This has to do with the operation of the transistor. Let me explain again what the operation of the transistor is:

When the NPN transistor is ON, its BE junction is biased in the forward direction. This means now holes from the basis cross the junction, become minority carriers in the emitter, and diffuse to the emitter contact.

Likewise, electrons from the emitter cross the BE junction, and become minority carriers in the basis region of the transistor. Now,  it becomes interesting. There happen mainly 2 things in the basis region of the transistor:

(1.) The minority carrier electrons recombine in the basis region.

(2.) They get attracted by the CB junction, which is biased in reverse direction. However, the minority carrier electrons cross the CB region, become majority carriers in the collector, and form the collector current.

Now, there are basically 3 current components associated with the forward biased BE junction:

(1.) The current of holes crossing the BE junction, and getting into the emitter

(2.) The current of the electrons crossing the BE junction, and recombining in the basis

(3.) The current of electrons crossing the BE junction and also crossing the CB junction.

The basis current is (1)+(2), the collector current is (3). The Beta = (3)/((1)+(2))

Now, the transistor is constructed to have high beta (high gain). (usually, at least). This is done through:

- Highly doped emitter to reduce the current component (1.)

- Thin basis to reduce the current component (2.)

- Lightly doped collector in order to have high voltage withstand capability. (This limits the max electrical field inside the reverse biased BC junction, and allows high collector voltage.)

Now, if you use the transistor in reverse, the emitter becomes the collector and the collector becomes the emitter, and its quite simple to see what happens:

(1). As the collector doping is less than the emitter doping, the hole current in the collector (now the emitter) increases, leading to lower beta.

(2). The voltage withstand capability goes down, as now the BE junction is reverse biased, and the emitter is not constructed for high withstand voltage.

(3). The effect of the thin basis remains. For that reason, if the forward beta is high, then the reverse beta will also be higher.

Cheers, Henri.

Dear Henri,

I am really grateful for your perfect, worthwhile, and prompt answer. I highly appreciate your convincing answer.

Regards,

Morteza Shabanzadeh

http://mortezash.blogfa.com/

I fully agree with Henri's explanation. However, one major advantage of operating the npn transistor with its emitter and collector roles reversed is that the saturation voltage drop across collector and emitter is lower than that obtained in the normal mode, inspite of having lower current gain. This is why such devices were used as analog switches in low voltage control electronics in discrete applications.

Dear Professor Biswas,

I highly appreciate your point of view regarding my question.

Regards,

Morteza Shabanzadeh

In default NPN transistor,the emitter part is highly doped so to withstand a large variation of AC signal( you can say high swing)and for normal mode of operation(like an amplifier).If the E and C are reverse biased we cant predict the same thing..or you can just assume by taking collector as emitter and vice versa.So we will lack of current gain factor.

Perhaps the most interesting observation is that the collector current is almost identical in reverse mode.  This makes for a very interesting question to ask students who might assume that collector current is limited by electron supply.  Note also that the built-in field in the base due to base doping  variation usually operates such as to oppose electron drift in the base for reverse mode (!).

There are also some advantages for temperature operation as the effective emitter does not suffer from band-gap narrowing which arises for heavily doped semiconductor.

Dear All,

Please accept my explanation is only a summary. If you want to study more in depth, you should look for basic book, web page on the topic of "solid state physics", "semiconductors solid state physics", or "silicon solid state physics".

A slightly similar use is the reverse bias of the base emitter junction, or.  I sometimes use this as a 8.8V zener if I need one. For the transistor I use most the BCW33 the breakdown voltage is about 8.8V If I remember correctly.  It seems to me connecting the collector to the base would give you a zener that could pass a higher current, for NPN base connected to collector connected to 0V, emitter to V+ thus "bias the collector and emitter of an NPN Transistor inversely".

NPN (or PNP) transistor can be used as an amplifier or a switch depends on how we bias the junctions. 

The inverted configuration was used in chopper amplifiers  long back to get extremely low Vsat. (less than 10mV compared to 100mV to 200mV for normal mode). today chopper amplifiers the FET as there is no Von for them.  

If you operate in inverse mode, the collector current stays almost the same (can you work out why? - look at the theory for the collector current..)

Regarding the base current, you are now injecting into a low doped region - so what happens to the current?

I agreed with all the answers given above Morteza Shabanzadeh sir. Reversing the role of emitter and collector with decrease the forward current since collector(which is now playing the role of emitter) is not heavily doped to emit the charge carriers like the emitter region. This is the main reason, why you get a reduced gain in this case. There is one more reason, emitter being heavily doped(acting as collector now) will have a very low breakdown voltage, so you can not operate it at higher voltage level.

Dear Morteza Shabanzadeh ,

If the npn transistor is symmetrical in construction its forward and inverse operation will be identical.

If the bipolar transistor is not symmetrical in structure, it will show different characteristics in both directions.

Most practical bipolar transistors are designed with unsymmetrical construction because the two different functions of the emitter junction and the collector junction. The emitter junction has the function to have high injection efficiency to make the forward current amplification factor high. The collector junction has to withstand the breakdown voltage of the transistor which dictates that its bulk must be relatively low doped. Therefore the transistor must have nn+ layer structure while the emitter must have only n+ layer structure.

It results in low reverse current amplification factor of the transistor.

So, the transistor is characterized in forward and reverse direction.

For more information about the structure and characteristics as well as the models of the bipolar transistors please refer to the book in the link:Book Electronic devices with physical insight

Ic stays about the same because it depends on the Gummel number in the base. IB increases as it depends (roughly) on the doping in the emitting region. So gain is greatly decreased