| Positive temperature co-efficient of resistivity or PTCR materials area family of semiconductors which exhibit special properties in relation to electrical conductivity. In particular, their electrical resistivity increases with increasing temperature. However, the unique behaviour of PTCR materials is more complicated than just having the resistivity increase as the operating temperature increases. PTCR materials behaviour is characterised by a slow increase in resistivity up to a certain temperature, which is dependent on the actual material. This temperature is called the Curie temperature. When a PTCR material reaches its Curie temperature, its resistivity increases by several orders of magnitude over a very small temperature range (see figure 1). Thus, the amount of current that can flow is very small compared to that which can flow at significantly lower temperatures. After this sharp rise, the resistivity approaches an almost constant value. It should be noted that the increase in temperature can be induced in some part by the flow of current through the material. | 
| | Figure 1. Electrical resistance versus temperature behaviour for positive temperature co-efficient of resistivity (PTCR), negative temeparature co-efficient of resistivity (NTCR) materials and critical temperature thermistors | Materials exhibiting PTCR properties are generally semiconducting titanate ceramics including barium titanate, lead titanate and strontium titanate. In their pure forms they are insulators, however when small amounts of dopants are added they become semiconducting. Other characteristics of PTCR materials include: · A polycrystalline n-type semiconductor · They have surface acceptor sites at their grain boundaries · They are ferroelectric · They have ohmic electrodes | 
