Semiconductor Diode 🌟

A semiconductor diode is simply a p–n junction fitted with metal contacts, so you can connect it to a circuit. It has two terminals, and the arrow on its circuit symbol points the way conventional current flows when you forward-bias the device. 🚀 :contentReference[oaicite:0]{index=0}

1 . How the p–n Junction Forms 🔍

  • Right after joining p- and n-type regions, electrons drift from n to p while holes drift from p to n. As they swap places they leave behind charged ions that set up an electric field. The field grows until drift current balances diffusion current—this is equilibrium, so no net current flows. 💡 :contentReference[oaicite:1]{index=1}
  • The resulting electric field creates a barrier potential (often called the built-in potential). It stops most majority carriers from crossing the junction. :contentReference[oaicite:2]{index=2}
  • You cannot make a true p–n junction by simply pressing two polished wafers together—the surfaces are far rougher than atomic spacing, so charge carriers see a discontinuity. ⚠️ :contentReference[oaicite:3]{index=3}

2 . Forward Bias ⚡

  • Connect the p-side to the battery’s + terminal and the n-side to –. The applied voltage opposes the barrier, so the depletion layer shrinks and the barrier height drops to
    \( V_{\text{barrier (forward)}} = V_0 – V \). 📉 :contentReference[oaicite:4]{index=4}
  • Electrons race from n to p and holes race from p to n—a process we call minority-carrier injection. Those carriers then diffuse across their new regions, producing the diode’s forward current (usually measured in mA). 💨 :contentReference[oaicite:5]{index=5}
  • A small forward voltage gives a tiny current; crank up the voltage and many more carriers have enough energy to cross, so current rises steeply. 📈 :contentReference[oaicite:6]{index=6}

3 . Reverse Bias 🚦

  • Swap the battery leads (n-side to +, p-side to –). Now the applied voltage aids the barrier, so the depletion layer widens and the barrier grows:
    \( V_{\text{barrier (reverse)}} = V_0 + V \). 🛡️ :contentReference[oaicite:7]{index=7}
  • Only a trickle current flows, supplied by minority carriers that the field swiftly sweeps across. This reverse-saturation current sits in the μA range and hardly changes with voltage. 😴 :contentReference[oaicite:8]{index=8}
  • Push the reverse voltage past the breakdown voltage \(V_{\text{br}}\) and the current shoots up sharply. Unless you limit it, the diode overheats and fails. 🔥 :contentReference[oaicite:9]{index=9}

4 . V-I Characteristic 📊

  • Plotting current vs. voltage gives the familiar diode curve. In forward bias current stays tiny until the cut-in (threshold) voltage: about 0.2 V for germanium and 0.7 V for silicon. Past that point, current grows exponentially for tiny voltage increases. 🚀 :contentReference[oaicite:10]{index=10}
  • In reverse bias the curve stays almost flat (reverse-saturation region) until breakdown. :contentReference[oaicite:11]{index=11}

5 . Dynamic Resistance 🧮

Engineers love the small-signal, or dynamic, resistance:

\( r_d = \dfrac{\Delta V}{\Delta I} \) :contentReference[oaicite:12]{index=12}

  • Typical forward-bias value (near 0.8 V, 20 mA) ≈ 10 Ω. 💡 :contentReference[oaicite:13]{index=13}
  • Typical reverse-bias value (–10 V, –1 mA) ≈ 1 × 107 Ω—showing how strongly the diode resists reverse current. 🛑 :contentReference[oaicite:14]{index=14}

6 . Why Diodes Matter 🎯

  • Because forward-bias resistance is tiny while reverse-bias resistance is huge, a diode works like a one-way valve for charge—perfect for turning AC into DC in rectifier circuits (next section in your textbook!). ⚙️ :contentReference[oaicite:15]{index=15}

Important Concepts for NEET 📌

  1. Cut-in voltage: Remember 0.7 V (Si) and 0.2 V (Ge). They often appear in circuit questions. ✨ :contentReference[oaicite:16]{index=16}
  2. Barrier potential change: Forward bias lowers it to \(V_0 – V\); reverse bias raises it to \(V_0 + V\). 🧩
  3. Reverse-saturation current: Tiny, voltage-independent, but sets the stage for breakdown. ⏳ :contentReference[oaicite:17]{index=17}
  4. Breakdown voltage \(V_{\text{br}}\): Beyond this, reverse current skyrockets—watch out for device failure. ⚡ :contentReference[oaicite:18]{index=18}
  5. Dynamic resistance \(r_d\): Calculate it with \( \Delta V / \Delta I \); compare forward (≈ Ω) and reverse (≈ MΩ) values to grasp diode behavior. 🧮 :contentReference[oaicite:19]{index=19}

🎉 You now know how a tiny slice of silicon can steer whole currents!