Special diodes and optoelectronic devices

In short

Special diodes are p-n (or metal-semiconductor) junctions engineered so that one normally unwanted effect becomes the useful one: reverse breakdown in a Zener, junction capacitance in a varactor, tunnelling in a tunnel diode, majority-carrier conduction in a Schottky, and photon-carrier interaction in LEDs, photodiodes and solar cells. Optoelectronic devices convert electricity into light (LED, laser diode) or light into electricity (photodiode, solar cell, phototransistor, LDR), and isolate circuits optically (optocoupler). The topic is examined in GATE EC/IN, ISRO, BARC, DRDO, BSNL JTO and university Electron Devices papers, and is used daily in power supplies, LED lighting, fibre-optic links, solar PV, remote controls and industrial drive isolation. Indian employers who test it include BEL, BHEL, ISRO/SAC, DRDO labs like SSPL, Signify/Havells/Syska (lighting), Tata Power Solar/Waaree/Adani Solar, Sterlite and Tejas (photonics), and semiconductor design houses.

Explained

Zener diode and voltage reference

• A heavily doped p-n junction operated deliberately in reverse breakdown, where voltage stays nearly constant while current varies widely - hence a voltage reference/regulator.
• Zener breakdown: narrow depletion region (heavy doping) gives a very high field (~10⁸ V/m, i.e. ~10⁶ V/cm) that tears electrons directly out of covalent bonds (field emission/tunnelling). Dominates below about 5 V.
• Avalanche breakdown: in lightly doped (wider) junctions, carriers accelerate and ionise atoms by collision, multiplying current. Dominates above about 7 V. Between 5 and 7 V both act together.
• Temperature coefficient: Zener mechanism is negative TC (breakdown voltage falls with temperature), avalanche is positive. Near 5-6 V they cancel, so 5.6 V and 6.2 V devices are the most temperature-stable references.
• Parameters: V_Z (at a stated test current I_ZT), dynamic/Zener resistance r_z = ΔV_Z/ΔI_Z (a few ohms to tens of ohms), I_ZK (knee, minimum current for regulation), I_ZM (maximum from power rating), P_D with derating above 25 °C.
• Shunt regulator design: series resistor R_S carries load plus Zener current; the Zener must stay between I_ZK and I_ZM for all input and load extremes. Line and load regulation improve as r_z falls.
• Other uses: over-voltage clipping/limiting, wave shaping, meter protection, surge protection (TVS diodes are avalanche diodes optimised for pulse energy), level shifting.
• Limitations: poor as a precision reference (±5% tolerance typical, drift with temperature and current); band-gap references and TL431 shunt regulators are preferred where accuracy matters.

Varactor, tunnel, Schottky and PIN diodes

• Varactor (varicap, tuning diode): reverse-biased junction used as a voltage-controlled capacitor; C_j = C_0/(1 + V_R/V_bi)ⁿ, n ≈ 1/2 for abrupt and 1/3 for graded junctions; hyperabrupt doping gives n up to ~2 for wide tuning. Used in VCOs, PLLs, FM modulators, RF tuners, parametric amplifiers. Quality figure: Q and tuning ratio C_max/C_min.
• Tunnel (Esaki) diode: both sides degenerately doped (≈10¹⁹-10²⁰ cm⁻³), depletion width ~10 nm, so carriers tunnel at small forward bias. Current rises to I_P at V_P, falls to I_V at V_V (the negative differential resistance, NDR region as bands slide out of alignment), then rises as normal diode current. Peak-to-valley ratio I_P/I_V (≈5-20 in Ge, higher in GaAs) is the figure of merit; used as microwave oscillator, amplifier, high-speed switch; obsolete in most modern designs but a standing exam favourite. Conducts in reverse immediately (no reverse breakdown knee).
• Schottky barrier diode: metal (Pt, W, Mo, Al) on lightly doped n-silicon forms a rectifying barrier; conduction is by majority carriers only, so there is no minority-carrier storage and no reverse-recovery charge - switching is in picoseconds. Forward drop 0.2-0.45 V (Si), ~0.3 V typical; reverse leakage higher and reverse voltage lower than p-n. Used in SMPS output rectifiers, mixers/detectors up to tens of GHz, Schottky-clamped TTL, and as anti-saturation clamp. SiC Schottkys reach 650-1700 V with near-zero recovery.
• Ohmic contact is the non-rectifying counterpart: heavy doping under the metal makes the barrier so thin that carriers tunnel through in both directions.
• PIN diode: thick intrinsic layer between p and n. At zero/reverse bias it is a small fixed capacitance (good RF switch OFF state); forward bias floods the i-region with carriers, making it a current-controlled resistor. Used as RF switch, attenuator, phase-shifter in radar and phased arrays, and as a photodetector (wide i-region = large absorption volume, low capacitance, fast response).
• Step-recovery, IMPATT, Gunn and backward diodes: IMPATT uses avalanche plus transit-time delay to give negative resistance for mm-wave power; Gunn uses the transferred-electron effect in GaAs bulk (not a junction); both appear in microwave questions.

Light-emitting diode (LED) and laser diode

• LED: forward-biased direct band-gap junction; injected electrons and holes recombine radiatively, emitting photons of energy hν ≈ E_g, so λ(µm) ≈ 1.24/E_g(eV).
• Materials: GaAs (IR, 870-900 nm), GaAsP/AlGaAs (red-yellow), GaP:N (green), InGaN (blue, green, near-UV; Nakamura-Akasaki-Amano, Nobel 2014), AlGaInP (red-amber), InGaAsP (1310/1550 nm for fibre). Silicon and Ge are indirect band-gap, so they cannot emit efficiently.
• White LED: blue InGaN chip + yellow phosphor (YAG:Ce), or RGB mixing. Rated by luminous efficacy (lm/W) - commercial lamps 90-150 lm/W; CRI and CCT (in kelvin) describe colour quality.
• Drive: LEDs are current-driven devices with a steep I-V curve; use a series resistor or, better, a constant-current driver. Typical indicator: 10-20 mA at 1.8-3.5 V (V_F rises with photon energy: ~1.8 V red, ~3.2 V blue/white).
• Advantages: long life (25,000-50,000 h), high efficacy, instant on, no mercury, low voltage, rugged, dimmable. Limitations: needs current limiting, V_F and output fall with junction temperature (needs heat sinking), ESD sensitive, droop at high current density.
• Laser diode: adds optical feedback (cleaved-facet Fabry-Perot cavity, or DFB/DBR gratings) and a double-heterostructure to confine carriers and light; above a threshold current I_th, stimulated emission dominates, giving coherent, narrow-linewidth, highly directional output and a steep L-I slope. VCSELs emit vertically, used in optical mouse, Li-Fi and data-centre links.
• LED vs laser diode: LED is incoherent, broad spectrum (30-50 nm), wide beam, slower (MHz-few hundred MHz), cheaper; laser is coherent, ~1 nm or less, fast (GHz), needs temperature and current control plus monitor photodiode for APC.

Photodetectors: photodiode, APD, phototransistor, solar cell

• Photoconduction/photovoltaic basics: photons with hν ≥ E_g create electron-hole pairs; cut-off wavelength λ_c(µm) = 1.24/E_g(eV) - Si ≈ 1.1 µm, Ge ≈ 1.8 µm, InGaAs ≈ 1.7 µm (fibre windows), GaAs ≈ 0.87 µm.
• Photodiode is used in reverse bias (photoconductive mode): reverse current = dark current + photocurrent proportional to incident optical power; wide depletion region collects carriers quickly → fast, linear response. In photovoltaic mode (zero bias) there is no dark current, so noise is lowest but speed and linearity are poorer.
• Responsivity R = I_ph/P_opt (A/W); quantum efficiency η = electrons out / photons in; R = ηqλ/(hc). Si PIN: R ≈ 0.5-0.6 A/W at 850 nm.
• Avalanche photodiode (APD): high reverse bias (50-300 V) multiplies carriers by impact ionisation, giving internal gain M ≈ 10-100; far more sensitive than a PIN, but noisier (excess noise factor) and temperature sensitive, needing gain stabilisation, and its gain-bandwidth product limits speed.
• Phototransistor: light generates base current, amplified by β, so responsivity is 100-1000× a photodiode's - but slower (large base-collector capacitance) and much less linear. Photo-Darlington gives still higher gain.
• LDR / photoconductive cell: CdS (or CdSe) bulk resistor whose resistance falls from megohms in dark to a few hundred ohms in light; peak response near the human eye, so used in street-light and camera-exposure control. Slow (ms-s), with a "memory" (light history) effect; CdS is restricted under RoHS-type rules for many products.
• Solar cell: large-area p-n junction in the fourth quadrant - it delivers power. Key terms: short-circuit current I_SC, open-circuit voltage V_OC, maximum power point (V_mp, I_mp), fill factor FF = V_mp I_mp/(V_OC I_SC) (0.7-0.85 for good cells), efficiency η = P_max/(G × A) with irradiance G and area A, at STC: 1000 W/m², AM1.5, 25 °C.
• Silicon cell V_OC ≈ 0.55-0.75 V; commercial mono-PERC/TOPCon module efficiency is roughly 20-23% (improving year on year - check current datasheets). Cells are series-connected for voltage, with bypass diodes across sub-strings to prevent hot-spots under shading, and blocking diodes to stop reverse flow from battery to array.
• Indian context: BIS/IEC 61215 and IEC 61730 certification is required for modules, with ALMM (Approved List of Models and Manufacturers) for government-supported projects, and PM Surya Ghar / PM-KUSUM driving rooftop and pump demand.

Optocouplers, displays and other optoelectronic devices

• Optocoupler/optoisolator: LED plus photodetector (photodiode, phototransistor, photo-Darlington, photo-SCR/TRIAC, or photo-MOS) in one package; transfers signal while isolating grounds. Specified by current transfer ratio CTR = I_C/I_F (typ. 50-600% for transistor types), isolation voltage (2.5-7.5 kV_rms), creepage, and bandwidth. Used for gate drive of IGBTs, SMPS feedback (PC817 + TL431), PLC digital inputs, 4-20 mA loops and medical isolation.
• Opto-TRIAC/photo-SCR couplers with zero-cross detection switch mains loads from microcontroller logic.
• Optical sensors: reflective and slotted opto-interrupters for encoders, paper-feed and tachometers; IR LED + photodiode pair at 38 kHz carrier for remote controls (TSOP receivers).
• Displays: seven-segment LED displays (common anode or common cathode, multiplexed to save pins); dot-matrix and RGB addressable LEDs; LCD is a passive, polarisation-based light valve - needs backlight, uses microwatts, no emission of its own; OLED is self-emissive organic LED, high contrast, no backlight, limited lifetime for blue.
• Comparison, light emitters vs detectors: emitters are forward-biased and current-driven; detectors are reverse-biased (or zero-biased) and voltage/charge-read through a transimpedance amplifier.
• Thermistors, varistors (MOV) and TVS diodes are the adjacent protection family often asked alongside Zeners: a MOV absorbs large surge energy but degrades; a TVS clamps fast and precisely.

Quick comparison of the special diodes

Key points

• Zener vs avalanche - tunnelling breakdown below ~5 V with negative TC; impact-ionisation breakdown above ~7 V with positive TC; 5.6 V is the near-zero-TC sweet spot.
• Zener resistance r_z - slope of the breakdown characteristic; smaller r_z gives better line and load regulation.
• Shunt regulator rule - size R_S so that I_Z lies between I_ZK and I_ZM across both input-voltage and load-current extremes.
• Varactor - voltage-controlled capacitor; C_j ∝ (V_bi + V_R)⁻ⁿ, n = 1/2 abrupt, 1/3 graded, up to ~2 hyperabrupt.
• Tunnel diode - degenerate doping, very thin depletion layer, negative differential resistance between V_P and V_V; figure of merit I_P/I_V.
• Schottky diode - metal-semiconductor, majority carriers only, no stored charge, low V_F (0.2-0.45 V), higher leakage, picosecond switching.
• PIN diode - intrinsic layer gives low capacitance and high reverse voltage; an RF switch when reverse-biased, a variable resistor when forward-biased.
• Direct vs indirect band gap - only direct-gap materials (GaAs, InGaN, InGaAsP) emit light efficiently; Si and Ge do not, which is why there is no silicon LED.
• λ = 1.24/E_g - with λ in µm and E_g in eV; sets LED colour and photodiode cut-off wavelength.
• LEDs are current-driven - always use a series resistor or constant-current driver; V_F and light output fall as junction temperature rises.
• Laser threshold - below I_th a laser diode behaves like a weak LED; above it, stimulated emission gives coherent, narrow-band light with a steep L-I slope.
• Responsivity R = ηqλ/hc - A/W; limited by quantum efficiency and increases with wavelength up to the cut-off.
• Photodiode modes - reverse bias (photoconductive) is fast and linear with dark current; zero bias (photovoltaic) is low-noise but slow.
• APD gain M - impact-ionisation multiplication of 10-100, at the cost of excess noise and strong temperature/bias dependence.
• Phototransistor - photodiode plus β gain: far more sensitive, far less linear and much slower than a photodiode.
• Solar cell figures - I_SC, V_OC, FF = V_mp I_mp/(V_OC I_SC), η at STC (1000 W/m², AM1.5, 25 °C); V_OC falls about 2 mV/°C per cell with heating.
• CTR - optocoupler current transfer ratio I_C/I_F; degrades with LED ageing, so designs must have CTR margin.
• Bypass vs blocking diode - bypass diodes protect shaded cells from hot-spots; blocking diodes prevent battery discharge back into the array at night.

Formulas and examples

Example 1 - Zener shunt regulator. A 6.2 V, 500 mW Zener regulates a 12 V ±10% supply feeding a load that varies from 0 to 40 mA. Take I_ZK = 5 mA and r_z = 8 Ω.

• I_ZM = P_D/V_Z = 0.5/6.2 = 80.6 mA.
• Worst case for the Zener current is maximum input with no load: R_S must pass ≤ 80.6 mA. With V_in(max) = 13.2 V: R_S ≥ (13.2 − 6.2)/0.0806 = 7/0.0806 ≈ 86.9 Ω.
• Worst case for regulation is minimum input with full load: with V_in(min) = 10.8 V, R_S ≤ (10.8 − 6.2)/(0.005 + 0.040) = 4.6/0.045 ≈ 102.2 Ω.
• Choose R_S = 100 Ω (standard value, inside 86.9-102.2 Ω). Check: at 13.2 V, no load, I_Z = 7/100 = 70 mA → P = 70 mA × 6.2 V = 0.434 W (< 0.5 W, so derating must be checked for ambient above 25 °C). At 10.8 V, 40 mA load, I_Z = 46 − 40 = 6 mA (> I_ZK, so regulation holds).
• Output change from the Zener slope: ΔI_Z = 70 − 6 = 64 mA, so ΔV_out ≈ 8 × 0.064 = 0.51 V - about 8%, which shows why a plain Zener is a poor precision reference.

Example 2 - LED wavelength, resistor and power. A GaAsP LED has E_g = 1.9 eV and is driven at 15 mA from 5 V with V_F = 2.0 V.

• λ = 1.24/1.9 = 0.653 µm = 653 nm → red.
• Series resistor R = (5 − 2.0)/0.015 = 200 Ω; the next E12 value, 220 Ω, gives I_F = 3/220 = 13.6 mA.
• Resistor power = I²R = (0.0136)² × 220 ≈ 0.041 W → a 1/8 W resistor is adequate; LED power = 2.0 × 13.6 mA ≈ 27 mW.
• If the external quantum efficiency is 2%, photons emitted per second = 0.02 × (13.6 mA/1.602×10⁻¹⁹ C) ≈ 0.02 × 8.49×10¹⁶ ≈ 1.7×10¹⁵ photons/s; each photon carries 1.9 × 1.602×10⁻¹⁹ = 3.04×10⁻¹⁹ J, so optical output ≈ 0.52 mW.

Example 3 - Photodiode responsivity and amplifier. A silicon PIN photodiode has η = 0.75 at λ = 850 nm and receives −20 dBm of optical power.

• R = ηqλ/(hc) = (0.75 × 1.602×10⁻¹⁹ × 850×10⁻⁹)/(6.626×10⁻³⁴ × 3×10⁸) = (1.0213×10⁻²⁵)/(1.9878×10⁻²⁵) ≈ 0.514 A/W.
• −20 dBm = 10 µW, so I_ph = 0.514 × 10×10⁻⁶ ≈ 5.14 µA.
• A transimpedance amplifier with R_f = 100 kΩ gives V_out = 5.14 µA × 100 kΩ ≈ 0.514 V.
• Shortcut worth remembering: R ≈ ηλ(µm)/1.24, i.e. 0.75 × 0.85/1.24 ≈ 0.514 A/W - the same answer.

Example 4 - Solar module fill factor and efficiency. A module measured at STC gives V_OC = 45.2 V, I_SC = 11.5 A, V_mp = 37.8 V, I_mp = 10.8 A; module area is 2.0 m².

• P_max = 37.8 × 10.8 = 408.2 W.
• FF = 408.2/(45.2 × 11.5) = 408.2/519.8 ≈ 0.785.
• η = 408.2/(1000 × 2.0) = 20.4%.
• At 55 °C cell temperature with a power coefficient of −0.35%/°C, output ≈ 408.2 × [1 − 0.0035 × (55 − 25)] = 408.2 × 0.895 ≈ 365 W - the reason real rooftop arrays under-deliver their nameplate on hot afternoons.

Example 5 - Varactor tuning range. A varactor has C_0 = 60 pF at zero bias, V_bi = 0.7 V, abrupt junction (n = 1/2), tuned with L = 100 nH.

• At V_R = 2 V: C_j = 60/√(1 + 2/0.7) = 60/√3.857 = 60/1.964 ≈ 30.6 pF → f_0 = 1/(2π√(100×10⁻⁹ × 30.6×10⁻¹²)) = 1/(2π × 1.749×10⁻⁹) ≈ 91.0 MHz.
• At V_R = 10 V: C_j = 60/√(1 + 10/0.7) = 60/√15.286 = 60/3.910 ≈ 15.3 pF → f_0 ≈ 1/(2π × 1.237×10⁻⁹) ≈ 128.7 MHz.
• Tuning ratio C_max/C_min = 30.6/15.3 = 2.0, giving a frequency ratio √2 ≈ 1.41 - capacitance ratio four times larger is needed to double the frequency.

Where it is used

• Power supplies and electronics manufacturing: Zener and TVS clamps, Schottky output rectifiers, PC817-type optocouplers in every SMPS and mobile charger produced by Indian EMS firms (Dixon, Amber, Syrma, Kaynes) - a routine hardware-engineer interview area.
• LED lighting and displays: Signify, Havells, Wipro Lighting, Syska, Bajaj; constant-current drivers, thermal design, BIS/IS 16102 and IS 16105 compliance for LED lamps and BEE star labelling.
• Solar PV: Tata Power Solar, Waaree, Adani Solar, Vikram Solar; cell/module testing (I-V curves, FF, EL imaging), bypass and blocking diodes, IEC 61215/61730 and ALMM, rooftop work under PM Surya Ghar.
• Fibre optics and telecom: Sterlite Technologies, Tejas Networks, BSNL/Jio networks; laser diode transmitters with APC, PIN/APD receivers, 1310/1550 nm InGaAs devices, optical budget calculations.
• Industrial automation and drives: opto-isolated gate drivers for IGBTs, optocoupled PLC inputs, opto-interrupters in encoders - roles at Siemens, ABB, L&T, Delta, Bharat Bijlee.
• Defence, space and research: BEL, DRDO's SSPL, ISRO/SAC use PIN diodes in radar phase shifters, varactors in RF tuning, space-qualified solar cells and photodetectors; semiconductor-fab and ATMP roles under the India Semiconductor Mission.

