To achieve high performance thermoelectric materials and devices, thermoelectric transistors, which integrate thermoelectric effects with transistor technology, represent a promising approach. Here p type Bi0.5Sb1.5Te3 and n type Bi2Te2.97Se0.03 are used as the constituent materials for an NPN transistor. By applying forward bias to the emitter and reverse bias to the collector to form a common-base triode configuration, the thermoelectric effect, transistor effect, and interfacial effects within the NPN heterostructure are coupled. This NPN heterostructure induces temperature increase (heat release) at the forward biased end and temperature decrease (heat absorption) at the reverse biased end. Therefore, this device becomes a new type of thermoelectric transistor cooler. Furthermore, a DC equivalent circuit method is introduced to analyze the cooling performance of the thermoelectric transistor cooler. The results show that when the emitter voltage of the NPN thermoelectric transistor cooler is 0.01925V, the collector voltage is 0.2124V, the corresponding base region width is 0.17nm, the maximum temperature difference of 242.89K can be obtained. Even if the base region width is limited to a level of 10 nanometers, for example when the base region width is 12.78nm, the maximum temperature differenced can still reach 174.15K. The results show that the thermoelectric transistor, which synergistically combines the thermoelectric effect with transistor technology, can effectively enhance the maximum temperature difference typical thermoelectric cooling achieves. Also, thermoelectric transistors are smaller than conventional thermoelectric cooling devices. Therefore, they will play a key role in cooling miniature electronic components.