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 as described in the text
(a) Partially filled band.
Credit: based on Callister
overlapping bands as described in the text
(b) Overlapping bands.
Credit: based on Callister
insulators as described in the text
(c) Insulators have a wide band gap (>2 eV) with few electrons excited across the band gap.
Credit: based on Callister
semiconductors as described in the text
(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 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

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.