- Draw the energy band diagram for forward-biased and reverse-biased p–n junction diode.
- What are an abrupt and a linearly-graded p–n junction?
- Draw and explain the I–V characteristics of p–n junction.
- Compare the I–V characteristics of ideal and practical p–n junction diode.
- Compare cut-off voltage (Vγ) of Ge, Si and GaAs.
The Correct Answer and Explanation is :
1. Energy Band Diagram for Forward-Biased and Reverse-Biased p–n Junction Diode
The energy band diagrams of a p-n junction diode change under forward and reverse bias:
- Forward Bias: When a forward voltage is applied (p-side positive, n-side negative), the depletion region narrows, and the built-in potential barrier reduces. This allows charge carriers (electrons and holes) to move across the junction easily, leading to current flow.
- Reverse Bias: When a reverse voltage is applied (p-side negative, n-side positive), the depletion region widens, and the barrier potential increases, preventing the majority carrier flow. Only a small leakage current flows due to minority carriers.
Now, I’ll generate an image showing these band diagrams.
2. Abrupt and Linearly-Graded p–n Junctions
- Abrupt p-n Junction: In this type, the doping concentration changes suddenly from p-type to n-type at the junction. The transition is sharp, and the electric field distribution is relatively high within the depletion region.
- Linearly-Graded p-n Junction: Here, the doping concentration changes gradually across the junction rather than abruptly. The electric field varies linearly, and the depletion region is wider than in an abrupt junction.
3. I–V Characteristics of a p-n Junction
The current-voltage (I-V) characteristics of a p-n junction diode show:
- Forward Bias: At small voltages, the current is negligible, but after the threshold voltage (cut-in voltage), current increases exponentially.
- Reverse Bias: A small leakage current flows due to minority carriers. However, after a certain breakdown voltage, a significant current flows (avalanche or Zener breakdown).
I’ll generate an image illustrating the I-V characteristics of a p-n junction diode.
I was unable to generate the I-V characteristics image due to content policy restrictions. However, I can still describe it:
- The x-axis represents voltage (V), and the y-axis represents current (I).
- In forward bias, the current is negligible initially but rises exponentially after the threshold voltage (~0.7V for Si, ~0.3V for Ge).
- In reverse bias, only a small leakage current flows until the breakdown voltage is reached.
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4. Comparison: Ideal vs. Practical I–V Characteristics
| Feature | Ideal Diode | Practical Diode |
|---|---|---|
| Forward Bias | Conducts immediately at 0V | Conducts after cut-in voltage (0.7V for Si) |
| Reverse Bias | No current flow | Small leakage current exists |
| Breakdown Region | No breakdown occurs | Breakdown happens at high voltage |
| Turn-on Voltage | Zero | Finite value (~0.7V for Si, ~0.3V for Ge) |
5. Comparison of Cut-off Voltage (Vγ) for Ge, Si, and GaAs
| Material | Cut-off Voltage (Vγ) |
|---|---|
| Germanium (Ge) | ~0.3V |
| Silicon (Si) | ~0.7V |
| Gallium Arsenide (GaAs) | ~1.4V |
Explanation:
- Germanium (Ge) has a low cut-in voltage (~0.3V), making it suitable for low-power applications but also more temperature-sensitive.
- Silicon (Si) is widely used due to its moderate voltage (~0.7V) and thermal stability.
- Gallium Arsenide (GaAs) has a high cut-in voltage (~1.4V) but offers high-speed performance and better efficiency in optoelectronic applications.
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