If you bond the drain and gate of the FET, you obtains a diode connected transistor

Dear Mohammad Khaleqi sir, I know that shorting drain and gate will yield a diode connected transistor but want to know whether it will behave as a diode or just its current voltage relation is similar to that of diode.

The bipolar transistor is basically two diodes. When connected as a diode (for example, with a base shorted to collector or emitter), the transistor will work as a diode - it will rectify, with a current-voltage relationship of a diode. No amplification is possible in this configuration. However, using the transistor as a diode could be problematic since the transistors designed in a way that the currents flowing through the base are much smaller than the curents flowing through collector.

Thank you Anton Sheinin sir for the response. Can you please explain how the last point "using the transistor as a diode could be problematic since the transistors designed in a way that the currents flowing through the base are much smaller than the currents flowing through collector" is related to diode connected transistor?

Dear Amit Das ,

First of all, "diode-connected" originally meant B-C shorted BJT (which effectively causes a pn junction behavior), but later other transistors (e.g. MOSFET) were also used with similar connection, which found an application especially in current mirrors, some active-loaded amplifiers, voltage references and DC voltage shifters (most of them essential blocks in integrated circuit design).

On the other hand, when you need solely a diode in an integrated circuit, it may not be directly available in a bipolar process (since the process is optimized for an npn transistor). Under these conditions, IC designers sought for alternatives for which usually BE junction of an npn was a good one, which could be used as a Zener diode as well (because of small breakdown voltage, as a consequence of high emitter doping). If, however, you need a large-breakdown-voltage diode (to withstand large reverse voltages when in OFF state), then BC junction is a better choice.

I recalled a figure from a book ("Bipolar and MOS Analog Integrated Circuit Design" by Alan Grebene, p.125) which was giving all possibilities of obtaining a diode from an npn BJT, along with summarized properties for each alternative. I found the figure and attached it to this answer.

I hope my comments are helpful for you.

Best regards...

Thank you so much Ali Zeki sir for such an informative and detailed answer. Actually I was looking for the same thing which you have answered, thanks sir.

It was a pleasure, dear Amit Das .

Dear Amit Das ,

welcome,

I would like to precise the meaning of some technical terms.

Diode is a device with two electrodes which are distinguished from each other. For example the anode and the cathode. The I-V characteristics of the diode has a rectifier shape in the sense it low ohmic in the forward direction and high ohmic in the reverse direction.

They may differ in their physical structure such as:

The vacuum diode,

The p-n diode,

The Schottky metal semicondcutor diode

There are devices which are composed from diode as basic building block such as the bipolar transistor where the transistor is an pnp or npn structure with two junctions and three terminals. This transistor can act as diode by different electrode connections. They are really p-n diodes.

When connecting the transistor as a diode it will not work as a transistor.

That is not as an amplifier. To work as an amplifier, one of the junctions must be forward biased and the other will be reverse biased.

In case of operating the MOSFET in a diode configuration one short the the D and G.. In this case the diode characteristics will be

ID= F(VGS, VDS= VGS)

Which has a rectifier diode similar characteristics.

The junction FET can be used as diode by making VGD=VGS.

You can get more information from the book:Book Electronic devices with physical insight

Best wishes

Both are right