Silicon Transistor Gallium
Posted on :: Updated on :: hardware

Silicon, semi-conductor

Silicon is a semi conductor, something in between a conductor (like metal) or insulator (like rubber). Pure silicon doesn't conduct electricity. However, when you can add some impurities, i.e, tiny amounts of other elements, it starts conducting electricity, this is known as doping.

N type (N for negative), adds tiny amount of phosphorus. This creates some free electrons.

P type (P for positive), adds tiny amount of boron. This creates extra holes (positive).

If you bring these together, some electrons from N type flow to holes in P type at the junction.

Transistor

Transistors are switch or amplifiers. A small current from emitter to base, drives a big current from emitter to collector.

transistor|300

npn-transistor|400

How it works

  • When you sandwich NPN silicon together, some electrons from the N type side neutralises holes in P type at the junction, this is known as depletion region.
  • Since, N side loses electrons, it becomes positively charged and base becomes negatively charged.
  • Things stabilises after a bit, and no current flows through. Transistor is said to be in switched off state, even if there is potential difference ( the battery in Emitter to Collector circuit).
  • When you apply small current or ~0.7 V between emitter and base. Extra electrons enter emitter side, this shrinks the depletion region between emitter and base.
  • However, because of positively charged collector, most of these electrons move to collector directly, rather than moving out of base (99%). This switches on the transistor.
  • This is helped by the fact that emitters are heavily doped (more free electrons) and base is very thin.

Silicon ceiling, gallium

Electrons exists in 2 different states:

  • Locked in bonds that tie atom together, this is known as valence band
  • Others are free to move around, known as conduction band

Band gap is the energy an electron requies in valence band to jump over to conduction band. For silicon, this is 1.1 electron volts (eV).

With technical advancements, we can now place billions of transistor in a small chip in your iphone. This leads to increased heat, and it's becomes feasible to jump the bands. After 150° C, silicon stops being a reliable switch.

In general for traditional computing, you work around this constraint, by adding cooling fans etc.

But there're are cases like 5G base stations, which has to constantly transmit at 3 GHz frequencies, sometimes to 30 GHz for millimeter wave 5G. Silicon based chips struggle beyond 5 GHz, so the quality of stations degrade.

Enter gallium, gallium by itself, is not a semi conductor. But, Gallium Arsenide & Gallium Nitride has some interesting electronic properties. Gallium Arsenide (GaAs) has bandgap of 3.4 eV. It can operate till a temperature of 400° C. Also it switches faster than silicon based chip. This leads to less energy waste as heat, and needs less cooling.

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