AE Notes

Diode Characteristics

Study of diode V-I characteristics including forward bias, reverse bias, breakdown region, temperature effects, and diode parameters for silicon and germanium diodes.

Introduction

The V-I characteristic of a diode is the graphical representation of the relationship between the voltage applied across the diode and the current flowing through it. Understanding these characteristics is essential for selecting diodes for specific applications and predicting circuit behavior.

Complete V-I Characteristic Curve

\| Region 3	╱ Region
\| Region 1	Reverse Bias
-I \|⋰ ← Region 2	Breakdown

Three Operating Regions

Region 1: Reverse Bias (V < 0)

Silicon	Is = 1-100 nA
Germanium	Is = 1-100 µA

Region 2: Breakdown (V < -VBR)

- Avalanche breakdown	High-energy carriers create more pairs
- Zener breakdown	High field directly breaks covalent bonds
Breakdown voltages	5V to 1000V+ depending on doping

Region 3: Forward Bias (V > 0)

a) V < Vγ	Very small current (exponentially growing but < 1 mA)
b) V > Vγ	Large current, approximately linear on I-V plot
- Silicon	Vγ ≈ 0.6 - 0.7 V
- Germanium	Vγ ≈ 0.2 - 0.3 V
- GaAs	Vγ ≈ 1.0 - 1.2 V

Diode Parameters

Parameter	Symbol	Si Typical	Ge Typical
Forward voltage	VF	0.7 V	0.3 V
Reverse saturation current	Is	10 nA	1 µA
Breakdown voltage	VBR	50-1000 V	25-400 V
Knee voltage	Vγ	0.6 V	0.2 V
Ideality factor	n	1-2	1-2
Dynamic resistance	rd	5-50 Ω	2-20 Ω
Junction capacitance	Cj	1-50 pF	2-100 pF
Reverse recovery time	trr	4-50 ns	100-500 ns

Dynamic (AC) Resistance

The small-signal resistance of a forward-biased diode:

Example

rd = nVT / ID = n × 26mV / ID Where ID = DC operating current Example: At ID = 5 mA (n=1): rd = 26/5 = 5.2 Ω Example: At ID = 1 mA (n=1): rd = 26/1 = 26 Ω

Temperature Effects

Forward Voltage Temperature Coefficient

Example

dVF/dT ≈ -2 mV/°C (at constant current) VF(T) = VF(T0) - 0.002 × (T - T0) Example: If VF = 0.7V at 25°C, then at 75°C: VF = 0.7 - 0.002 × 50 = 0.6V

Reverse Current Temperature Dependence

Example

Is doubles for every 10°C rise: Is(T) = Is(T0) × 2^((T-T0)/10) Example: If Is = 10 nA at 25°C, at 85°C: Is = 10 × 2^(60/10) = 10 × 64 = 640 nA

V-I Curves at Different Temperatures

Example

I ^ | T3>T2>T1 | / / / | / / / | // / | /|/ | / / | / / As T↑: Curve shifts LEFT | / / (lower forward voltage needed) |// ●──────────────────> V 0.5 0.6 0.7

Diode Equivalent Circuit Models

Ideal Diode Model

Forward	Short circuit (VF = 0, any current)
Reverse	Open circuit (I = 0, any voltage)
ON	──────── (wire)
OFF	────╳──── (open)

Constant Voltage Drop Model

Forward	Voltage source (VF = 0.7V for Si)
Reverse	Open circuit
ON	───\|0.7V\|────
OFF	────╳────────

Piecewise Linear Model

Forward	Voltage source + resistance
Reverse	Open circuit
ON	───\|Vγ\|──[rd]───
OFF	────╳────────────

Numerical Examples

Example 1: Operating Point

Problem: A silicon diode with Is = 5 nA is in a circuit with 5V supply and 1 kΩ series resistor. Find the diode current and voltage.

Solution:

Using iterative method (load line analysis):

KVL: 5 = I×1000 + VD

Iteration 1: Assume VD = 0.7V

Example

I = (5 - 0.7)/1000 = 4.3 mA

Check with diode equation:

Example

VD = nVT × ln(I/Is + 1) = 0.026 × ln(4.3×10⁻³/5×10⁻⁹ + 1) = 0.026 × ln(860001) = 0.026 × 13.66 = 0.355...

Wait — let me recalculate properly:

Example

VD = nVT × ln(I/Is) = 0.026 × ln(4.3×10⁻³/5×10⁻⁹) = 0.026 × ln(8.6×10⁵) = 0.026 × 13.66 = 0.685V

Iteration 2: VD = 0.685V

Example

I = (5 - 0.685)/1000 = 4.315 mA VD = 0.026 × ln(4.315×10⁻³/5×10⁻⁹) = 0.026 × 13.67 = 0.685V ✓

Result: ID ≈ 4.3 mA, VD ≈ 0.685V

Example 2: Temperature Effect

Problem: A silicon diode carries 10 mA at VF = 0.65V at 25°C. Find VF at 85°C at the same current.

