Semiconductor Diode: Working, V-I Characteristics, Types & Rectifiers (with MCQs)

The semiconductor diode is the simplest electronic device — a one-way valve for current — yet it powers every phone charger, protects circuits from reverse voltage and lights our homes as LEDs. Diode theory (barrier potential, forward/reverse bias, V-I curve, breakdown, rectifiers) is asked in almost every BCS (EEE), PGCB, BPDB, BREB, DPDC, NTRCA and university electronics exam. This post explains it step by step with an animation, formulas, solved problems and MCQs.

⚡ Key takeaways

• A diode is a p-n junction with two terminals: anode (p) and cathode (n).
• A depletion region and barrier potential form at the junction: ≈ 0.7 V for Si, ≈ 0.3 V for Ge.
• Forward bias (p positive) narrows the depletion region → large current after the cut-in voltage. Reverse bias widens it → only tiny saturation current.
• Shockley equation: I = I_S(e^V/ηV_T − 1), V_T ≈ 26 mV at room temperature.
• Uses: rectifiers, clippers, clampers, voltage regulation (Zener), LEDs, photodiodes, protection.

📑 Contents

1. Formation of the p-n junction
2. Forward and reverse bias (animation)
3. V-I characteristic and diode equation
4. Reverse breakdown: Zener and avalanche
5. Important parameters and models
6. Types of diodes
7. Rectifiers at a glance
8. Advantages of semiconductor diodes
9. Solved problems
10. Practice MCQs
11. বাংলায় দ্রুত রিভিশন

1. Formation of the p-n Junction

When p-type (many holes) and n-type (many electrons) silicon are joined, electrons diffuse into the p-side and holes into the n-side and recombine. This leaves behind immobile ions — negative acceptor ions on the p-side, positive donor ions on the n-side — forming the depletion region (no free carriers). The ions create an electric field and a barrier potential V₀ that stops further diffusion: about 0.7 V for silicon and 0.3 V for germanium at room temperature. See PN junction formation.

2. Forward and Reverse Bias (Animation)

• Forward bias: positive terminal to p (anode), negative to n (cathode). The applied voltage opposes the barrier, the depletion region narrows, and once V exceeds the cut-in (knee) voltage current rises steeply.
• Reverse bias: positive to n, negative to p. The depletion region widens and only a very small reverse saturation current I_S (nA for Si) flows, carried by minority carriers.

Animation 1 – As bias goes from −5 V to +0.75 V, the depletion region (hatched) narrows and current flows only above ≈ 0.7 V.

3. V-I Characteristic and Diode Equation

I = I_S (e^{V / ηV_T} − 1), V_T = kT/q ≈ 25.85 mV at 300 K

• η (ideality factor) ≈ 1 for Ge and 1–2 for Si.
• Forward region: current increases exponentially after cut-in.
• Reverse region: I ≈ −I_S until breakdown.
• Temperature effects: I_S roughly doubles every 10 °C; forward voltage drops about 2 mV/°C.

4. Reverse Breakdown: Zener and Avalanche

Feature	Zener breakdown	Avalanche breakdown
Doping	Heavy (thin depletion region)	Light/moderate
Voltage	Low (≲ 5 V)	Higher (≳ 6–7 V)
Mechanism	Strong field pulls electrons from covalent bonds (tunnelling)	Carriers gain energy and ionise atoms by collision — multiplication
Temperature coefficient	Negative	Positive

More detail: PN Junction Breakdown: Zener vs Avalanche.

5. Important Parameters and Models

• Forward current rating I_F(max), peak inverse voltage (PIV), power rating, reverse recovery time t_rr.
• Static resistance R_DC = V/I; dynamic (AC) resistance r_d = ΔV/ΔI ≈ ηV_T/I_D ≈ 26 mV / I_D.
• Junction (transition) capacitance in reverse bias and diffusion capacitance in forward bias.
• Models: ideal (short when forward, open when reverse); constant-drop (0.7 V); piecewise linear (0.7 V + r_d).

6. Types of Diodes

Diode	Key feature	Application
Rectifier (1N4007)	High current & PIV, 50/60 Hz	Power supplies
Zener	Operates in reverse breakdown at fixed VZ	Voltage regulation, reference
LED	Emits light when forward biased (GaAs, GaN…)	Lighting, displays, indicators
Photodiode	Reverse current proportional to light	Optical receivers, sensors
Schottky	Metal–semiconductor, low drop (0.2–0.4 V), very fast	SMPS, RF detectors
Varactor	Capacitance varies with reverse voltage	VCOs, tuning
Tunnel	Heavily doped, negative-resistance region	Microwave oscillators (see negative resistance)
PIN	Intrinsic layer between p and n	RF switches, attenuators, photodetectors

7. Rectifiers at a Glance

Parameter	Half-wave	Full-wave centre-tap	Bridge
Number of diodes	1	2	4
VDC (ideal)	Vm/π	2Vm/π	2Vm/π
Ripple factor	1.21	0.482	0.482
Efficiency (max)	40.6 %	81.2 %	81.2 %
PIV per diode	Vm	2Vm	Vm
Ripple frequency (50 Hz input)	50 Hz	100 Hz	100 Hz

8. Advantages of Semiconductor Diodes (over Vacuum Diodes)

• Very small size and weight; no heater, so no warm-up time and lower power consumption.
• High efficiency and low forward voltage drop.
• Long life, mechanical ruggedness and low cost.
• Operate at low voltages and can be integrated in ICs.
• Limitations: sensitive to temperature, limited reverse voltage and current ratings, can be damaged by surges.

