Intrinsic semiconductors-conductivity

Carrier concentration in intrinsic semiconductors

Pure Si is a very poor conductor. It is rarely used in the intrinsic form, except for specific optical sensor applications, at liquid N2 temperatures. Here is it usually lightly doped with Li to control the carrier concentration. For higher conductivity, using equation 3, we either need a material with a higher mobility or a higher intrinsic carrier concentration (i.e. lower band gap). Mobility and band gap values for 3 commonly used semiconductors are listed in table 1. Based on mobilities, GaAs would be the material to choose for higher conductivity but GaAs also has a higher band gap so that the intrinsic carrier concentration would be low. The carrier concentration and conductivity for pure GaAs can be calculated in a manner similar to that done for Si. The effective mass values for GaAs are m^∗ e = 0.067 me and m^∗ h = 0.50 me. Using

conductivity the dominant term is the carrier concentration and consequently the band gap. Since the carrier concentration has an exponential dependence on Eg lowering Eg will have the greatest effect on conductivity. Based on table 2, Ge would be the semiconductor material of choice due to its high conductivity and low band gap. In fact, the first transistors were made using Ge. But later, Si has come to dominate the microelectronics industry. The reason for this is conductivity in Si can be increased by adding specific impurities to the material. This is also coupled to the fact that Si is the second most abundant material on earth (27%) while Ge is only present in the parts per million amount.

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