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SMA Physics • Physics Lab Class XII • Exp 24

⚡ P-N Junction Diode I–V Characteristics

📖 Manual
📐 Active Circuit & Microscopic Depletion Layer Animation
💡
Forward Bias: Increase $V_F$ from $0$ to $1.2\text{ V}$. Current remains minimal until the knee voltage ($V_k \approx 0.7\text{ V}$ for Si, $0.3\text{ V}$ for Ge), then shoots up steeply in milliamperes (mA).
Reverse Bias: Switch to Reverse Bias. Current is tiny microamperes ($\mu\text{A}$) until breakdown ($V_z \approx 35\text{ V}$).
⚙️ Biasing Mode & DC Power Supply
1. Select Biasing Mode
2. DC Voltage Potentiometer Range: 0.0 to 1.2 V (mA range)
Applied Voltage: 0.00 V
Semiconductor Material: Silicon (Vk ≈ 0.7 V)
📊 Digital Meter Readings
Diode Voltage ($V$)
0.00 V
Diode Current ($I_F$ mA)
0.00 mA
Knee Voltage ($V_k$)
0.70 V
Dynamic Resistance ($r_d$)
-- Ω

📋 Observation Tables: Forward & Reverse Bias

Table A: Forward Bias ($V_F$ vs $I_F$)

Obs # Forward Voltage $V_F$ (V) Forward Current $I_F$ (mA)
No forward readings logged yet.

Table B: Reverse Bias ($V_R$ vs $I_R$)

Obs # Reverse Voltage $V_R$ (V) Reverse Current $I_R$ (μA)
No reverse readings logged yet.

📈 Diode 4-Quadrant Characteristic Curve

Quadrant 1: Forward Bias ($+V_F, +I_F$) • Quadrant 3: Reverse Bias ($-V_R, -I_R$)
🎯 Aim of the Experiment

To draw the $I-V$ characteristic curve for a p-n junction diode in forward bias and reverse bias.

📐 Working Formulas & Diode Parameters
$$I = I_s \left(e^{\frac{V}{\eta V_T}} - 1\right)$$ $$\text{Dynamic Forward Resistance } r_f = \frac{\Delta V_F}{\Delta I_F} \quad (\approx 10\text{ to } 100\ \Omega)$$ $$\text{Dynamic Reverse Resistance } r_r = \frac{\Delta V_R}{\Delta I_R} \quad (\approx \text{Mega-ohms } M\Omega)$$
🔬 Key Phenomena Explained
  • Knee Voltage / Cut-in Voltage ($V_k$): The forward voltage at which diode conduction rapidly starts ($0.7\text{ V}$ for Silicon, $0.3\text{ V}$ for Germanium).
  • Depletion Layer Behavior: In forward bias, external field opposes the barrier potential, compressing and thinning the depletion region. In reverse bias, external field widens the depletion layer.
  • Breakdown Voltage ($V_z$): At high reverse voltages ($V_z \approx 35\text{ V}$ for Si, $25\text{ V}$ for Ge), covalent bonds rupture and avalanche breakdown occurs, causing current to rise steeply.
❓ Viva Voce Interactive Quiz