2. PN Junction Diodes

Learning Objectives

• Explain how joining P-type and N-type semiconductors creates a depletion region and a built-in potential
• Describe what happens to the depletion region and current flow under forward and reverse bias
• Read and interpret the V-I characteristic curve of a PN junction diode
• Distinguish between the main diode types (rectifier, Zener, LED, photodiode) and their typical uses
• Calculate approximate diode behavior using the 0.7 V (silicon) or 0.3 V (germanium) approximation
• Identify common diode circuit applications such as rectification and voltage regulation

Quick Answer

A PN junction diode is formed by joining P-type and N-type semiconductor material into a single crystal. At the junction, electrons and holes diffuse across and recombine, leaving behind fixed dopant ions that create a thin depletion region and a built-in electric field of about 0.6–0.7 V for silicon. Forward bias (positive terminal to P-side) opposes this field, narrows the depletion region, and lets current flow easily once the voltage exceeds the turn-on threshold. Reverse bias widens the depletion region and blocks current except for a tiny leakage current. This one-way behavior makes diodes essential for rectification, signal demodulation, voltage regulation (Zener diodes), and light emission (LEDs).

How a PN Junction Forms

Take a single crystal of silicon and dope one half with acceptor atoms like boron (P-type, majority carriers are holes) and the other half with donor atoms like phosphorus (N-type, majority carriers are electrons). The moment these two regions meet, nature does not wait for you to apply a voltage — diffusion begins immediately because each side has a huge concentration gradient of its own carriers relative to the other.

Electrons near the junction on the N-side diffuse into the P-side and recombine with holes there. Holes near the junction on the P-side diffuse into the N-side and recombine with electrons. Every recombination event removes a mobile carrier and exposes a fixed, immobile dopant ion: a positive donor ion left behind in the N-side, a negative acceptor ion left behind in the P-side. This strip of exposed ions on both sides of the junction, now empty of mobile carriers, is the depletion region (or depletion layer).

The exposed ions create an electric field pointing from the N-side toward the P-side. This field opposes further diffusion — it pushes any electron trying to cross back into the N-side and any hole trying to cross back into the P-side. Diffusion and this opposing drift reach a balance at equilibrium, and the field settles into a built-in potential (barrier potential) of about 0.6–0.7 V for silicon and about 0.3 V for germanium. No external circuit is needed for any of this — it happens simply because P-type and N-type materials were brought into contact.

Visual Learning

Forward and Reverse Bias

Forward Bias

Connect the positive terminal of a battery to the P-side (anode) and the negative terminal to the N-side (cathode). The applied voltage pushes holes toward the junction from the P-side and electrons toward the junction from the N-side, directly opposing the built-in field. Once the applied voltage exceeds the built-in potential (~0.7 V for silicon), the depletion region collapses to a very thin layer, majority carriers cross the junction freely, and current rises steeply — increasing exponentially with a very small further increase in voltage, following the diode equation.

Reverse Bias

Connect the positive terminal to the N-side (cathode) and the negative to the P-side (anode). Now the applied voltage pulls majority carriers away from the junction, widening the depletion region and increasing the effective barrier. Only a very small reverse saturation current flows, carried by minority carriers that are thermally generated near the junction — typically in the nanoamp to microamp range for silicon. If reverse voltage is increased far enough, the diode reaches its breakdown voltage, where avalanche or Zener breakdown causes a sharp, controlled rise in current — this is deliberately exploited in Zener diodes for voltage regulation but is normally destructive in ordinary rectifier diodes.

The V-I Characteristic Curve

The V-I curve captures everything above in one picture. In the forward region, current stays negligible until the applied voltage nears the turn-on voltage (about 0.7 V for silicon, 0.3 V for germanium), after which current rises exponentially — a small increase in voltage produces a large increase in current. In the reverse region, current stays essentially flat and very small (the reverse saturation current) across a wide range of reverse voltages, until breakdown voltage is reached, at which point current increases sharply for almost no further increase in voltage. Engineers read this curve to pick a safe operating region: normal use stays well below breakdown voltage and above the knee voltage when conduction is wanted.

Types of PN Junction Diodes

Diode Type	Special Feature	Typical Use
Rectifier diode	Standard PN junction optimized for forward conduction and reverse blocking	Converting AC to DC in power supplies
Zener diode	Heavily doped junction with a sharp, controlled reverse breakdown	Voltage regulation and reference circuits
Light Emitting Diode (LED)	Direct-bandgap material (GaAs, GaN, etc.) releases photons on recombination	Indicators, displays, lighting
Photodiode	Reverse-biased junction generates current proportional to incident light	Light sensors, optical receivers
Schottky diode	Metal-semiconductor junction instead of PN, very low forward drop and fast switching	High-frequency rectification, fast switching
Varactor diode	Reverse-biased junction capacitance varies with applied voltage	Tuning circuits, voltage-controlled oscillators

Diode Applications in Circuits

Rectification is the most common use: a single diode passes only the positive (or negative) half of an AC waveform, producing pulsating DC; four diodes arranged as a bridge rectifier convert the full AC waveform, both half-cycles, into pulsating DC that a capacitor can smooth further.

