AE Notes

Introduction to Diodes

Comprehensive introduction to semiconductor diodes covering construction, working principle, types, symbol conventions, and basic applications in analog circuits.

What is a Diode?

A diode is a two-terminal semiconductor device that allows current to flow in one direction (forward bias) while blocking current in the opposite direction (reverse bias). It is the simplest and most fundamental semiconductor device, formed by joining P-type and N-type semiconductors.

Diode Symbol and Terminals

████████	█
Body	Band	Lead
(Anode)	(K)

Working Principle

Forward Bias (Conducting State)

│ ○	○ │ ← ● ● │ Carriers pushed toward junction
│ ○	○ │ ← ● ● │ → Current flows
Condition	V_Anode > V_Cathode

Reverse Bias (Blocking State)

│ ← ○ ○ │ ● ●	│ Carriers pulled away from junction
│ ← ○ ○ │ ● ●	│ → Depletion widens → No current
Condition	V_Cathode > V_Anode

Types of Diodes

Type	Symbol	Key Feature	Application
Standard rectifier	▶\		General purpose	Power rectification
Zener diode	▶\	⟋	Controlled breakdown	Voltage regulation
LED	▶\	→	Emits light	Indicators, lighting
Photodiode	▶\	←	Light sensitive	Sensors, communication
Schottky	▶\	S	Low VF (0.2-0.4V)	High-speed switching
Varactor	▶\	C	Variable capacitance	Tuning circuits
Tunnel diode	▶\	T	Negative resistance	Oscillators, high-speed
PIN diode	▶\	P	High-speed switching	RF switches

V-I Characteristics Summary

10	/
5	/
1	· (Si: 0.7V, Ge: 0.3V)

Diode Equation (Shockley Equation)

I = Is × (e^(V/nVT) - 1) Parameters: ┌────────────────────────────────────────────────────┐ │ Is = Reverse saturation current (1-100 nA for Si) │ │ V = Applied voltage across diode │ │ n = Ideality factor (1 for ideal, 1-2 practical) │ │ VT = Thermal voltage = kT/q ≈ 26 mV at 27°C │ │ k = Boltzmann constant = 1.38 × 10⁻²³ J/K │ │ T = Temperature in Kelvin │ │ q = Electron charge = 1.6 × 10⁻¹⁹ C │ └────────────────────────────────────────────────────┘

Diode Circuit Analysis

Simple Diode Circuit

Example

R = 1kΩ Vs ──[████]──┬──▶|──┬── GND 5V │ │ VR VD

Using constant voltage model (VD = 0.7V for Si):

Example

VR = Vs - VD = 5 - 0.7 = 4.3V I = VR/R = 4.3/1000 = 4.3 mA

Load Line Analysis

Example

I = (Vs - VD) / R ← Load line equation I (mA) ^ 5 |· Load line: I = (5-V)/1000 | · | · 4.3|─ ─ ─● ← Q-point (operating point) | · · | · · Diode curve | · · +──────────●───●───> V 0.7 5

Numerical Example

Problem: Two silicon diodes are connected in series with a 10V supply and 2.2 kΩ resistor. Find the current and voltage across each diode.

Example

10V ──[2.2kΩ]──▶|──▶|── GND D1 D2

Solution:

Step 1: Both diodes are forward biased (current flows from + to -)

Example

VD1 = VD2 = 0.7V (silicon diodes)

Step 2: Apply KVL

Example

10 = I × 2200 + VD1 + VD2 10 = I × 2200 + 0.7 + 0.7 10 - 1.4 = I × 2200 I = 8.6/2200 = 3.91 mA

Step 3: Verify

Example

VR = 3.91 × 10⁻³ × 2200 = 8.6V VR + VD1 + VD2 = 8.6 + 0.7 + 0.7 = 10V ✓

Diode Ratings and Specifications

Parameter	Description	Typical Range
IF(max)	Maximum forward current	100 mA - 30 A
VR(max)	Maximum reverse voltage	50 - 1000 V
VF	Forward voltage at rated IF	0.6 - 1.2 V
IR	Reverse leakage current	1 nA - 100 µA
PD(max)	Maximum power dissipation	250 mW - 50 W
trr	Reverse recovery time	4 ns - 5 µs
TJ(max)	Maximum junction temperature	150 - 200°C

Common Diode Part Numbers

Part Number	Type	IF(max)	VR(max)	Application
1N4148	Signal	200 mA	100 V	General switching
1N4007	Rectifier	1 A	1000 V	Power supply
1N5819	Schottky	1 A	40 V	Low-loss rectification
1N4733	Zener (5.1V)	—	—	Voltage regulation
BAT54	Schottky	200 mA	30 V	RF detection

Interview Questions

What happens if you connect a diode in reverse across a battery?

Only a very small leakage current (nA for Si) flows, limited to thermally generated minority carriers. The diode effectively blocks current. If the battery voltage exceeds the breakdown voltage, the diode may be damaged.

Why does a real diode need approximately 0.7V to conduct?

This voltage overcomes the built-in potential barrier of the PN junction. The built-in potential depends on the band gap and doping levels. Silicon's band gap of 1.12 eV creates a barrier of approximately 0.6-0.7V that must be overcome for significant diffusion current.

What is the difference between a signal diode and a power diode?

Signal diodes (e.g., 1N4148) are small, fast-switching, low-current devices. Power diodes (e.g., 1N4007) handle high current and voltage but switch slower due to larger junction area and more stored charge.

Can a diode be used as a temperature sensor?

Yes. The forward voltage of a diode decreases linearly at approximately -2 mV/°C when operated at constant current. This predictable temperature coefficient makes diodes useful as inexpensive temperature sensors.

