Special Purpose Diodes Notes FE Sem 1 BEE | MU

Module 5 deals with two special purpose PN junction diodes that are found in nearly every electronic product. The Zener diode is designed to operate in reverse breakdown and is used to hold a voltage constant, which makes it the simplest voltage regulator. The light emitting diode (LED) converts electrical energy directly into light and is used in indicators, seven-segment and dot-matrix displays, and modern lighting. The syllabus states that numericals are not expected, so focus on characteristics, operation, structure and applications with neat diagrams.

Summary

A Zener diode is a heavily doped silicon PN junction diode designed to operate safely in the reverse breakdown region. Because of heavy doping, its depletion layer is very thin and the breakdown voltage Vz is sharp and well defined, available from about 2 V to over 200 V. Its symbol is like a diode with a bent (Z-shaped) bar. In forward bias it behaves like an ordinary silicon diode (about 0.7 V drop).

Zener characteristics: in reverse bias, only a tiny leakage current flows until the reverse voltage reaches Vz; then the current rises sharply while the voltage across the diode stays nearly constant at Vz. The knee current Izk (minimum current to stay in regulation) and the maximum current Izm (set by the power rating Pz = Vz Izm) define the useful operating range. The small change of voltage over this range is described by the Zener (dynamic) resistance rz = delta Vz/delta Iz.

Breakdown mechanisms: Zener breakdown occurs in heavily doped junctions with breakdown below about 5 V; the intense electric field (about 3 x 10^7 V/m) across the thin depletion layer pulls electrons directly out of covalent bonds. It has a negative temperature coefficient. Avalanche breakdown occurs in lightly doped junctions at higher voltages (above about 6 V); minority carriers accelerated by the field gain enough energy to knock out further electrons by collision, causing carrier multiplication. It has a positive temperature coefficient. Diodes around 5 to 6 V show both effects and have nearly zero temperature coefficient. All such diodes are commercially called Zener diodes.

Zener diode as a voltage regulator: the Zener is connected in reverse bias in parallel with the load, and a series resistor Rs is connected between the unregulated supply Vin and the diode. As long as the diode stays in breakdown, the load voltage VL = Vz stays constant. If Vin rises, the extra current flows through the Zener and the extra voltage drops across Rs; if load current rises, Zener current falls by the same amount so the total current through Rs and the drop across it remain nearly the same. Thus it provides both line regulation and load regulation. Is = (Vin - Vz)/Rs and Iz = Is - IL. The regulator works only while Iz lies between Izk and Izm.

Basics of LED: a light emitting diode is a forward-biased PN junction made of a direct band-gap compound semiconductor. In forward bias, electrons from the N region cross the junction and recombine with holes in the P region; in a direct band-gap material the energy released is emitted as a photon (electroluminescence) rather than as heat. The wavelength depends on the band gap: lambda = h c/Eg (lambda in nm about 1240/Eg in eV). Materials: GaAs (infrared), GaAsP (red, orange, yellow), GaP (red, green), AlGaInP (red, orange, yellow), InGaN (blue, green); white LEDs use a blue InGaN chip with a yellow phosphor. Silicon and germanium are indirect band-gap materials and are not used for LEDs. Typical forward voltage is 1.8 to 3.5 V depending on colour, and current is limited with a series resistor.

Structure of LED: the P-N junction is formed on a substrate, with the P layer kept very thin (or the junction near the surface) so that the light generated near the junction escapes with little re-absorption. A metal contact (anode) is made on the top P layer in a small area or as a ring/finger pattern so that most of the surface is free for light emission, and the cathode contact is made at the bottom. The chip sits in a reflecting cup on the cathode lead and is encapsulated in a dome-shaped epoxy lens that directs light upward. The longer lead is the anode and the flat edge on the package marks the cathode. LEDs are operated only in forward bias; their reverse breakdown voltage is low (about 5 V).

Applications of LEDs in indicative displays: power-on and status indicators in electronic equipment, seven-segment displays (seven LED bars a to g plus a decimal point, in common-anode or common-cathode configuration) for digital clocks, calculators and meters, dot-matrix displays (for example 5 x 7) for alphanumeric messages and scrolling boards, bar-graph displays, traffic signals, and infrared LEDs in remote controls and opto-couplers.

