how a transistor can be converted into a diode?

Answer 1

The base-emitter junction is a diode. Use those 2 terminals.

2nd answer is technically correct, though the more common answer (and probably what the asker is looking for) is the base-emitter junction, since that's how an NPN transistor is typically biased (and even an emitter-base breakdown voltage of ~5V is adequate for many applications). And while the phototransistor setup is interesting, it's not the basic principle the asker was looking for.

3rd answer neglects to consider that Ic (the collector current) can be 0. So if that terminal is unused, Ib=Ie.

Answer 2

I am not sure if a transistor can be used as a diode as a in a transistor Ie = Ib + Ic. If only emmiter and base terminals are used how will the equation be fulfilled. More over base is thin and less doped. The ref to the web site speaks about bipolar transistors. and not conversion of transistor to doide.

Answer 3

Either the base-emitter or the base-collector junction will act as a diode. The choice depends on what you want the diode to do.

The base-collector junction is a better diode if you need a high reverse voltage capability with decent switching speed. This is generally why you need a diode in the first place.

On the other hand, the base-emitter junction has lower reverse leakage current, up to a reverse bias potential of 4 to 5 volts (maximum). After that the b-e junction goes into zener breakdown and will self-destruct if the current is not limited to a safe value.

The b-e junction, because of its low reverse leakage, is sometimes used to protect the differential inputs of an operational amplifier.

A common application of diode operation of a transistor is to connect a photo-transistor as a photo-diode.

Photo-transistors, which are usually NPN types, are operated with the base-collector junction reverse biased (as in normal common-emitter amplifiers) by applying a voltage between the collector and emitter. The base is usually left unconnected or returned to the emitter through a large-valued resistor. A photon absorbed in the P-type base region creates an electron-hole pair there. The electrons are minority carriers in the thin base region and are swept toward the collector by the reverse biased collector-base junction. Electrons entering the emitter re-combine with the holes. The result is a beta-amplified current in the collector of the transistor. Such an application is very sensitive to light, but not particularly linear in its response to intensity of the light. It is used extensively in optical isolators where the phototransistor is illuminated by an LED driven by a binary (on/off) type signal.

However, the actual photo-current, the minority-carrier electrons generated in the base region, is quite linear with light intensity over at least five decades of magnitude. The lower limit generally is determined by the so-called "dark current" of the reverse biased collector-base junction with 15 to 30 volts bias.

Using two NPN photo-transistors with one biased from a positive supply connected to its collector and the other from a negative supply connected to its base, and connecting the base of the first one to the collector of the second one, allows the two dark currents to be subtracted for increased sensitivity and reduction of drift as a result of temperature changes. The photo-current is measured (with a high input impedance, low bias current, op-amp) at the connection between the base of the first transistor and the collector of the second transistor.

One photo-transistor has to be masked from light of course and both should be mounted on or in a heat sink so they are at the same temperature. Clip the emitter lead to avoid noise pickup.

About forty years ago (before phototransistors were inexpensively available) I used to cut the tops off metal-can NPN transistors and use them as photo-transistors to measure the speed of slot-cars. We even re-wound our own motor armatures back then. Does anybody still race slot cars?

Answer 4

Using one of the junction out of the two either emitter -base or base -collector.

Answer 5

Every time I post a question, even if its the simplest one, they can't offer me a proper informed answer on this website. What happened to people that actually take the time to write an answer??

Answer 6

Funny, I was wondering the same thing myself