# Can a transistor's emitter and collector be used interchangeably?

I'm trying to understand the standard TTL NAND Gate in TTL level. But confused with the transistor Q1 operation in the diagram below:

Apparently when A and B are together HIGH, the Q1 base-emitter junction is reverse biased. Collector-base junction is on the other hand forward biased. So the current flows through R1 and turns on the transistor Q2. So far so good up to here.

Here is my question: How can a BJT(probably npn) transistor such as Q1 can operate in such way? I mean when I learn about transistors the base emitter collector connections are very important. But from this Q1 I conclude that one can use base and collector interchangeably. Is that right?

• A BJT is simultaneously both a transistor and a pair of diodes. Oct 23, 2014 at 0:47
• I dont understand how current flows this way in Q1. Could you expund on how the q1 operates in my diagram? Oct 23, 2014 at 0:50
• Current cannot flow from the base to the emitter, so it flows from the base to the collector instead. Oct 23, 2014 at 0:54
• i think im confused because normally the base is a very small voltage just to trigger the transistor and the real current flows from collector to emitter. but in this case i noticed the base is directly connected to the Vcc. as you put it it acts like diode here. Oct 23, 2014 at 0:58
• Theoretically bipolar transistors are only 2 opposed PN junctions, which would normally mean they can be used interchangeably - however usually the collector is made wider than the emitter [in turn wider than base] which makes the transistor asymmetric. As for the base and collector, they definitely don't have the same working and can't be switched around in terms of biasing. The configuration (common base, emitter, collector) can change though. Oct 23, 2014 at 1:35