MATSE 81
Materials In Today's World

Energy Bands (Continued)

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As shown in the figures below, there are four possible band configurations.

partially filled band with a gap and then an empty band
(a) Partially filled band.
Credit: based on Callister
two filled bands. Top filled band overlaps with a smaller unfilled band
(b) Overlapping bands.
Credit: based on Callister

In the cases of (a) and (b), empty energy states are readily available and electrons (with a little bit of thermal energy) are able to speed through the material, similar to the cars pictured below.

few cars on a road
Cars driving. 
Credit: Vincent van Zeijst - Own work, CC BY-SA 3.0, Wikimedia Commons
filled bands followed by a large gap & then an empty conduction band. An arrow from the filled bands can't get to the empty states & returns
(c) Insulators have a wide band gap (>2 eV) with few electrons excited across the band gap.
Credit: based on Callister
filled bands followed by a small gap & then an empty conduction band. An arrow from the filled bands goes to the empty states
(d) Semiconductors have a narrow band gap (<2 eV) with more electrons excited across the band gap.
Credit: based on Callister

In cases of (c) and (d), insulator and semiconductor, the bands are completely filled and electrons have no mobility, because like the cars in the figure below, the electrons cannot move because there are no available open spaces to move to.

traffic jam with many lanes of traffic
Traffic jam.
Credit: Australian cowboy - Own work via Wikimedia Commons

In the case of semiconductors, applying a voltage can boost the electrons across the gap. This would be like kicking one of the cars from the traffic jam over a medium to an unpacked highway. Thus, the semiconductor can be changed from being an insulator (off) to a conductor (on). We will look at one aspect of this behavior in the next section.