Solution:

Example

ΔT = 85 - 25 = 60°C ΔVF = -2 mV/°C × 60°C = -120 mV VF(85°C) = 0.65 - 0.12 = 0.53V

Example 3: Dynamic Resistance

Problem: Find the AC resistance of a diode operating at DC currents of 0.1 mA, 1 mA, and 10 mA.

Solution:

At 0.1 mA	rd = 26/0.1 = 260 Ω
At 1 mA	rd = 26/1 = 26 Ω
At 10 mA	rd = 26/10 = 2.6 Ω

Higher bias current → lower dynamic resistance → better for large-signal operation.

Interview Questions

Why is the knee voltage of silicon higher than germanium?

Silicon has a larger band gap (1.12 eV vs 0.67 eV), requiring more energy for carrier injection across the junction. This results in a higher built-in potential and therefore a higher forward voltage needed to initiate significant conduction.

What causes the reverse saturation current and why is it temperature-dependent?

Is is caused by thermally generated minority carriers in the vicinity of the junction that are swept across by the built-in field. Since minority carrier generation depends exponentially on temperature (through nᵢ), Is approximately doubles every 10°C.

Explain the difference between avalanche and Zener breakdown.

Avalanche breakdown occurs in lightly-doped junctions with wide depletion regions — accelerated carriers collide with atoms creating more carriers (chain reaction). Zener breakdown occurs in heavily-doped junctions with narrow depletion regions — the strong electric field directly ruptures covalent bonds.

How does the ideality factor (n) affect diode characteristics?

The ideality factor accounts for recombination in the depletion region. n=1 indicates ideal diffusion current. n=2 indicates dominant recombination current. Real diodes have 1<n<2. Higher n means the curve is less steep (more voltage needed for same current).

What is the significance of reverse recovery time?

When a diode switches from forward to reverse bias, stored minority carriers must be removed before the junction can block reverse voltage. This takes time (trr), during which reverse current flows briefly. Fast recovery diodes (trr < 50ns) are needed in high-frequency switching.

Summary

Diode V-I characteristics define three distinct operating regions: forward conduction (exponential), reverse blocking (near-zero current), and breakdown. Understanding temperature effects, dynamic resistance, and equivalent circuit models enables accurate circuit analysis and proper diode selection for applications ranging from power rectification to high-speed switching.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Diode Characteristics.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Analog Electronics topic.

Search Terms

analog-electronics, analog electronics, analog, electronics, semiconductor, fundamentals, diode, characteristics

Continue learning this concept

Semiconductor FundamentalsPN Junction DiodeDetailed explanation of PN junction formation, depletion region, built-in potential, forward and reverse bias operation, and V-I characteristics of the junction diode.Semiconductor FundamentalsAtomic Structure and ElectronicsUnderstanding atomic structure, electron configuration, energy bands, and their relevance to semiconductor physics and analog electronics.Semiconductor FundamentalsConductors, Insulators, and SemiconductorsDetailed comparison of conductors, insulators, and semiconductors based on energy band theory, resistivity, temperature behavior, and applications in electronics.Semiconductor FundamentalsExtrinsic SemiconductorsComprehensive study of extrinsic semiconductors including N-type and P-type doping, donor and acceptor impurities, majority and minority carriers, and compensation doping.Semiconductor FundamentalsIntrinsic SemiconductorsComplete study of intrinsic (pure) semiconductors covering carrier generation, recombination, Fermi level, mass action law, and temperature dependence of conductivity.Semiconductor FundamentalsSemiconductor MaterialsStudy of semiconductor materials including silicon, germanium, gallium arsenide, and compound semiconductors with their properties, crystal structures, and applications.

PreviousConductors, Insulators, and Semiconductors NextExtrinsic Semiconductors

AE Notes

Diode Characteristics

Study of diode V-I characteristics including forward bias, reverse bias, breakdown region, temperature effects, and diode parameters for silicon and germanium diodes.

Introduction

Complete V-I Characteristic Curve

\| Region 3	╱ Region
\| Region 1	Reverse Bias
-I \|⋰ ← Region 2	Breakdown

Three Operating Regions

Region 1: Reverse Bias (V < 0)

Silicon	Is = 1-100 nA
Germanium	Is = 1-100 µA

Region 2: Breakdown (V < -VBR)

- Avalanche breakdown	High-energy carriers create more pairs
- Zener breakdown	High field directly breaks covalent bonds
Breakdown voltages	5V to 1000V+ depending on doping

Region 3: Forward Bias (V > 0)

a) V < Vγ	Very small current (exponentially growing but < 1 mA)
b) V > Vγ	Large current, approximately linear on I-V plot
- Silicon	Vγ ≈ 0.6 - 0.7 V
- Germanium	Vγ ≈ 0.2 - 0.3 V
- GaAs	Vγ ≈ 1.0 - 1.2 V

