Electronic components, Electronics, Materials, Semiconductors

Comparison between silicon and germanium

Fulfilling requests, this post is a comparison between silicon and germanium, as base materials for semiconductors.

Similarities between silicon and germanium

silicon and germanium
Both elements are semimetals, belong to 4A family of carbon, and have 4 valence electrons. The atomic number Source: Hyperphysics.

Why is silicon more used on electronics?

The main factor is abundance, silicon used for semiconductor manufacturing comes from sand, whose main component is silicon dioxide (SiO_{2}).

silicon on earth crust
After oxygen, silicon is the most abundant element on Earth’s crust. Source: BC Campus.

Another important factor is thermal properties. Silicon has a thermal conductivity of 148 W/(m\cdot K) (Watts per (meters times Kelvin)), while Germanium has 59.9 W/(m\cdot K). It shows that silicon transmits heat more efficiently. Also, it has more thermal stability than germanium, allowing it to operate in higher temperatures without degradation, germanium can operate until 70ºC (158ºF), while silicon can reach 150ºC (302ºF).

Cutoff current

The cutoff current circulates between the BJT transistor‘s terminals, when it’s in cutoff state, because BJTs aren’t ideal.

When there is no base current in a BJT transistor, a cutoff current circulates between collector and emitter. On common base configuration, this current circulates between collector and base.

In a germanium transistor, the cutoff current would be from 10 to 100 times bigger than in a silicon transistor, because the doped crystal made of this element has less free electrons than a doped germanium crystal at the same temperature. In addition to that, cutoff current variation due to temperature is lower on silicon.

Peak Inverse Voltage (PIV)

Every diode has a maximum reverse voltage that can withstand without being damaged, a silicon diode can withstand a reverse voltage of up to 1000V, while in a germanium diode, PIV is much smaller.

Some germanium applications

  • Germanium oxide is inside the optical fiber’s nucleus and internal coating, to increase the refraction index, 
  • Lens and windows with germanium are used in infrared cameras, remote sensing and infrared spectroscopy.
  • High purity germanium crystals can detect X-rays and gamma rays. A high energy photon produces an electron-hole pair when it enters in a germanium crystal. But the latter have to be cooled at liquid nitrogen temperature (-196°C/-320.8ºF), to minimize thermal noise.
  • Germanium has more electrons and holes (empty space without electrons) mobility than silicon. Because of that, were introduced CMOS with silicon and germanium.
This is a CMOS transistor, developed by Purdue University, in 2014. It has a germanium (Ge) layer, so the transistors on chips can be smaller. Source: Purdue University.

About Pedro Ney Stroski

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