance of the depletion region becomes insignificant. At $0.3\text{ V}$ to $0.6\text{ V}$ (or up to $0.7\text{ V}$ for silicon), the resistance of the depletion region becomes absolutely insignificant, allowing current to flow freely through it.

Fig: Forward Biasing .

2. Reverse Bias

When the P-type region is connected to the negative terminal of the battery and the N-type region is connected to the positive terminal, it is called reverse bias.

Explanation: In this condition, the applied electric field and the built-in electric field are both in the same direction. The resultant electric field is enhanced, resulting in a more resistive, thicker depletion region. Increasing the applied voltage further results in an even thicker and more resistant depletion region.

Fig: Reverse Biasing .

I-V Characteristics of a Diode

I-V Characteristics of P-N Junction: The relationship between the voltage across the junction and the current through the circuit is known as the V-I characteristics of a P-N junction or semiconductor diode.

The V-I characteristics of the P-N junction can be explained in three cases:

  1. Zero bias or unbiased: At zero bias, no electric current flows through the diode because there is no external voltage applied to enable the movement of electrons or holes.
  2. Forward bias: In forward bias, when the diode voltage ($V_d$) reaches approximately $0.7\text{ V}$ for silicon and $0.3\text{ V}$ for germanium, current starts flowing. The current increases gradually at first, creating a non-linear curve until the diode surpasses the potential barrier, after which it operates normally and the curve steepens linearly with increasing external voltage.
  3. Reverse bias: In reverse bias, only a small leakage current flows, represented to the left of the origin in the graph. This current remains low until the diode breaks down, at which point it can be destroyed unless a high series resistance limits the current.
Fig: Block diagram of an automobile seat belt alarm system using a 3-input AND gate.

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