Voltage regulation uses a Zener diode reverse-biased into its breakdown region. Because the voltage across a Zener diode stays nearly constant over a wide current range once it is in breakdown, it holds a stable reference voltage even as the supply or load current varies.

Clipping and clamping circuits use diodes to limit a waveform to a certain voltage range (clipping) or shift a waveform's DC level without changing its shape (clamping) — both rely on the diode simply switching between "on" (~0.7 V drop) and "off" (essentially open) states.

Protection circuits place a diode across an inductive load (like a relay coil) to give the collapsing magnetic field's induced current a safe path, protecting switching transistors from voltage spikes (a "flyback" or "freewheeling" diode).

Key Terms

Term	Definition	Related Concept
PN Junction	Interface formed where P-type and N-type semiconductor regions meet	Diode structure
Depletion Region	Narrow zone at the junction cleared of mobile carriers by recombination	Built-in potential
Built-in Potential	Equilibrium voltage barrier (~0.7 V silicon) created by the depletion region's electric field	Forward/reverse bias
Forward Bias	External voltage applied to narrow the depletion region and enable conduction	Anode positive relative to cathode
Reverse Bias	External voltage applied to widen the depletion region and block conduction	Reverse saturation current
Reverse Saturation Current	Small current from thermally generated minority carriers under reverse bias	Leakage current
Breakdown Voltage	Reverse voltage at which current rises sharply due to avalanche or Zener effect	Zener diode design
Turn-on Voltage	Approximate forward voltage where conduction becomes significant (0.7 V Si, 0.3 V Ge)	V-I characteristic
Rectification	Conversion of AC to pulsating DC using diode's one-way conduction	Bridge rectifier

Common Mistakes

Misconception: A diode conducts as soon as any positive forward voltage is applied, like a perfect switch. Why it's wrong: Real diodes need the applied voltage to overcome the built-in potential before significant current flows. Below about 0.5–0.6 V for silicon, forward current is negligible; conduction only becomes practically significant near 0.7 V. Correct understanding: Treat a silicon diode as "off" below roughly 0.6 V and "on" with an approximately constant 0.7 V drop above that — the diode equation describes the exact exponential transition, but the 0.7 V approximation is accurate enough for most circuit analysis.

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Misconception: Under reverse bias, absolutely no current flows through a diode. Why it's wrong: A small reverse saturation current always flows, caused by thermally generated minority carriers drifting across the junction. It is small (nanoamps to microamps for silicon) but never exactly zero, and it roughly doubles for every 10°C rise in temperature. Correct understanding: Reverse current is small enough to treat as zero in most introductory circuit calculations, but it becomes significant in precision analog circuits and increases sharply with temperature, which matters for thermal stability.

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Misconception: Reverse breakdown always destroys the diode. Why it's wrong: Breakdown itself is not inherently destructive — it is a well-defined, repeatable electrical phenomenon. Damage occurs only if the current through the diode during breakdown is not limited, causing excessive power dissipation and heat. Correct understanding: Zener and avalanche diodes are specifically designed to operate continuously in reverse breakdown, as long as an external resistor limits the current to a safe value; ordinary rectifier diodes are simply not built to survive breakdown current levels.

Comparison and Connections

Aspect	Forward Bias	Reverse Bias
Depletion region	Narrows	Widens
Barrier height	Reduced	Increased
Current magnitude	Large, rises exponentially with voltage	Very small (leakage), until breakdown
Dominant carriers	Majority carriers	Minority carriers (thermally generated)
Practical diode state	"On" — acts like a low resistance	"Off" — acts like an open circuit

Diode Type	Bias Used in Normal Operation	Key Distinguishing Behavior
Rectifier diode	Alternates forward/reverse	Simple one-way conduction
Zener diode	Reverse (in breakdown)	Constant voltage in breakdown region
LED	Forward	Emits light on recombination
Photodiode	Reverse	Current proportional to light intensity

Practice Questions

Recall

1. What is the approximate built-in potential of a silicon PN junction, and what physical mechanism creates it? Guidance: About 0.6–0.7 V, created by diffusion of majority carriers across the junction leaving behind fixed dopant ions whose electric field opposes further diffusion.

2. Name three types of PN junction diodes and one key application for each. Guidance: Rectifier diode — AC to DC conversion; Zener diode — voltage regulation; LED — light emission/indicators (or photodiode — light detection).

Understanding

3. Explain why the depletion region widens under reverse bias but narrows under forward bias. Guidance: Forward bias pushes majority carriers toward the junction, opposing the built-in field and shrinking the ion-exposed region. Reverse bias pulls majority carriers away from the junction, exposing more ions on both sides and widening the depletion region.

4. Why does forward current rise exponentially rather than linearly once the diode turns on? Guidance: The diode current follows the Shockley diode equation, where current depends exponentially on applied voltage divided by the thermal voltage — this comes from the exponential dependence of carrier injection across the junction on the applied bias.