Applications of LEDs in lighting displays: LED bulbs, tube lights and street lights, automobile head lamps and tail lamps, torches, backlighting of LCD screens in TVs, laptops and mobiles, large outdoor video walls and advertisement boards (RGB LED pixels), and decorative lighting. Advantages: low power consumption and high luminous efficiency, long life (tens of thousands of hours), small size, fast switching, rugged, low operating voltage, no mercury, available in many colours. Limitations: light output and life fall at high temperature, needs current limiting, sensitive to reverse voltage.

Mind map

How a Zener regulator holds the output constant

Important definitions

Formulas

Key points to remember

• Numericals are not expected in this module; characteristics, operation, regulator circuit, structure and applications are asked.
• A Zener diode is always connected in reverse bias for regulation; in forward bias it acts as an ordinary diode.
• Heavy doping gives a thin depletion layer and a low, sharp breakdown voltage.
• Zener breakdown (below about 5 V) has a negative temperature coefficient; avalanche breakdown (above about 6 V) has a positive temperature coefficient.
• The series resistor Rs is essential; without it the Zener current is unlimited and the diode burns.
• In the regulator, Zener current and load current share a nearly constant total current Is.
• LEDs are always forward biased and need a series current-limiting resistor.
• LED colour depends on the semiconductor band gap, not on the colour of the plastic package.
• Si and Ge are indirect band-gap materials and give heat rather than light, so LEDs use compound semiconductors like GaAs, GaAsP, GaP and InGaN.
• White LEDs are generally blue LEDs coated with a yellow phosphor.
• Seven-segment displays come as common-anode (segments lit by a LOW) and common-cathode (segments lit by a HIGH) types.

Solved examples

Example 1. (Illustrative, numericals not asked in the MU paper.) A 10 V Zener diode is used with a series resistor of 200 ohm and a load of 1 kilo-ohm across a 20 V supply. Find the current through the series resistor, the load current, the Zener current and the power dissipated in the Zener.

Check breakdown: open-circuit load voltage without Zener = 20 x 1000/1200 = 16.7 V, which is greater than 10 V, so the Zener is in breakdown and VL = 10 V.

Is = (20 - 10)/200 = 0.05 A = 50 mA.

IL = 10/1000 = 10 mA.

Iz = Is - IL = 50 - 10 = 40 mA.

Pz = Vz Iz = 10 x 0.040 = 0.4 W.

Example 2. (Illustrative.) In the above regulator, what happens to Iz if the supply rises to 24 V with the same load? What does this show?

Is = (24 - 10)/200 = 70 mA; IL = 10 mA (unchanged since VL = 10 V); Iz = 60 mA.

The extra 20 mA flows through the Zener while the load voltage remains 10 V; the extra 4 V drops across Rs. This illustrates line regulation.

Example 3. (Illustrative.) A red LED with forward voltage 2 V and rated current 20 mA is to be operated from a 5 V supply. Find the series resistor.

R = (VS - VF)/IF = (5 - 2)/0.020 = 150 ohm.

Power in the resistor = I^2 R = 0.02^2 x 150 = 0.06 W, so a standard 1/4 W resistor is adequate.

Example 4. (Illustrative.) GaAs has a band gap of about 1.43 eV. Find the wavelength of the emitted light and state whether it is visible.

lambda = 1240/Eg = 1240/1.43 = 867 nm.

Visible light lies roughly between 400 nm and 700 nm, so 867 nm is infrared. GaAs LEDs are therefore used in TV remote controls and opto-couplers, not as visible indicators.

Example 5. (Conceptual.) A seven-segment display is to show the digit 7. Which segments must be lit, and what logic level is applied to them for common-anode and common-cathode displays?

Segments are labelled a (top), b (top right), c (bottom right), d (bottom), e (bottom left), f (top left), g (middle).

Digit 7 needs segments a, b and c.

Common-cathode: cathodes tied to ground; a HIGH (logic 1) on a, b, c lights them.

