# Transistor

Discussion in 'Homework Help' started by Iwan, Aug 22, 2004.

1. ### Iwan Thread Starter Member

Jul 14, 2004
23
0
Hi....

can someone explain to me how PNP transistor works?I know about NPN transistor.
When i see the structure the NPN have great amplifying in the emitter.But the question about PNP the current are flow to base right.Is the structure of PNP different with NPN?Is the base region are bigger than emitter or collector?
Plz someone explain the PNP transistor to me .

Thanks

2. ### Mjollnir Member

Apr 22, 2004
27
0
I just studied this last semester

with the PNP, Current enters tru the emitter and leaves tru the base and collector

Base region is highly negative doped, and very thin, so hardly any recombination occurs and most carriers drift across it into the deleption region on the C collector side. E and C are postively doped

IE = IC + IB = (beta +1) IB still holds, except now current enters tru the emitter

in order for the PNP to be in forward active region, u need to bias it such that VBE = -0.7V and VCE = -0.2V

this makes the Base Emitter P-N in forward bias. The VCE biasing setup an electric field to sweep the carriers out of the deleption region on the collector side, also to prevent Collector Base P-N junction conducting in forward bias.

3. ### mozikluv AAC Fanatic!

Jan 22, 2004
1,437
1
hi

let me add a little about the workings of BJT NPN & PNP. everytime we see a bjt transistor symbol we always use the arrow symbol to identify the transis whether it's npn or pnp.

but we sometimes forget that the arrow also represents the current flow direction and when we look at a schem it makes it easy to follow how the current flows in a circuit. that arrow direction guides us how the control current and the working current flows.

for npn, the arrow points away from the base and collector so the control current flows into the base and goes into the collector but goes out the emitter and the working current flows into the collector and goes out the emitter. while for pnp, control current flows into the emitter and goes out of the base and the working current flows into the emitter and flows out the collector.

when an arrowhead appears in any semiconductor symbol it always represent a PN junction and it points from P to N. as a general rule for PN junctions current is permitted to pass from P to N but blocked if it tries to pass from N to P.