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In the 1960s, the germanium PNP transistor was the cheapest and best for use at ordinary temperatures. The leakage current from collector to base in this type of device is larger than for the silicon transistor, and also varies more with temperature. The effect of these deficiencies can be lessened by proper biasing and feedback, which can make the circuit both serviceable and reliable. Neither germanium nor PNP transistors are as common today.
The voltages used on a PNP transistor are inverted when compared with vacuum tubes. Further, the behaviour of vacuum tubes is usually described in terms of voltages whereas transistors are better described in terms of current.