Common-anode: anodes tied to +V; a LOW (logic 0) on a, b, c lights them. Series resistors limit each segment's current.

Important questions and answers

Q1. Explain the V-I characteristics of a Zener diode with a neat diagram. (5 marks)

Forward region: when forward biased, the Zener behaves like an ordinary silicon diode; current is negligible below about 0.7 V and rises sharply thereafter.

Reverse region: a very small reverse saturation (leakage) current flows as reverse voltage increases. When the reverse voltage reaches Vz, breakdown occurs and current rises sharply while the voltage across the diode remains almost constant at Vz.

Knee current Izk: minimum current at which regulation begins. Maximum current Izm: limited by power rating Pz = Vz Izm. Between Izk and Izm the curve is almost vertical; its small slope gives the Zener resistance rz = delta Vz/delta Iz.

The diode is not damaged in breakdown as long as current is limited by an external resistor. Diagram: I-V axes with forward curve in first quadrant and sharp vertical breakdown at -Vz in the third quadrant, with Izk and Izm marked.

Q2. Differentiate between Zener breakdown and avalanche breakdown. (4 marks)

Doping: Zener breakdown occurs in heavily doped junctions; avalanche in lightly doped junctions.

Depletion layer: thin for Zener; wide for avalanche.

Voltage: Zener breakdown occurs below about 5 V; avalanche at higher voltages (above about 6 V).

Mechanism: in Zener breakdown, the strong electric field directly breaks covalent bonds (field emission); in avalanche breakdown, accelerated carriers ionise atoms by collision, producing multiplication.

Temperature coefficient: negative for Zener (breakdown voltage falls with temperature); positive for avalanche.

Characteristic: Zener breakdown is very sharp; avalanche breakdown curve is slightly less sharp.

Q3. Explain the working of a Zener diode as a voltage regulator. (6 marks)

Circuit: unregulated DC input Vin, series resistor Rs, Zener diode connected in reverse bias across the load RL.

When Vin exceeds Vz, the Zener breaks down and the load voltage is held at VL = Vz. The current through Rs is Is = (Vin - Vz)/Rs, which divides as Is = Iz + IL.

(a) Line regulation (varying Vin, fixed RL): if Vin increases, Is increases; since VL = Vz is fixed, IL is unchanged and the increase appears as an increase in Iz, with the extra voltage dropped across Rs. If Vin decreases, Iz decreases. VL stays constant.

(b) Load regulation (fixed Vin, varying RL): if RL decreases, IL increases; Iz decreases by the same amount so Is and the drop across Rs stay constant, and VL remains Vz. If RL increases, Iz increases.

Limits: regulation holds only while Iz remains above Izk (otherwise the diode leaves breakdown) and below Izm (otherwise it overheats). Rs must be chosen to satisfy both conditions over the expected range of Vin and IL.

Q4. Explain the principle of operation of an LED. (4 marks)

An LED is a PN junction diode made of a direct band-gap compound semiconductor such as GaAs, GaAsP, GaP or InGaN. When it is forward biased, electrons are injected from the N region into the P region and holes from P into N. Near the junction, free electrons in the conduction band fall into holes in the valence band (recombination). In a direct band-gap material the energy released, approximately equal to the band gap Eg, is emitted as a photon of wavelength lambda = hc/Eg. This process is called electroluminescence. The colour of light depends on the band gap, which depends on the material and its composition. In Si and Ge the energy is released mostly as heat, so they are not used.

Q5. Draw and explain the structure of an LED. (5 marks)

An N-type layer is grown on a substrate and a thin P-type layer is diffused or grown on top of it, forming the junction close to the surface. The P layer is made thin so that light generated near the junction escapes with minimum re-absorption.

The anode contact is a small metal pad (or a ring/finger pattern) on the top P layer so that most of the top surface emits light; the cathode contact is a metal film at the bottom of the N layer/substrate. A reflector cup under the chip redirects light upward.

The chip and leads are encapsulated in a transparent or tinted epoxy dome, which protects the chip and acts as a lens to focus the light. The longer lead is the anode and the flat side of the package rim marks the cathode. Symbol: diode symbol with two arrows pointing away from it.

