# Basic NPN transistor questions

#### JackD2

Joined Sep 15, 2012
3
I've just had my first lecture on transistors and we have been asked a couple of questions to see if we understand the basics. However, I'm not confident even with these.

1) For an NPN transistor without any external voltage, what is the electric field direction within the B-E and B-C junctions?

My understanding is that the electric field directions are from E to B and from C to B.

2) For an NPN transistor without any external voltage, what is the diffusion current direction within the B-E and B-C junctions?

My understanding is that the diffusion current was from E to B and then from B to C.

Are either or both of these correct?

#### bountyhunter

Joined Sep 7, 2009
2,512
Without external voltage applied, there isn't current flowing. The CB and BE junctions forma depletion region where the free N dopants fill in some adjacent holes. I believe to get current to flow you will need to apply voltage.

#### JackD2

Joined Sep 15, 2012
3
Thanks for your reply. It makes sense that there wouldn't be any overall current flow.

I might be wrong but I think the question is asking what the electric field and diffusion current directions are, regardless of the fact that the various currents enter equilibrium, leading to no net current flow.

#### atferrari

Joined Jan 6, 2004
4,798
What is "diffusion" current?

#### #12

Joined Nov 30, 2010
18,224
Modern schools make us old-timers look stupid!

#### bountyhunter

Joined Sep 7, 2009
2,512
What is "diffusion" current?
Carriers do diffuse into the adjacent regions which is what form the depletion zone..... but it establishes an equilibrium. I never heard of any "diffusion current" since there is no path for continuous current flow other than out the transistor leads.

#### bountyhunter

Joined Sep 7, 2009
2,512
Modern schools make us old-timers look stupid!
Or maybe they just find complicated new labels for simple old concepts.

#12

#### vk6zgo

Joined Jul 21, 2012
677
Or maybe they just find complicated new labels for simple old concepts.
More likely!

#### bretm

Joined Feb 6, 2012
152
With no applied voltage, diffusion is short-lived before equilibrium occurs. But it is charges that are moving, so technically that's current. With an applied voltage the diffusion is continuous so it makes more sense to call it a current then.

#### Audioguru

Joined Dec 20, 2007
11,248
Modern schools make us old-timers look stupid!
I think modern teachers are stupid.
My old teachers did not talk about "electric field directions'" nor "diffusions".

#### MrChips

Joined Oct 2, 2009
31,067
Diffusion of mobility carriers is discussed in solid-state physics.
You don't need this to understand circuit design.

#### bountyhunter

Joined Sep 7, 2009
2,512
I've just had my first lecture on transistors and we have been asked a couple of questions to see if we understand the basics. However, I'm not confident even with these.

1) For an NPN transistor without any external voltage, what is the electric field direction within the B-E and B-C junctions?

My understanding is that the electric field directions are from E to B and from C to B.

2) For an NPN transistor without any external voltage, what is the diffusion current direction within the B-E and B-C junctions?

My understanding is that the diffusion current was from E to B and then from B to C.

Are either or both of these correct?
Transistor theory:

http://www.tpub.com/neets/book7/25a.htm

#### Austin Clark

Joined Dec 28, 2011
412
Diffusion of mobility carriers is discussed in solid-state physics.
You don't need this to understand circuit design.
You know me, I'm all for learning about that sort of thing, but I agree. This really shouldn't be taught in a class about electronics design. Maybe touched on, or just thrown in for fun, but not part of the curriculum. Not that it isn't important, but there are many things that should take priority over this.

Unless, of course, the OP isn't in a design class.

#### Kermit2

Joined Feb 5, 2010
4,162
But aren't 'holes' being used as current carriers in some courses?

So asking about direction of diffusion current without recourse to which type of current, be it free electron, or intrinsic hole movement.

Kermit says to answer 'both directions' unless the question can be more specific.

(Take that! New math. Pfft!)

#### Audioguru

Joined Dec 20, 2007
11,248
I have designed hundreds or thousands of circuits in my career but I have never built a transistor or IC.
I understand how a transistor circuit or an IC circuit works, mainly from the spec's in the datasheets.

I have never thought about which way conventional or electron current flows. I simply think that the emitter of an NPN transistor is N so it must connect to negative and the emitter of a PNP transistor is P so it must connect positive.

I simply think that a diode conducts when its anode is positive and its cathode is negative.

#### bountyhunter

Joined Sep 7, 2009
2,512
But aren't 'holes' being used as current carriers in some courses?

Yes, I recall an upper division class absurdly titled "Basic Electronics" that went into all that and the functions inside the variuos types of semiconductor devices. That class was a bugger.