What is transistor negative temperature coefficient?

Answer 1

for a given current and temperature range, if you raise the temperature, voltage will be lower as a consequence.

usually you have ~-2mV/K, this is common in semiconductors (ie you will have the same behaviour for diodes).

For example, consider a transistor which drains 2mA and its output voltage is 200mV: if you increase its internal (junction) temperature by 2 kelvin degrees and keep its current to 2mA, output voltage will drop to 196mV.

this is due to thermal excitation, which conveys more energy in the crystal lattice, so that the number of free carriers increases.

for higher temperatures, some other effects arise and temperature coefficient will become positive.