Transistor as diode

[Simon123]

Hy!

A lot of times BJTs are used as diodes because of their small leakage current.
But sometimes i see the BE junction being used and sometimes the BC junction is used.
What are advantages/disadvantages of one over the other besides Reverse breakdown voltage of BE is smaller compared to BC.
I have also read that BE has smaller recovery time, but i dont undestand why some people say BC junction is better?

[KrudyZ]

The BE junction makes for a pretty good zener diode (~6-7V) in reverse.
In forward direction with the collector tied to the base, you get the full advantage of the current gain of the transistor to get a much sharper knee and high current capabilities.
The BC junction has lower leakage since that junction in a transistor is almost always in reverse bias, so that's what it is optimized for.

[Zero999]

Another application is in a current mirror, where the same transistor type is used as a diode, purely in the hope its base-emitter voltage will be matched to the other transistor.

https://en.wikipedia.org/wiki/Current_mirror

[Cerebus]

Typical figures for a small signal NPN bipolar transistor:

Junction	Recovery Time	Capacitance	Breakdown Voltage	Reverse leakage current
Collector Base Emitter Open	200 ns	3 pF	30-60V	4 pA
Emiiter Base CB Shorted	1 ns	5 pF	6-8V	10 pA

[mrkev]

...purely in the hope its base-emitter voltage will be matched to the other transistor.

Not purely Once oupon a time, there used to be a device with two pre-matched transitors in one package (those are pretty rare nowadays). But you can simply add emiter resistors.
On the other hand, current mirrors like this are very often used in on-chip circuits, where it's much easier to match two transistors than to make one with precise parameters.

And to the original post, exactly as KrudyZ wrote, the BE junction in reverse is often used as a low cost, very-low capacitance (and leakage current) zener for hi-frequency amateur circuits (usually as protection from overvoltage).

[David Hess]

Most available small signal diodes are either gold doped switching diodes or schottky diodes.  Low leakage and high conductance diodes exist but cost much more and have poor availability compared to a small signal bipolar transistor and fast low leakage high conductance diodes like a base-emitter junction have not been available in decades; if you want low leakage and fast recovery, there is nothing better.

Of course, why would you need them?  High impedance shunt protection circuits, analog waveform generation, analog processing, sampling, and RF applications can make use of them.