On which factor does the electrical conductivity of an intrinsic semiconductor depend? It doesn't have an excess of charge carriers in fact, does it?
Conductivity of intrinsic semiconductor is due to their own internal charge carriers. The bonding between between two electrons of two neighboring atoms is covalent, therefor at NTP, there is no free charge carrier for conduction. When it is heated, some covalent bonds break due to heat and thus some electron get free for conduction. As soon as one electron gets free, there is a deficiency of electrons at its preceding position which acts as a positive charge or a hole, The number of holes is equal to number of electrons. At normal temperature, only $1$ ou of $10^9$ bonds break and therefore, conductivity is very low about few milli amps.