Help on understanding NPN transistor

In summary, the conversation discusses the different operational modes of a bipolar junction transistor (BJT) and how they are affected by the biasing of the base-collector and base-emitter junctions. It also clarifies the definition of positive and negative collector current and how it relates to the direction of current flow. The conversation concludes with the understanding that the collector current can be negative, but it is physically impossible to achieve in certain operational modes.
  • #1
mkkrnfoo85
50
0
Hey,

I would like help understanding this scenario:

(1) base-collector voltage, V_BC, is reverse-biased
(2) base-emitter voltage, V_BE, is forward-biased
(3) collector current, I_C, is negative

Normally, if I_C was positive, I would assume the transistor to be in the forward-active region. However, can you still consider it in the forward-active region if the collector current is negative? Does this scenario make sense?

Thanks,

Mark
 
Engineering news on Phys.org
  • #2
Never heard of a negative current.. It is only a convention wrt some current flow direction.

Basically electrons fire across the forward biased emitter-base and then about 99% of them carry on across the reverse biased base-collector. A small base current controls a much larger collector current.
 
  • #3
Did you instead meant to say that the collector current is flowing out of the collector junction? Hmm, that sounds as though the BJT is working in the reverse active mode. The gain for this is much for smaller though due to disproportionate doping for the collector and emitter regions. Check your textbook for details on that mode of operation.
 
  • #4
If the BE junction is forward biased, and the CB junction is reverse biased, shouldn't Ic be positive? The transistor should work in the active region.
 
  • #5
mkkrnfoo85 said:
Hey,

I would like help understanding this scenario:

(1) base-collector voltage, V_BC, is reverse-biased
(2) base-emitter voltage, V_BE, is forward-biased
(3) collector current, I_C, is negative

Normally, if I_C was positive, I would assume the transistor to be in the forward-active region. However, can you still consider it in the forward-active region if the collector current is negative? Does this scenario make sense?

Thanks,

Mark

Well Mark that is the one "quadrant" of collector operation that is physically impossible to obtain. All three other possiblities at the collector can indeed occur. I'll summarize them (NPN).

1. I_c +ive, BC junction reverse biased = active region

2. I_c +ive, BC junction slightly forward biased = saturation region.

3. I_c -ive, BC forward biased = reverse operation.

In modes 1 and 2 above the BE junction should be forward biased. In mode 3 it either operates as an inefficient reversed NPN (that is, with C and E functions interchanged) if the BE junction is reverse biased, otherwise it pretty much degenerates into two diodes with a common annode (that being the base).BTW. I should note that the definition of +ive collector current is when the current flows into the device. This is a standard definition. Perhaps you were unaware of this and were mistakenly thinking negative Ic corresponded to the case where Ic crosses the BC junction in the direction from N to P? In this case then that's actually the positive direction for Ic and you would be looking a simple active region operation.
 
  • #7
hey, thanks everyone. it helped improve my conceptual understanding of bjt transistors.

-Mark
 

1. What is an NPN transistor and how does it work?

An NPN transistor is a type of semiconductor device that is used for amplification and switching of electrical signals. It consists of three layers of doped materials, namely the emitter, base, and collector. The flow of current through the transistor is controlled by the application of a voltage at the base, which allows for amplification or switching of the current at the collector.

2. How do I read the specifications of an NPN transistor?

The specifications of an NPN transistor typically include the maximum collector current (IC), the maximum collector-emitter voltage (VCEO), and the maximum power dissipation (PD). These values are important for determining the transistor's capabilities and limitations in a circuit.

3. What are the differences between NPN and PNP transistors?

The main difference between NPN and PNP transistors lies in their polarity and current flow. In an NPN transistor, the current flows from the collector to the emitter, while in a PNP transistor, the current flows from the emitter to the collector. Additionally, the doped layers of an NPN transistor are arranged as P-N-P, while a PNP transistor has an N-P-N arrangement.

4. How do I choose the appropriate NPN transistor for my circuit?

The choice of an NPN transistor for a circuit depends on its required specifications, such as the desired collector current and voltage. Additionally, factors such as the gain (hFE) and frequency response should also be considered. It is important to select a transistor that can handle the maximum current and voltage in the circuit while also providing the desired amplification or switching capabilities.

5. Can I use an NPN transistor in place of a PNP transistor?

No, NPN and PNP transistors are not interchangeable. As mentioned earlier, their polarity and current flow are opposite, so using an NPN transistor in place of a PNP transistor would result in incorrect functioning of the circuit. It is important to choose the appropriate type of transistor for a specific circuit.

Similar threads

Understanding a heat sensor circuit?
    • Electrical Engineering
Replies
3
Views
795
Why does a transistor in saturation act like a short circuit?
    • Electrical Engineering
Replies
5
Views
2K
Understanding the base emitter voltage in a common emitter NPN BJT
    • Electrical Engineering
Replies
16
Views
4K
NPN transistor and how it operates....
    • Electrical Engineering
Replies
11
Views
1K
Exploring the Role of Charge Injection in the Operation of an NPN Transistor
    • Electrical Engineering
Replies
3
Views
1K
How to select a transistor?
    • Electrical Engineering
Replies
2
Views
1K
I don't understand transistors in combinations .... Or maybe basics ....
    • Electrical Engineering
2
Replies
68
Views
4K
Will the bleed resistors enable a shorter path to ground or not?
    • Electrical Engineering
3
Replies
80
Views
3K
How can a transistor behave as a switch in the saturated region?
    • Introductory Physics Homework Help
Replies
5
Views
591
Hacked pulsed laser driver using Arduino
    • Electrical Engineering
Replies
32
Views
3K

Hot Threads

Recent Insights

Back
Top