A full lesson: a cartoon movie to start, then the Zener diode and its breakdown, the Zener voltage regulator, the LED and its colour, the photodiode, and the solar cell with its I-V curve, NEET shortcuts, problems to solve yourself, and 12 MCQs.
One garden lamp, four special diodes: a solar cell makes power from sunlight, a photodiode notices that it is dark, LEDs turn electrical energy into light, and a Zener diode keeps the voltage steady.
Dadi bought a solar lamp for the terrace. Nobody switches it on or off. All day its little dark-blue panel sits in the sun; the moment the sky turns orange in the evening, it lights up by itself, and it glows the whole night. Meera wanted to know how a lamp with no wire and no switch "knows" what to do.
Open such a lamp and you find four different p-n junction diodes, each used in a different way: a solar cell (no bias at all), a photodiode (reverse bias, as a light sensor), LEDs (forward bias, giving light) and a Zener diode (reverse bias, in breakdown, as a voltage guard). The way a diode is biased decides its job.
"Same p-n junction, four jobs. Learn which bias each one uses and half of this lesson's NEET questions are already done."
"A solar cell and a photodiode both turn light into current. Why do we need two different devices?"
A photodiode is a sensor: it is reverse biased by a battery and its small current tells us how bright the light is. A solar cell is a source: it has no battery, a large area, and it delivers power to a load.
A Zener diode is a p-n junction with heavily doped p and n regions. The depletion layer becomes very thin (less than 10⁻⁶ m), so even about 5 V of reverse bias makes a huge field (about 5 × 10⁶ V/m) across it. At a sharp, fixed reverse voltage, the Zener voltage V_Z, the current shoots up. Slide the applied voltage, or drag the dot along the curve.
The strong field pulls valence electrons out of their bonds (internal field emission), so breakdown happens at a low, well-defined voltage.
The voltage across the Zener stays at V_Z over a wide range of current. That is the property we use.
For NEET, "breakdown in a heavily doped diode by field emission" is the Zener effect.
"For an ordinary diode, breakdown means damage. A Zener is designed to live in breakdown. It is always used in reverse bias; in forward bias it is just an ordinary diode."
A series resistor R (200 Ω) from the unregulated supply, then the Zener (V_Z = 6 V) reverse biased, in parallel with the load R_L. Change the input or the load and watch the three currents. The output does not move, as long as the Zener stays in breakdown.
Whatever voltage is above V_Z is dropped across R.
The load sits in parallel with the Zener, so it gets V_Z.
The Zener takes the rest. If the input rises, I and I_Z rise; the output stays at V_Z. Regulation stops if I_Z would become zero (input too low or load too heavy).
"The Zener is like a pressure-release valve on a pressure cooker. Extra pressure escapes through it, so the pressure inside stays fixed. Extra current goes through the Zener, so the output voltage stays fixed."
"Input 15 V, R = 200 Ω, V_Z = 6 V. What is the smallest load resistance for which the output is still 6 V?"
I = 9/200 = 45 mA. The load may take at most all of it: R_L(min) = 6 V / 45 mA ≈ 133 Ω. Try it with the slider.
A light emitting diode is a heavily doped p-n junction that glows in forward bias. Electrons from the n side and holes from the p side meet at the junction and recombine; each recombination releases energy close to the band gap E_g as a photon. Slide the band gap, or choose a material.
The photon carries away the energy the electron loses when it falls from the conduction band into a hole in the valence band.
hc = 6.63 × 10⁻³⁴ × 3 × 10⁸ J m = 1240 eV nm. Example: E_g = 2.0 eV → λ ≈ 620 nm, red-orange.
Larger gap → shorter wavelength → towards blue. Si (1.1 eV) and Ge (0.7 eV) cannot give visible light; GaAsP, GaP, GaN-type compounds are used.
Fast on-off switching, rugged, nearly monochromatic light (narrow band of wavelengths), no warm-up. That is why every bulb is now an LED bulb.
The forward voltage at which an LED starts glowing is roughly E_g/e. Its reverse breakdown voltage is low (about 5 V), so do not reverse it.
"The colour of an LED is not paint on the plastic. It is the band gap of the material written in light. Bigger gap, bluer light."
A photodiode is used in reverse bias. A photon with hν > E_g absorbed near the junction makes an electron-hole pair. The junction field sweeps the electron to the n side and the hole to the p side: a photocurrent that grows with the light intensity. Change the brightness and the colour of the light.
The reverse current without light is very small, so the extra current due to light is a large fraction of it and easy to measure. In forward bias, the same photocurrent hides inside a large current.
λ_max (nm) = 1240/E_g (eV). Silicon (1.1 eV) senses up to about 1100 nm.
Reverse voltage and reverse current, both negative. Higher intensity → curve further down; nearly flat with voltage.
"In the dark the photodiode is almost silent. One flash of light and the current jumps. That is how the garden lamp knows it is evening, and how your TV remote talks to the TV."
