# How does middle layer of a transistor get a charge in order to get reverse biased or forward biased accordingly?

#### Doubtician

Joined Mar 19, 2022
11
I dont know much about electronics.While studying about transistors,i had a doubt about the middle layer.If the emitter and middle layer becomes forward biased, and collector and middle layer becomes reverse biased,how does middle layer have both the charge in order to get reverse and forward biased at the same time?Pardon me if i understood it the wrong way.

#### Ian0

Joined Aug 7, 2020
5,535
It's not the layers that are biassed, it's the junctions between them.
If base voltage is greater than emitter voltage, the base-emitter junction is forward biassed. If collector voltage is greater than base voltage then the base-collector junction is reverse biassed.

#### Delta Prime

Joined Nov 15, 2019
1,311
Hello there,welcome to AAC!
I need to see exactly what you are seeing or studying can you post a link a picture or just draw it on a piece of paper.

#### Doubtician

Joined Mar 19, 2022
11
Hello there,welcome to AAC!
I need to see exactly what you are seeing or studying can you post a link a picture or just draw it on a piece of paper.
From this video.....

#### dl324

Joined Mar 30, 2015
14,702
Welcome to AAC!
If the emitter and middle layer becomes forward biased, and collector and middle layer becomes reverse biased,how does middle layer have both the charge in order to get reverse and forward biased at the same time?

A bipolar junction transistor has two junctions. In active mode, the BE junction is forward biased and the CB is reverse biased.

#### Papabravo

Joined Feb 24, 2006
18,820
It is the battery or voltage sources that create the potential differences across the junctions. The base is more positive than the emitter, so that junction is forward biased. The collector is similarly more positive than the base, so the base collector junction is reverse biased.

$$V_b>V_e\implies V_{be}>0\implies \text{Forward Biased}$$

$$V_b < V_{c}\implies V_{bc}<0\implies \text{Reverse Biased}$$

#### Doubtician

Joined Mar 19, 2022
11
So,in a PN junction diode, the junction is biased and not the diode as whole ,right?

#### Papabravo

Joined Feb 24, 2006
18,820
So,in a PN junction diode, the junction is biased and not the diode as whole ,right?
What happens at a pn-junction is a bit more complicated than a simple model can convey. The simple explanation is that the external potential (aka voltage) changes the impediments to current flow in such a way that a semi-conductor becomes an actual conductor.

#### dl324

Joined Mar 30, 2015
14,702
So,in a PN junction diode, the junction is biased and not the diode as whole ,right?
Trying to model a transistor as diodes will only lead to more confusion.

This diagram is from an ECE322 course at CMU: