Compound semiconductors (GaN, GaAs, SiC) overview

WHAT is a compound semiconductor?

Why "on average 4"? Silicon (group IV) has 4 valence electrons per atom → perfect covalent lattice. Gallium (III, 3 electrons) alone can't do this. But pair Ga (3) with As (5): the average is $(3+5)/2 = 4$ . The lattice is happy again, but now the atoms are different, which shifts the energy bands.

WHY do we bother? (the three killer properties)

Everything about a semiconductor for devices comes down to three numbers:

Bandgap $E_g$ — energy to free an electron. Sets the color of light it emits and how much heat/voltage it survives.
Electron mobility $\mu$ — how fast electrons move per unit field. Sets switching speed.
Breakdown field $E_{crit}$ — field before the material arcs. Sets max voltage per micron.

Why this matters: Silicon's $E_g=1.12$ eV → $\lambda \approx 1100$ nm (infrared, invisible) AND silicon is indirect gap so it barely emits light at all. To get visible/blue light you need a wide direct-gap compound → GaN. That's the entire reason blue LEDs (and the 2014 Nobel Prize) required GaN.

Figure — Compound semiconductors (GaN, GaAs, SiC) overview

The three headline materials

Material	$E_g$ (eV)	Gap type	$\mu_e$ (cm²/V·s)	$E_{crit}$ (MV/cm)	Superpower
Si (reference)	1.12	indirect	1400	0.3	cheap, mature
GaAs	1.42	direct	8500	0.4	fast RF, IR lasers
GaN	3.4	direct	2000 (bulk)	3.3	blue LED, RF power, HEMT
SiC (4H)	3.26	indirect	900	3.0	high-power, high-heat

HOW are they grown? (why they're expensive)

Common mistakes

Recall Feynman: explain it to a 12-year-old (click to reveal)

Silicon is like a cheap plastic ruler — fine for everyday stuff but it melts and bends easily. Compound semiconductors are like mixing two metals to make a super-alloy: GaN and SiC are the tough ones that don't mind being red-hot or holding back a lot of electricity (great for electric-car chargers). GaAs is the sprinter — electrons race through it, perfect for phone signals and shooting out light beams. And GaN can make blue light, which plain silicon can never do — that's why your white LED bulbs and blue lasers exist. The catch: mixing two atoms perfectly is really hard, so they cost more.

Active recall

What defines a compound semiconductor?

A semiconductor of two or more elements (III–V or IV–IV) whose valence electrons average ~4 per atom.

Why must the valence electrons average 4?

To fill the tetrahedral covalent bonding (4 bonds/atom), like silicon; e.g. Ga(3)+As(5) averages 4.

Formula for emitted wavelength from a bandgap?

\lambda = hc/E_g

, i.e.

\lambda[\text{nm}] \approx 1240/E_g[\text{eV}]

Why can't silicon make efficient visible LEDs?

Its gap is only 1.12 eV (infrared) AND it's indirect-gap, so photon emission is very inefficient.

Direct vs indirect gap — which emits light well and why?

Direct (electron & hole aligned in momentum) emits photons efficiently; indirect needs a phonon, so emission is weak.

Which listed material has the highest electron mobility, and its use?

GaAs (~8500 cm²/V·s) → high-frequency RF amps and IR lasers.

Why do GaN and SiC dominate high-power electronics?

Wide bandgap → high breakdown field (~10× Si) → thinner devices block the same voltage with lower loss; SiC also handles heat well.

Approximate scaling of breakdown field with bandgap?

E_{crit} \propto E_g^{\,n}

n\approx2

, because impact ionization needs the carrier to gain ~

E_g

of energy.

Why are compound semiconductors expensive to make?

Two elements with different vapor pressures + no cheap native substrate → require epitaxy (MOCVD); lattice mismatch creates yield-killing defects.

Define lattice mismatch and why it matters.

f=(a_{layer}-a_{sub})/a_{sub}

; large

|f|

creates strain → dislocations that degrade devices.

Which of GaN/SiC emits light, and why not the other?

GaN (direct gap) emits; SiC is indirect so it's a poor emitter despite being wide-gap.

GaN's

E_g

and its two flagship applications?

~3.4 eV; blue/UV LEDs & lasers, and RF/power HEMTs.

Connections

Silicon crystal structure — the group-IV baseline these compounds improve on
Energy bands and bandgap — where $E_g$ , direct/indirect comes from
Doping and carrier concentration — how mobility & carriers are engineered
LEDs and laser diodes — direct application of $\lambda = hc/E_g$
Power electronics & MOSFETs — where SiC/GaN breakdown advantage pays off
Epitaxy and crystal growth — MOCVD, lattice mismatch, defects
High Electron Mobility Transistor (HEMT) — GaN's signature device

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, silicon sasta hai lekin bechara "kamzor" material hai — chhota bandgap, dheema, aur zyada heat/voltage nahi jhel paata. Compound semiconductors matlab do ya zyada elements ko mila ke banaya gaya material — jaise Gallium + Nitrogen (GaN), Gallium + Arsenic (GaAs), ya Silicon + Carbon (SiC). Trick yeh hai ki average valence electrons 4 rehne chahiye (jaise Ga=3 aur As=5 ka average 4), tabhi covalent lattice sahi banta hai. Iss mixing se hum bandgap, speed aur breakdown field ko "tune" kar sakte hain jo silicon me possible hi nahi.

Teen numbers yaad rakho: bandgap $E_g$ (light ka color + kitna voltage/heat sehta hai), mobility $\mu$ (electron kitni tezi se chalta hai → switching speed), aur breakdown field (kitna voltage per micron jhel lega). Formula simple hai: $\lambda = 1240/E_g$ nm — isliye silicon ka gap chhota (1.12 eV) hone se woh infrared hi deta hai, blue light kabhi nahi. Blue LED banane ke liye wide direct gap chahiye — isliye GaN aaya aur Nobel Prize mila.

Roles yaad karne ka mantra: "GaAs Runs, GaN Lights, SiC Fights." GaAs ki mobility sabse zyada (~8500) → 5G/RF amplifiers aur IR lasers ke liye best. GaN wide direct gap → blue LED, lasers, aur high-frequency power HEMT. SiC wide gap + best heat handling → EV inverters aur grid ke high-power, high-temperature kaam. Ek galti mat karna: wide bandgap ka matlab "zyada conductive" nahi — ulta, room temperature pe kam carrier free hote hain, iska fayda hai high voltage/heat jhelne me, aur direct hone par light dena.

Yeh mehenge kyun? Kyunki do alag elements ko atomically-perfect layer banana mushkil hai (As bhaap ban ke ud jaata hai!), aur native GaN/SiC wafers cheap nahi. Isliye epitaxy (MOCVD) use hoti hai, aur agar substrate ka lattice mismatch $f=(a_{layer}-a_{sub})/a_{sub}$ bada ho to defects ban jaate hain jo device kharab kar dete hain.

Compound semiconductors (GaN, GaAs, SiC) overview

WHAT is a compound semiconductor?

WHY do we bother? (the three killer properties)

The three headline materials

HOW are they grown? (why they're expensive)

Common mistakes

Active recall

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Materials & Atomic Structure

Connections