A solar cell is a p-n junction with no external bias. Three steps: generation of electron-hole pairs by light, separation by the junction field, collection at the contacts. Change the sunlight and the load, or drag the operating point along the curve. The pink rectangle is the power P = V × I.
Short-circuit current ∝ light intensity; open-circuit voltage (about 0.5–0.6 V for Si) grows only slowly.
The cell does not draw current; it supplies current to the load. So the curve is drawn with I pointing down.
The solar spectrum peaks near 1.5 eV. A high absorption, good conductivity and low cost also matter.
"A photodiode is a light sensor with a battery. A solar cell is a light battery with no battery. Same physics: generation, separation, collection."
| Device | Bias | I-V quadrant | Job |
|---|---|---|---|
| Zener | reverse, in breakdown | third (sharp knee at −V_Z) | voltage regulator |
| LED | forward | first | electricity → light |
| Photodiode | reverse | third | light sensor |
| Solar cell | none | fourth | light → electric power |
Three lines, every time. Check I_Z > 0, otherwise the Zener is off and V_out = V_in R_L/(R + R_L).
LED colour from E_g; the longest wavelength a photodiode can detect is λ_max = 1240/E_g.
E_g = 2.8 eV → ν = 2.8 × 1.6 × 10⁻¹⁹ / 6.63 × 10⁻³⁴ ≈ 6.8 × 10¹⁴ Hz.
Fill factor (beyond syllabus) is how "square" the curve is: about 0.7–0.85 for good cells.
"One table, four rows: device, bias, quadrant, job. Make it on your rough sheet before you read the options. And keep 1240 ready: it turns electron volts into nanometres in one division."
Cover the solution, write the formula, and solve on paper. Take h = 6.63 × 10⁻³⁴ J s, e = 1.6 × 10⁻¹⁹ C, hc = 1240 eV nm.
V_in = 15 V, series resistor R = 200 Ω, V_Z = 6 V and R_L = 600 Ω. Find the current through R, the load current, the Zener current and the power dissipated in the Zener.
I = (V_in − V_Z)/R, I_L = V_Z/R_L, I_Z = I − I_L, P_Z = V_Z I_Z.
In the same circuit (V_in = 15 V, R = 200 Ω, V_Z = 6 V), the input changes between 12 V and 18 V with R_L = 600 Ω. Find the range of the Zener current. Then find the smallest R_L for which the output stays 6 V at V_in = 15 V.
Recalculate I at each input. Regulation needs I_Z ≥ 0, so I_L ≤ I.
An LED is made of a material with E_g = 1.9 eV. Find the wavelength of its light. What band gap is needed for a blue LED of wavelength 450 nm?
λ (nm) = 1240/E (eV).
A photodiode is made of a semiconductor with E_g = 2.8 eV. Find the minimum frequency and the maximum wavelength it can detect. Will it respond to orange light of 600 nm?
ν_min = E_g/h; compare the photon energy of 600 nm light with E_g.
A silicon solar cell has I_sc = 30 mA and V_oc = 0.60 V. At its maximum power point, V = 0.52 V and I = 28.6 mA. Find the maximum power and the fill factor.
P = V I; FF = P_max/(V_oc I_sc).
Take hc = 1240 eV nm and h = 6.63 × 10⁻³⁴ J s. Write the formula first, then click.
Score: 0 / 12
| If the question says… | Write this first | The trap |
|---|---|---|
| "Zener as a voltage regulator" | reverse bias, in breakdown, parallel to R_L | Choosing forward bias, or "reverse bias below breakdown" |
| "current through the Zener" | I_Z = (V_in − V_Z)/R − V_Z/R_L | Giving (V_in − V_Z)/R, which is the total current |
| "Zener diode is heavily doped, so…" | thin depletion layer, high field | Saying the depletion layer is wide |
| "LED of band gap E_g" | λ = 1240/E_g nm | Mixing up: larger gap gives longer λ (it gives shorter) |
| "photodiode is operated in" | reverse bias | Answering forward bias "so that current flows easily" |
| "minimum frequency / maximum λ detected" | ν = E_g/h, λ = 1240/E_g | Forgetting to change eV into joules (× 1.6 × 10⁻¹⁹) |
| "I-V of a solar cell is drawn in" | fourth quadrant | Choosing the first quadrant like an ordinary forward-biased diode |
| "best band gap for a solar cell" | about 1.5 eV | Choosing the largest gap "to get more voltage" |
A diode is either ON or OFF. Lesson 9 uses exactly that two-state idea to build logic gates: OR, AND, NOT, NAND and NOR, the building blocks of every digital circuit.
Values used: hc = 1240 eV nm, h = 6.63 × 10⁻³⁴ J s, e = 1.6 × 10⁻¹⁹ C; E_g(Si) = 1.1 eV, E_g(GaAs) = 1.43 eV; solar-cell model with I_sc = 30 mA and V_oc = 0.60 V at full sunlight.
Follow the four steps: understand it, remember it, try it, then prove it with past papers.