p-n Junction — Formation, depletion layer, biasing
Semiconductor
11
JEE Qs
8%
Hard
75
min
Focus on the qualitative understanding of charge carrier movement, electric field changes, and potential barrier modification under different biasing conditions, along with the resulting I-V characteristics curve.
🧮 Key Formulas
✅ Key Points for JEE
- 1p-n junction formation involves diffusion of majority carriers across the junction, creating a region of immobile charged ions (depletion layer) and establishing a potential barrier.
- 2The depletion layer is devoid of mobile charge carriers (electrons and holes) but contains immobile positive donor ions in the n-region and negative acceptor ions in the p-region.
- 3Forward biasing reduces the potential barrier and the width of the depletion layer, allowing majority carriers to flow and leading to an exponential increase in current after a certain knee voltage.
- 4Reverse biasing increases the potential barrier and the width of the depletion layer, drastically reducing current flow to a small, constant reverse saturation current (due to minority carriers) until breakdown occurs.
- 5Understanding the I-V characteristics for both forward and reverse bias, including the knee voltage (threshold voltage) and reverse breakdown voltage, is crucial for analyzing diode circuits.
⚠️ Common Mistakes
- ✕Confusing the role of immobile ions (fixed charges) in the depletion region with mobile charge carriers (electrons and holes) causing current.
- ✕Incorrectly stating that the depletion region contains a high concentration of mobile carriers; it is largely devoid of them.
- ✕Misinterpreting the direction of current flow or the change in potential barrier under forward vs. reverse bias.
- ✕Not understanding that the reverse saturation current is primarily due to minority carriers drifting across the junction, not majority carriers.
- ✕Failing to connect the decrease/increase in depletion layer width with the applied bias and the potential barrier height.
📝 Practice Questions
See allQ27.Consider the following statements: A. The junction area of solar cell is made very narrow compared to a photo diode. B. Solar cells are not connected with any external bias. C. LED is made of lightly doped p-n junction. D. Increase of forward current results in continuous increase of LED light intensity. E. LEDs have to be connected in forward bias for emission of light. Choose the correct answer from the options given below: (1) B, E Only (2) B, D, E Only (3) A, C Only (4) A, C, E Only
Q43.Which of the following circuits has the same outpur as that of the given circuit? (1) (2) (3) (4)
Q33.Which of the following circuits represents a forward biased diode? Choose the correct answer from the options given below : (1) (A) and (D) only (2) (B), (D) and (E) only (3) (C) and (E) only (4) (B), (C) and (E) only
Q26. A B Y 0 0 1 0 1 1 1 0 0 1 1 1 To obtain the given truth table, following logic gate should be placed at G: 2025 (22 Jan Shift 2) JEE Main Previous Year Paper (1) OR Gate (2) AND Gate (3) NOR Gate (4) NAND Gate
Q28.The output of the circuit is low (zero) for : (A) X = 0, Y = 0 (B) X = 0, Y = 1 (C) X = 1, Y = 0 (D) X = 1, Y = 1 Choose the correct answer from the options given below : (1) (B), (C) and (D) only (2) (A), (B) and (C) only (3) (A), (C) and (D) only (4) (A), (B) and (D) only
Q45. In the circuit shown here, assuming threshold voltage of diode is negligibly small, then voltage VAB is correctly represented by : (1) VAB would be zero at all times (2) (3) (4)
NCERT Chapters
- Class 12 Physics Ch 14: Semiconductor Electronics: Materials, Devices and Simple Circuits