---
title: "Si versus GaAs semiconductors"
subject: "Electrical Engineering Material"
chapter: 6
key: "semester_3/electrical_engineering_material:si-vs-gaas"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/si-vs-gaas
site: BCT Engineering
---

# Si versus GaAs semiconductors

Electrical Engineering Material · Chapter 6: Semiconductors

- Asked in: 2073 Chaitra, 6 marks; 2072 Chaitra, 6 marks (asked 2 times)
- Video: [ECE Purdue Semiconductor Fundamentals L1.4: Materials Properties - Common Semiconductors](https://www.youtube.com/watch?v=1ni9QWMm0Us) — nanohubtechtalks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/si-vs-gaas
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-6

## Question

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

## 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.

## Answer

**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
```text
     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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is correct about the band gaps?
   A. Si is direct, GaAs is indirect
   B. Both are direct
   C. Both are indirect
   D. Si is indirect, GaAs is direct
   Answer: D. In GaAs the CB minimum and VB maximum are both at k = 0; in Si the CB minimum is at k0 not equal to 0. (Key point: Direct versus indirect)

2. Why is GaAs good for LEDs and lasers while Si is poor?
   A. Electrons drop directly across the gap and emit photons, with no phonon needed for momentum
   B. GaAs has a smaller band gap
   C. GaAs has a stable oxide
   D. Si has higher electron mobility
   Answer: A. A transition in Si needs a phonon to conserve momentum, which makes radiative recombination unlikely. (Key point: Light emission)

3. Why does GaAs have much higher electron mobility than Si?
   A. It has a larger effective mass
   B. Its sharply curved conduction band gives a small effective mass, 0.067 me
   C. It has a smaller lattice constant
   D. It is cheaper
   Answer: B. mu = e tau / m*, so a small m* gives high mobility (about 8500 versus 1350 cm^2/Vs). (Key point: Mobility and effective mass)

## Related in Electrical Engineering Material

- [Degenerate semiconductors; Si vs GaAs E-k](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adegenerate-si-gaas) — Electrical Engineering Material, chapter 6
- [Degenerate versus non-degenerate semiconductors](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adegenerate-nondegenerate) — Electrical Engineering Material, chapter 6, asked 5×
- [Direct versus indirect band gap semiconductors](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adirect-indirect-bandgap) — Electrical Engineering Material, chapter 6

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