Diode Parameters

Parameter	Symbol	Si Typical	Ge Typical
Forward voltage	VF	0.7 V	0.3 V
Reverse saturation current	Is	10 nA	1 µA
Breakdown voltage	VBR	50-1000 V	25-400 V
Knee voltage	Vγ	0.6 V	0.2 V
Ideality factor	n	1-2	1-2
Dynamic resistance	rd	5-50 Ω	2-20 Ω
Junction capacitance	Cj	1-50 pF	2-100 pF
Reverse recovery time	trr	4-50 ns	100-500 ns

Dynamic (AC) Resistance

The small-signal resistance of a forward-biased diode:

Example

rd = nVT / ID = n × 26mV / ID Where ID = DC operating current Example: At ID = 5 mA (n=1): rd = 26/5 = 5.2 Ω Example: At ID = 1 mA (n=1): rd = 26/1 = 26 Ω

Temperature Effects

Forward Voltage Temperature Coefficient

Example

dVF/dT ≈ -2 mV/°C (at constant current) VF(T) = VF(T0) - 0.002 × (T - T0) Example: If VF = 0.7V at 25°C, then at 75°C: VF = 0.7 - 0.002 × 50 = 0.6V

Reverse Current Temperature Dependence

Example

Is doubles for every 10°C rise: Is(T) = Is(T0) × 2^((T-T0)/10) Example: If Is = 10 nA at 25°C, at 85°C: Is = 10 × 2^(60/10) = 10 × 64 = 640 nA

V-I Curves at Different Temperatures

Example

I ^ | T3>T2>T1 | / / / | / / / | // / | /|/ | / / | / / As T↑: Curve shifts LEFT | / / (lower forward voltage needed) |// ●──────────────────> V 0.5 0.6 0.7

Diode Equivalent Circuit Models

Ideal Diode Model

Forward	Short circuit (VF = 0, any current)
Reverse	Open circuit (I = 0, any voltage)
ON	──────── (wire)
OFF	────╳──── (open)

Constant Voltage Drop Model

Forward	Voltage source (VF = 0.7V for Si)
Reverse	Open circuit
ON	───\|0.7V\|────
OFF	────╳────────

Piecewise Linear Model

Forward	Voltage source + resistance
Reverse	Open circuit
ON	───\|Vγ\|──[rd]───
OFF	────╳────────────

Numerical Examples

Example 1: Operating Point

Problem: A silicon diode with Is = 5 nA is in a circuit with 5V supply and 1 kΩ series resistor. Find the diode current and voltage.

Solution:

Using iterative method (load line analysis):

KVL: 5 = I×1000 + VD

Iteration 1: Assume VD = 0.7V

Example

I = (5 - 0.7)/1000 = 4.3 mA

Check with diode equation:

Example

VD = nVT × ln(I/Is + 1) = 0.026 × ln(4.3×10⁻³/5×10⁻⁹ + 1) = 0.026 × ln(860001) = 0.026 × 13.66 = 0.355...

Wait — let me recalculate properly:

Example

VD = nVT × ln(I/Is) = 0.026 × ln(4.3×10⁻³/5×10⁻⁹) = 0.026 × ln(8.6×10⁵) = 0.026 × 13.66 = 0.685V

Iteration 2: VD = 0.685V

Example

I = (5 - 0.685)/1000 = 4.315 mA VD = 0.026 × ln(4.315×10⁻³/5×10⁻⁹) = 0.026 × 13.67 = 0.685V ✓

Result: ID ≈ 4.3 mA, VD ≈ 0.685V

Example 2: Temperature Effect

Problem: A silicon diode carries 10 mA at VF = 0.65V at 25°C. Find VF at 85°C at the same current.

Solution:

Example

ΔT = 85 - 25 = 60°C ΔVF = -2 mV/°C × 60°C = -120 mV VF(85°C) = 0.65 - 0.12 = 0.53V

Example 3: Dynamic Resistance

Problem: Find the AC resistance of a diode operating at DC currents of 0.1 mA, 1 mA, and 10 mA.

Solution:

At 0.1 mA	rd = 26/0.1 = 260 Ω
At 1 mA	rd = 26/1 = 26 Ω
At 10 mA	rd = 26/10 = 2.6 Ω

Higher bias current → lower dynamic resistance → better for large-signal operation.

Interview Questions

Why is the knee voltage of silicon higher than germanium?

What causes the reverse saturation current and why is it temperature-dependent?

Explain the difference between avalanche and Zener breakdown.

How does the ideality factor (n) affect diode characteristics?

What is the significance of reverse recovery time?

Summary

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Diode Characteristics.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Analog Electronics topic.

Search Terms

analog-electronics, analog electronics, analog, electronics, semiconductor, fundamentals, diode, characteristics

Continue learning this concept

PreviousConductors, Insulators, and Semiconductors NextExtrinsic Semiconductors