9. Solved Problems

Problem 1. Find the dynamic resistance of a silicon diode at 1 mA and 10 mA (η = 1). r_d = 26 mV/1 mA = 26 Ω; at 10 mA = 2.6 Ω.

Problem 2. A bridge rectifier is fed from a 12 V (RMS) transformer secondary. Find V_m and the DC output (a) ideal, (b) with 0.7 V per diode.

1. V_m = 12√2 ≈ 16.97 V
2. (a) V_DC = 2V_m/π ≈ 10.8 V
3. (b) Two diodes conduct at a time: V_m′ = 16.97 − 1.4 = 15.57 V ⇒ V_DC ≈ 9.9 V

Problem 3. I_S = 10 nA at 25 °C. Estimate I_S at 55 °C. Three 10 °C steps ⇒ 10 × 2³ = 80 nA.

10. Practice MCQs (click to see answer)

1. Barrier potential of a silicon diode at room temperature is about: (a) 0.3 V (b) 0.7 V (c) 1.1 V (d) 0.1 V

✔ (b) 0.7 V.

2. In forward bias, the depletion region: (a) Widens (b) Narrows (c) Stays the same (d) Disappears permanently

✔ (b) Narrows.

3. Reverse saturation current is due to: (a) Majority carriers (b) Minority carriers (c) Ions (d) Photons only

✔ (b) Minority carriers.

4. Reverse saturation current approximately doubles for every: (a) 1 °C (b) 5 °C (c) 10 °C (d) 50 °C rise

✔ (c) 10 °C.

5. Zener breakdown has a temperature coefficient that is: (a) Positive (b) Negative (c) Zero (d) Infinite

✔ (b) Negative.

6. The diode with the lowest forward voltage drop is the: (a) Si rectifier (b) Schottky diode (c) LED (d) Zener

✔ (b) Schottky diode.

7. Efficiency of a full-wave rectifier is: (a) 40.6 % (b) 81.2 % (c) 100 % (d) 50 %

✔ (b) 81.2 %.

8. PIV of each diode in a centre-tapped full-wave rectifier is: (a) V_m (b) 2V_m (c) V_m/2 (d) √2V_m

✔ (b) 2V_m.

9. The thermal voltage V_T at room temperature is about: (a) 0.7 V (b) 26 mV (c) 1 V (d) 2.6 V

✔ (b) 26 mV.

10. A varactor diode is used as a: (a) Rectifier (b) Voltage-variable capacitor (c) Light source (d) Constant-current source

✔ (b) Voltage-variable capacitor.

11. বাংলায় দ্রুত রিভিশন

📝 পরীক্ষার আগে এক নজরে

• ডায়োড: p-n জাংশন; অ্যানোড (p) ও ক্যাথোড (n)। কারেন্ট একদিকে চলতে দেয়।
• জাংশনে ডিপ্লেশন অঞ্চল ও বাধা বিভব তৈরি হয় — সিলিকন ≈ ০.৭ V, জার্মেনিয়াম ≈ ০.৩ V।
• ফরোয়ার্ড বায়াস: ডিপ্লেশন সরু হয়, কাট-ইন ভোল্টেজের পর বড় কারেন্ট। রিভার্স বায়াস: ডিপ্লেশন চওড়া, খুব সামান্য স্যাচুরেশন কারেন্ট (সংখ্যালঘু বাহক)।
• শকলি সমীকরণ: I = I_S(e^V/ηV_T − 1); V_T ≈ ২৬ mV; ডায়নামিক রেজিস্ট্যান্স ≈ ২৬ mV/I।
• জেনার ব্রেকডাউন (কম ভোল্টেজ, ঋণাত্মক তাপ সহগ) বনাম অ্যাভালাঞ্চ (বেশি ভোল্টেজ, ধনাত্মক তাপ সহগ)।
• রেক্টিফায়ার: হাফ-ওয়েভ দক্ষতা ৪০.৬%, ফুল-ওয়েভ ৮১.২%; রিপল ফ্যাক্টর ১.২১ ও ০.৪৮২।

Related Posts

• PN Junction Formation: Depletion Region
• PN Junction Breakdown: Zener vs Avalanche
• FET vs BJT: Key Differences
• Bilateral vs Unilateral Elements

References: Boylestad & Nashelsky, Electronic Devices and Circuit Theory; Millman & Halkias, Integrated Electronics; Sedra & Smith, Microelectronic Circuits. Images and animations: original work by Totalecer.