Doped Semiconductors Group IVA semiconductors can be “doped” by adding small amounts of impurities with more or fewer than 4 valence electrons. e.g. add N or P impurity to Si. The extra electron will go into the conduction band, creating donor level, lowering band gap, increasing conductivity. Valence Band Conduction Band E EgEg Valence Band Conduction Band new E g Donor level This is called an n-type (negative) semiconductor due to additional electrons.

p-Type Semiconductors Adding an impurity with fewer electrons e.g. add B or Al to Si, results in the formation of an acceptor level above the valence band, easily accessible to the valence band. This also lowers the band gap. Valence Band Conduction Band E EgEg This is called a p-type (positive) semiconductor as “holes” (positive charge carriers) are left behind in the valence band. Valence Band Conduction Band new E g Acceptor level

Figure Crystal structures and band representations of doped semiconductors

Diodes If p– and n–type semiconductors are joined together, a “p–n” junction is obtained. If this is made part of a circuit, with the n-type end connected to a negative voltage, electrons will flow from donor groups across junction, holes will move other way, leads to flow of current. If circuit is connected up the other way, donor electrons will be attracted to positive terminal – no flow of current. Thus, a diode only allows current to flow in one direction.

Forward bias Reverse bias p-n junction Figure The p-n junction

LEDs When free electrons in a diode move into holes of p–type layer, the drop in energy from the conduction band to a lower orbital. This can result in the emission of light. The band gap energy can by “tuned” by composition of semiconductor, as discussed earlier. The energy difference can be made to correspond to visible light (E = hc/ ). This is called a Light Emitting Diode.

Resistance Temperature Resistance Temperature Resistance Resistance is the opposition to the flow of current (units Ohm –  ) Conductors Resistance increases with T, due to increasing atomic vibration hindering electron mobility Semiconductors Resistance decreases with T, as fraction of electrons able to overcome band gap increases

Superconductors Temperature Resistance For certain substances, resistance drops to zero below a critical temperature, T c Under such conditions, current can flow completely unimpeded. Potential uses of superconductors include power transmission (avoiding loss due to heating), and as strong magnets. Meissner effect: Superconductors are perfect diamagnets, so strongly repel magnetic fields with less than the critical field strength (H c ). So, to be superconducting, must be H c Breakthrough in late 1980’s: ceramic superconductors with T c > liquid nitrogen temperature (77 K).