Compound semiconductors (GaN, GaAs, SiC) overview

1.3.10 · Hardware › Materials & Atomic Structure

Compound semiconductor KIYA hota hai?

"Average mein 4" kyun? Silicon (group IV) ka har atom mein 4 valence electrons hain → perfect covalent lattice. Gallium (III, 3 electrons) akela yeh nahi kar sakta. Lekin Ga (3) ko As (5) ke saath pair karo: average hai $(3 + 5) /2 = 4$ . Lattice phir se khush, lekin ab atoms alag hain, jo energy bands ko shift kar deta hai.

Hum kyun bother karte hain? (teen killer properties)

Devices ke liye kisi bhi semiconductor ki saari baat teen numbers par aati hai:

Bandgap $E_{g}$ — electron ko free karne ki energy. Yeh set karta hai emit hone wali light ka color aur kitni heat/voltage material survive kar sakta hai.
Electron mobility $μ$ — electrons unit field per kitni tezi se move karte hain. Yeh set karta hai switching speed.
Breakdown field $E_{cr i t}$ — woh field jis par material arc karne lagta hai. Yeh set karta hai max voltage per micron.

Yeh kyun matter karta hai: Silicon ka $E_{g} = 1.12$ eV → $λ \approx 1100$ nm (infrared, invisible) AUR silicon indirect gap hai isliye yeh almost bilkul light emit hi nahi karta. Visible/blue light ke liye zaroori hai ek wide direct-gap compound → GaN. Isliye hi blue LEDs (aur 2014 Nobel Prize) ke liye GaN chahiye tha.

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

Teen headline materials

Material	$E_{g}$ (eV)	Gap type	$μ_{e}$ (cm²/V·s)	$E_{cr i t}$ (MV/cm)	Superpower
Si (reference)	1.12	indirect	1400	0.3	sasta, 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

Yeh KAISE grow kiye jaate hain? (kyun mehenge hain)

Common mistakes

Recall Feynman: ek 12-saal ke bacche ko explain karo (click to reveal)

Silicon ek saste plastic ruler ki tarah hai — roz ke kaam ke liye theek, lekin yeh asaani se pighal jaata aur jhuk jaata hai. Compound semiconductors do metals ko milakar ek super-alloy banane jaisa hai: GaN aur SiC tough wale hain jo red-hot hone se ya bahut zyada electricity rokne se nahi dartey (electric-car chargers ke liye great). GaAs sprinter hai — electrons isme race karte hain, phone signals aur light beams shoot karne ke liye perfect. Aur GaN blue light bana sakta hai, jo plain silicon kabhi nahi kar sakta — isliye tumhare white LED bulbs aur blue lasers exist karte hain. Catch yeh hai: do atoms ko perfectly mix karna bahut mushkil hai, toh yeh zyada cost karte hain.

Active recall

Compound semiconductor kya define karta hai?

Do ya zyada elements ka semiconductor (III–V ya IV–IV) jiske valence electrons average ~4 per atom hon.

Valence electrons ka average 4 kyun hona chahiye?

Tetrahedral covalent bonding fill karne ke liye (4 bonds/atom), silicon ki tarah; jaise Ga(3)+As(5) ka average 4 hai.

Bandgap se emitted wavelength ka formula?

λ = h c / E_{g}

, yaani

λ [nm] \approx 1240/ E_{g} [eV]

Silicon efficient visible LEDs kyun nahi bana sakta?

Iska gap sirf 1.12 eV hai (infrared) AUR yeh indirect-gap hai, toh photon emission bahut inefficient hai.

Direct vs indirect gap — kaun achhe se light emit karta hai aur kyun?

Direct (electron & hole momentum mein aligned) efficiently photons emit karta hai; indirect ko phonon chahiye, toh emission weak hai.

Kaunse listed material ka highest electron mobility hai, aur iska use?

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

GaN aur SiC high-power electronics mein kyun dominate karte hain?

Wide bandgap → high breakdown field (~10× Si) → thinner devices same voltage block karte hain lower loss ke saath; SiC heat bhi achhe se handle karta hai.

Breakdown field aur bandgap ki approximate scaling?

E_{cr i t} \propto E_{g}^{n}

n \approx 2

, kyunki impact ionization ke liye carrier ko ~

E_{g}

energy gain karni hoti hai.

Compound semiconductors banane mein mehenge kyun hain?

Do elements jinke different vapor pressures hain + koi sasta native substrate nahi → epitaxy (MOCVD) zaroori hai; lattice mismatch yield-killing defects create karta hai.

Lattice mismatch define karo aur yeh kyun matter karta hai.

f = (a_{l a y er} - a_{s u b}) / a_{s u b}

; bada

∣ f ∣

strain create karta hai → dislocations jo devices degrade karte hain.

GaN/SiC mein se kaun light emit karta hai, aur doosra kyun nahi?

GaN (direct gap) emit karta hai; SiC indirect hai toh wide-gap hone ke bawajood poor emitter hai.

GaN ka

E_{g}

aur uske do flagship applications?

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

Connections

Silicon crystal structure — woh group-IV baseline jis par yeh compounds improve karte hain
Energy bands and bandgap — jahan se $E_{g}$ , direct/indirect aata hai
Doping and carrier concentration — mobility & carriers kaise engineer kiye jaate hain
LEDs and laser diodes — $λ = h c / E_{g}$ ka direct application
Power electronics & MOSFETs — jahan SiC/GaN breakdown advantage pay off karta hai
Epitaxy and crystal growth — MOCVD, lattice mismatch, defects
High Electron Mobility Transistor (HEMT) — GaN ka signature device