Why is GaAs good for LEDs and lasers while Si is poor?

Electrons drop directly across the gap and emit photons, with no phonon needed for momentum

Si has higher electron mobility

Why does GaAs have much higher electron mobility than Si?

Its sharply curved conduction band gives a small effective mass, 0.067 me

It has a smaller lattice constant

Asked 2/28
2073 Chaitra · 6 marks
2072 Chaitra · 6 marks

Si versus GaAs semiconductors

Short answer

Silicon is indirect (1.12 eV), cheap, with stable SiO₂, used for ICs. GaAs is direct (1.42 eV), has high electron mobility (8500 cm²/Vs) and efficient light emission for LEDs/lasers, but lacks a good native oxide and is costlier.

Present a comparison between Si and GaAs semiconductors with the help of their basic properties and E-k diagram.

Silicon is an elemental, indirect band gap semiconductor used in almost all integrated circuits. Gallium arsenide is a III-V compound, direct band gap semiconductor used for optoelectronic and high-frequency devices.

Basic properties

Property (300 K)	Si	GaAs
Type	Elemental (group IV)	Compound (III-V)
Crystal structure	Diamond	Zinc blende
Lattice constant	0.543 nm	0.565 nm
Band gap $E_g$	1.12 eV	1.42 eV
Band gap type	Indirect	Direct
Electron mobility	about 1350 cm $^2$ /Vs	about 8500 cm $^2$ /Vs
Hole mobility	about 450 cm $^2$ /Vs	about 400 cm $^2$ /Vs
Electron effective mass	$1.08m_e$ (dos)	$0.067m_e$
Intrinsic $n_i$	about $10^{10}$ cm $^{-3}$	about $2\times10^6$ cm $^{-3}$
Relative permittivity	11.9	13.1
Native oxide	Stable SiO $_2$	None useful
Light emission	Very poor	Efficient (LEDs, lasers)
Cost and processing	Cheap, mature	Costly, brittle
Main uses	ICs, CMOS, power devices, solar cells	Microwave/RF ICs, LEDs, laser diodes, high-speed devices

E-k diagrams

     Si (indirect)          GaAs (direct)
  E                       E
  |      \   /  CB        |    \   /  CB
  |       \_/             |     \_/
  |        ^ min at k0    |      | min at k=0
  |   Eg  /  (phonon)     |   Eg | photon
  |    /\                 |     /\
  |   /  \   VB           |    /  \  VB
  +---0----k0---- k       +-----0------ k

Si: CB minimum at $k_0 \ne 0$ , VB maximum at $k = 0$ . A transition needs a phonon for momentum, so light emission is poor.
GaAs: CB minimum and VB maximum both at $k = 0$ . Electrons drop directly and emit photons ( $\lambda \approx 1.24/1.42 = 0.87$ $\mu$ m). Sharp CB curvature gives small $m_e^* = 0.067m_e$ and high mobility.

✓ Answer checked by Claude Opus 5.5 (high effort)

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