People have been using BJTs in various configurations with either of the junctions shorted. The following queries persist for these configurations. Links to previous questions are added

1)Base collector shorted This configuration is used when the bjt is to be configured as a diode. Now, what would be the ratio of the base current to the collector current is this configuration and does this have any relation to the standard Hfe

2)Base emitter shorted Mentioned as an alternative to using zeners. In that case can I use a BJT with base-collector shorted and reverse biased in replacement for a low voltage zener. (in place of standard zeners which have a soft knee, saw this link which mentions a sharp knee for this config). Say for example I use a BC817 as a replacement for a 5V zener. And in this case I expect the current rating to be the lower of Ic and that dictated by the power rating of the device

  • \$\begingroup\$ (1) This forms a diode with approximately the same saturation current and emission coefficient as the BJT. Since these determine the voltage, it's often used in current mirrors. The BJT will operate in active mode as the BC junction isn't forward-biased. (2) The avalanche mode of the BJT can be used as a random number generator, noise generator, and as a protection device. I don't see it often used for protection. But I also don't often see it used for random number generation or noise, either. So, consider it a bit of a rare case to be aware of. \$\endgroup\$ – jonk Nov 1 '19 at 5:27

With Base connected to Collector the transistor is on the knee going into the saturation zone. Depending on knee voltage the current gain may be a bit lower than HFE at standard Collector voltage.

With Base connected to Emitter the transistor is turned off unless the polarity is reversed, and then the Collector and Emitter swap roles. The current gain is very low in this mode but it still acts like the other case.

If the Base is not connected to the Emitter then the B-E reverse bias breakdown voltage can be exploited. The problem with this mode is that it is outside the absolute maximum specification (so if it damages anything it's your fault). The actual breakdown voltage is not usually specified either, so it cannot be relied on.


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