# transistor question

J

#### John Popelish

Jan 1, 1970
0
Scott said:
I'm using this transistor:
http://www.onsemi.com/pub/Collateral/BD243B-D.PDF

I'm building this circuit:
http://tacashi.tripod.com/elctrncs/inverter/inverter.htm

When looking at a schematic I see a standard circle with the 3 lines.

What are the three (four) pins?

center line:
Line with arrow:
Line above arrow:

For that matter... What are each of these parts called?

Scott

Here is a picture of the symbols with the lead identity.
http://www.americanmicrosemi.com/tutorials/bipolartransistor.htm

R

#### Robert C Monsen

Jan 1, 1970
0
Scott said:
I'm using this transistor:
http://www.onsemi.com/pub/Collateral/BD243B-D.PDF

I'm building this circuit:
http://tacashi.tripod.com/elctrncs/inverter/inverter.htm

When looking at a schematic I see a standard circle with the 3 lines.

What are the three (four) pins?

center line:
Line with arrow:
Line above arrow:

For that matter... What are each of these parts called?

Scott

here are the rules for bipolar transistors:

1) The leg with the arrow is the emitter.
2) The leg that goes perpendicular to the line is the base
3) The other leg is the collector (the one with no arrow)
4) If the arrow points into the line, its PNP, otherwise, its NPN.

This arrow thing is also used in the diagrams for MOSFET and JFET devices.
If the arrow on the gate is pointing towards the line, its an N-CHANNEL
device. Otherwise, its a P-CHANNEL device.

Here is the reason why this is the case:

Bipolar transistors are really diodes with an extra hunk of silicon added.
The symbol for a diode is an arrow going from positive to negative (the P-N
junction)

These arrows in transistors indicate where and which way the diode is
facing. So, for an NPN transistor, the arrow is pointing out of the
transistor. For these transistors, the arrow points from the base (which is
P type silicon) to the emitter (which is N type silicon). A diode is a PN
junction, so its really just the diode which is depicted by the little
arrow. The collector is the line which doesn't have an arrow. Its not really
used as a diode in the transistor; its 'reverse biased', and only when you
put current through the actual diode (the base to collector) does current
flow through the collector.

For a PNP transistor, its really the same, except that the diode is now from
the emitter (which is P type silicon) to the base (which is N type silicon).
A diode is always a P to N junction. Thus, the arrow points from the emitter
to the base, into the transistor. Again, the collector is not a diode in the
traditional sense, and thus doesn't have an arrow. Again, the collector is
'reverse biased', an NP junction, which requires current flowing through the
actual diode (the emitter-base junction) to cause current to flow across it.

Hope this lessens some confusion.

Regards,
Bob Monsen

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