What is the maximum current a diode can handle and what limits it?

Maximum current is limited by thermal dissipation. Power = VF × IF generates heat. If junction temperature exceeds the maximum (typically 150-200°C), the device is destroyed. Better heat sinking allows higher current.

Summary

Diodes are fundamental semiconductor devices enabling one-way current flow. Their operation is based on PN junction physics — forward bias reduces the barrier allowing current, while reverse bias increases it blocking current. Understanding diode characteristics, ratings, and models is essential for analog circuit design.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Introduction to Diodes.

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, diodes, diode, introduction, introduction to diodes

Continue learning this concept

DiodesDiode Applications in Analog ElectronicsComprehensive guide to practical diode applications including rectification, clamping, clipping, and specialized circuits.DiodesDiode Equation (Shockley Equation)Detailed derivation and analysis of the Shockley diode equation including temperature dependence, ideality factor, and applications in circuit analysis.DiodesLED Diodes: Principles, Characteristics, and ApplicationsComplete guide to Light Emitting Diodes covering semiconductor physics, color selection, brightness calculations, and practical circuit design.DiodesZener DiodeComplete study of Zener diodes covering breakdown mechanisms, V-I characteristics, voltage regulation circuits, design calculations, and practical applications.DiodesBridge RectifierComplete analysis of the full-wave bridge rectifier circuit including four-diode configuration, operation, comparison with center-tapped design, and practical applications.DiodesFull-Wave RectifierAnalysis of center-tapped full-wave rectifier including operation, output waveform, ripple factor, efficiency comparison with half-wave, and design considerations.

PreviousDiode Equation (Shockley Equation)NextFull-Wave Rectifier

AE Notes

Introduction to Diodes

Comprehensive introduction to semiconductor diodes covering construction, working principle, types, symbol conventions, and basic applications in analog circuits.

What is a Diode?

Diode Symbol and Terminals

████████	█
Body	Band	Lead
(Anode)	(K)

Working Principle

Forward Bias (Conducting State)

│ ○	○ │ ← ● ● │ Carriers pushed toward junction
│ ○	○ │ ← ● ● │ → Current flows
Condition	V_Anode > V_Cathode

Reverse Bias (Blocking State)

│ ← ○ ○ │ ● ●	│ Carriers pulled away from junction
│ ← ○ ○ │ ● ●	│ → Depletion widens → No current
Condition	V_Cathode > V_Anode

Types of Diodes

Type	Symbol	Key Feature	Application
Standard rectifier	▶\		General purpose	Power rectification
Zener diode	▶\	⟋	Controlled breakdown	Voltage regulation
LED	▶\	→	Emits light	Indicators, lighting
Photodiode	▶\	←	Light sensitive	Sensors, communication
Schottky	▶\	S	Low VF (0.2-0.4V)	High-speed switching
Varactor	▶\	C	Variable capacitance	Tuning circuits
Tunnel diode	▶\	T	Negative resistance	Oscillators, high-speed
PIN diode	▶\	P	High-speed switching	RF switches

V-I Characteristics Summary

10	/
5	/
1	· (Si: 0.7V, Ge: 0.3V)

Diode Equation (Shockley Equation)

Diode Circuit Analysis

Simple Diode Circuit

Example

R = 1kΩ Vs ──[████]──┬──▶|──┬── GND 5V │ │ VR VD

Using constant voltage model (VD = 0.7V for Si):

Example

VR = Vs - VD = 5 - 0.7 = 4.3V I = VR/R = 4.3/1000 = 4.3 mA

Load Line Analysis

Example

Numerical Example

Problem: Two silicon diodes are connected in series with a 10V supply and 2.2 kΩ resistor. Find the current and voltage across each diode.

Example

10V ──[2.2kΩ]──▶|──▶|── GND D1 D2

Solution:

Step 1: Both diodes are forward biased (current flows from + to -)

Example

VD1 = VD2 = 0.7V (silicon diodes)

Step 2: Apply KVL

Example

10 = I × 2200 + VD1 + VD2 10 = I × 2200 + 0.7 + 0.7 10 - 1.4 = I × 2200 I = 8.6/2200 = 3.91 mA

Step 3: Verify

Example

VR = 3.91 × 10⁻³ × 2200 = 8.6V VR + VD1 + VD2 = 8.6 + 0.7 + 0.7 = 10V ✓

Diode Ratings and Specifications

Parameter	Description	Typical Range
IF(max)	Maximum forward current	100 mA - 30 A
VR(max)	Maximum reverse voltage	50 - 1000 V
VF	Forward voltage at rated IF	0.6 - 1.2 V
IR	Reverse leakage current	1 nA - 100 µA
PD(max)	Maximum power dissipation	250 mW - 50 W
trr	Reverse recovery time	4 ns - 5 µs
TJ(max)	Maximum junction temperature	150 - 200°C

Common Diode Part Numbers

Part Number	Type	IF(max)	VR(max)	Application
1N4148	Signal	200 mA	100 V	General switching
1N4007	Rectifier	1 A	1000 V	Power supply
1N5819	Schottky	1 A	40 V	Low-loss rectification
1N4733	Zener (5.1V)	—	—	Voltage regulation
BAT54	Schottky	200 mA	30 V	RF detection

Interview Questions

What happens if you connect a diode in reverse across a battery?

Why does a real diode need approximately 0.7V to conduct?

What is the difference between a signal diode and a power diode?

Can a diode be used as a temperature sensor?

What is the maximum current a diode can handle and what limits it?

Summary

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Introduction to Diodes.

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, diodes, diode, introduction, introduction to diodes

Continue learning this concept

PreviousDiode Equation (Shockley Equation)NextFull-Wave Rectifier