Application

5. A silicon diode in a circuit has 5 V applied through a 1 kΩ resistor in the forward direction. Estimate the current flowing, using the 0.7 V approximation. Guidance: Voltage across resistor = 5 − 0.7 = 4.3 V. Current = 4.3 V / 1 kΩ = 4.3 mA.

6. A student wants to design a simple LED indicator circuit powered from a 9 V supply, using an LED with a 2 V forward drop and a desired current of 20 mA. What resistor value should they use? Guidance: Voltage across resistor = 9 − 2 = 7 V. R = 7 V / 0.02 A = 350 Ω (nearest standard value like 330 Ω or 390 Ω).

Analysis

7. Two diodes are available: one silicon (turn-on ~0.7 V) and one germanium (turn-on ~0.3 V). For a low-voltage detector circuit operating from a 1.5 V battery, which would likely perform better, and why? Guidance: Germanium, because its lower turn-on voltage allows it to conduct usefully even with a very low supply, whereas the silicon diode's larger drop leaves little headroom.

8. A Zener diode rated at 5.1 V breakdown is connected in reverse bias with a series resistor across a 12 V supply, powering a load. Explain how this configuration keeps the load voltage stable even if the supply voltage rises to 13 V. Guidance: As long as the Zener stays in breakdown, its voltage remains close to 5.1 V regardless of moderate changes in the current through it. The extra supply voltage is absorbed by the series resistor, which carries more current, while the Zener (and hence the load) voltage stays essentially fixed.

FAQ

Why does a silicon diode need about 0.7 V before it conducts, rather than 0 V? The 0.7 V is the energy per unit charge needed to push carriers over the built-in potential barrier created when the PN junction formed. This barrier exists regardless of any external circuit — it's a property of the junction itself. Below that voltage, the applied field only partially cancels the built-in field, so very few carriers have enough energy to cross, and forward current stays negligible.

What is the difference between reverse breakdown in a Zener diode and reverse breakdown in an ordinary diode? Physically, breakdown mechanisms are the same (Zener effect at lower voltages via direct tunneling, or avalanche effect at higher voltages via carrier multiplication). The difference is intentional design: Zener diodes are heavily doped and built with adequate power dissipation and a sharp, well-defined breakdown knee so they can operate continuously in this region without damage. Ordinary rectifier diodes are not designed for this and will typically be damaged if pushed into breakdown without careful current limiting.

Can a diode be used as a switch? Yes — in digital and switching circuits, a diode's sharp transition between "off" (essentially open circuit) under reverse bias and "on" (low resistance) under sufficient forward bias makes it useful as a simple, no-moving-parts switch, though it only switches based on voltage polarity/magnitude, not an independent control signal (unlike a transistor).

Why do LEDs need different forward voltages depending on color? The forward voltage of an LED corresponds closely to the bandgap energy of its semiconductor material, since that's the energy released per photon during recombination. Red LEDs (smaller bandgap materials like GaAsP) have lower forward voltages (~1.8–2.2 V), while blue and white LEDs (wider bandgap materials like GaN) need higher forward voltages (~3–3.4 V) because producing higher-energy (shorter wavelength) photons requires a larger bandgap.

What happens if you connect a diode backward in a circuit by mistake? For a rectifier diode, connecting it backward simply blocks current — it behaves as if the circuit were open, since it's now reverse biased. In a rectifier or power supply, this typically means the circuit produces no output. If the reverse voltage exceeds the diode's rated reverse breakdown voltage, it may be permanently damaged, so checking polarity markings (a band on the cathode end) before connecting a diode always matters.

Quick Revision

• A PN junction forms a depletion region and built-in potential purely by bringing P-type and N-type material into contact
• Silicon diodes have a built-in potential/turn-on voltage of about 0.6–0.7 V; germanium diodes are about 0.3 V
• Forward bias narrows the depletion region and allows exponentially rising current
• Reverse bias widens the depletion region and permits only a small leakage (reverse saturation) current
• Exceeding the breakdown voltage in reverse bias causes a sharp current rise (avalanche or Zener effect)
• The V-I characteristic curve shows negligible current until turn-on, then exponential rise; flat, tiny current in reverse until breakdown
• Rectifier diodes convert AC to pulsating DC; bridge rectifiers use four diodes to rectify both half-cycles
• Zener diodes are deliberately operated in reverse breakdown to provide a stable reference voltage
• LEDs emit light on forward-biased recombination; forward voltage depends on the semiconductor's bandgap
• Photodiodes are reverse-biased and generate current proportional to incident light intensity
• Reverse saturation current roughly doubles for every 10°C temperature rise
• Diode polarity matters: connecting one backward blocks conduction (or risks breakdown damage) in that orientation

Related Topics

Prerequisites: Introduction to Semiconductor Devices, N-type and P-type doping, Band theory basics

Related Topics: Bipolar Junction Transistors, Photonic Devices, Zener diode voltage regulation, Rectifier circuit design

Next Topics: Bipolar Junction Transistors, Field Effect Transistors, Power Semiconductors