Interview questions

• Q: What is the difference between Zener and avalanche breakdown? A: Zener breakdown is direct field-induced tunnelling in a heavily doped, narrow junction (below about 5 V, negative temperature coefficient); avalanche breakdown is carrier multiplication by impact ionisation in a lightly doped, wider junction (above about 7 V, positive temperature coefficient).
• Q: Why is a 5.6 V Zener used as a reference? A: Near 5-6 V the negative temperature coefficient of Zener breakdown and the positive coefficient of avalanche breakdown roughly cancel, giving the smallest drift with temperature.
• Q: Why does a Schottky diode switch faster than a p-n diode? A: Conduction is by majority carriers only, so there is no minority-carrier charge to be removed - the reverse-recovery time is essentially zero.
• Q: Why can't silicon be used to make an LED? A: Silicon has an indirect band gap, so recombination needs a phonon to conserve momentum and is mostly non-radiative; direct-gap materials like GaAs and InGaN emit efficiently.
• Q: Why does a tunnel diode show negative resistance? A: As forward bias increases, the filled states on the n-side move out of alignment with the empty states on the p-side, so tunnelling current falls even though voltage rises.
• Q: In which bias is a photodiode normally operated, and why? A: Reverse bias - it widens the depletion region, which increases the collection volume and reduces junction capacitance, giving fast and linear response; the penalty is dark current.
• Q: Difference between a photodiode and a phototransistor? A: A phototransistor amplifies the photo-generated base current by β, so it is 100-1000 times more sensitive but much slower and far less linear than a photodiode.
• Q: What does the fill factor of a solar cell tell you? A: How close the I-V curve is to a rectangle: FF = V_mp I_mp/(V_OC I_SC); low FF means high series resistance, low shunt resistance or poor diode quality.
• Q: Why do solar modules have bypass diodes? A: A shaded cell becomes reverse-biased by the rest of the string and dissipates power as a hot-spot; a bypass diode across each sub-string carries the current around it.
• Q: What is CTR in an optocoupler and why does it matter? A: Current transfer ratio I_C/I_F; it varies widely between parts and falls as the LED ages, so the design must work at the minimum CTR over life.
• Q: How is a laser diode different from an LED? A: A laser diode has an optical cavity and operates above a threshold current where stimulated emission dominates, producing coherent, narrow-linewidth, directional light; an LED emits spontaneous, incoherent, broad-spectrum light.
• Q: What limits the long-wavelength response of a silicon photodiode? A: Photon energy must exceed the band gap, so the cut-off is λ = 1.24/1.12 ≈ 1.1 µm; beyond that silicon is transparent and InGaAs or Ge detectors are used.

Common mistakes

• Treating a Zener as a constant-voltage source and ignoring r_z - then being surprised that output ripple and load regulation are poor.
• Sizing R_S for only one operating point; the design must be checked at maximum input/no load (Zener power) and minimum input/full load (I_ZK).
• Connecting an LED directly across a supply without current limiting, or computing the resistor from the supply voltage instead of (V_supply − V_F).
• Confusing the varactor's reverse-bias depletion capacitance with the forward-bias diffusion capacitance, and forgetting that f_0 varies as the square root of 1/C.
• Writing responsivity as independent of wavelength; R = ηqλ/hc rises with λ until the cut-off, then drops to zero.
• Quoting a photodiode's "gain": a PIN has unity gain - only APDs and phototransistors have internal gain.
• Mixing up bypass and blocking diodes in PV systems, or omitting the solar-cell temperature coefficient when predicting field output.

Related topics

Semiconductor diodes and applications · Semiconductor physics: energy bands, carriers and junctions · Oscillators and power supplies · Data converters: ADC and DAC · BJT and MOSFET · Operational amplifiers