Q6. State the applications of LEDs in indicative and lighting displays. (4 marks)

Indicative displays: power-on and status indicators in equipment and appliances; seven-segment displays in digital clocks, calculators, meters and lift indicators; dot-matrix displays for alphanumeric and scrolling messages in railway and bus stations; bar-graph level indicators; traffic signals; infrared LEDs in remote controls and opto-couplers.

Lighting displays: LED bulbs, tube lights and street lights; automobile head lamps, tail lamps and indicators; torches; backlights for LCD TVs, monitors and mobile phones; large RGB video walls, stadium screens and advertisement hoardings; decorative and architectural lighting.

Q7. Explain the seven-segment LED display. (4 marks)

A seven-segment display consists of seven bar-shaped LEDs labelled a, b, c, d, e, f, g arranged in the shape of the figure 8, often with an eighth LED for the decimal point. By lighting selected segments, any digit from 0 to 9 (and some letters) can be displayed; for example 1 uses b and c, 8 uses all seven.

Types: (1) common-cathode, where all cathodes are joined to ground and a segment glows when its anode is driven HIGH; (2) common-anode, where all anodes are joined to +V and a segment glows when its cathode is driven LOW. Each segment needs a current-limiting resistor. It is driven by a BCD to seven-segment decoder (e.g. 7447 for common-anode, 7448 for common-cathode). Used in clocks, calculators, meters and counters.

Q8. List the advantages and disadvantages of LEDs. (3 marks)

Advantages: low operating voltage and power; high efficiency (more light per watt than incandescent lamps); long life; small size and light weight; fast switching (nanoseconds); rugged and shock resistant; available in many colours; no warm-up time; no mercury.

Disadvantages: performance and life fall at high temperature, so heat sinking is needed for power LEDs; need a series resistor or constant-current driver; damaged by reverse voltage above about 5 V; higher initial cost of high-power lighting LEDs.

Q9. Compare an ordinary PN junction diode, a Zener diode and an LED. (4 marks)

Doping: ordinary diode moderately doped; Zener heavily doped; LED uses compound semiconductor with suitable doping.

Material: Si or Ge for ordinary diodes, Si for Zener diodes; GaAs, GaAsP, GaP, InGaN for LEDs.

Operating region: ordinary diode in forward bias (rectification); Zener in reverse breakdown; LED in forward bias.

Breakdown: ordinary diode is damaged if breakdown current is not limited; Zener is designed to operate in breakdown; LED has low reverse breakdown and must not be reverse biased.

Output: ordinary and Zener diodes release energy as heat; LED emits light.

Applications: rectifiers; voltage regulators and reference; indicators, displays and lighting.

Exam tips

• Draw the Zener V-I characteristic with Vz, Izk and Izm clearly marked; examiners look for these labels.
• For the regulator, draw the circuit and explain both line and load regulation separately.
• Learn the Zener versus avalanche breakdown comparison as a table (doping, voltage, mechanism, temperature coefficient).
• For LED structure, draw the cross-section showing thin P layer, small contact, reflector and epoxy lens.
• Remember a few LED materials with colours; this is a common short-answer question.
• Write applications under two headings, indicative displays and lighting displays, exactly as in the syllabus.

FAQs

Why does a Zener diode not get damaged in breakdown?

It is designed with heavy doping and adequate heat dissipation to operate in breakdown, and the external series resistor limits the current below Izm, so the power dissipated stays within its rating.

Can an ordinary diode be used as a Zener?

Not reliably. An ordinary diode has a high, poorly defined breakdown voltage and is not designed to dissipate the power at breakdown, so it may be damaged.

Why do LEDs need a series resistor?

Above its forward voltage an LED's current rises very steeply with a small rise in voltage. The resistor limits the current to the rated value, preventing overheating and failure.

Are numericals asked on Zener diode and LED in FE BEE?

No. The MU syllabus states numericals are not expected for this module; questions are on characteristics, operation, regulator, structure and applications.

These notes follow the University of Mumbai first-year engineering syllabus (NEP 2020, Basic Electrical & Electronics Engineering) and were prepared with AI help and checked.