Meter check of a transistor
Bipolar transistors are constructed of a
three-layer semiconductor "sandwich," either PNP or NPN. As such, they
register as two diodes connected back-to-back when tested with a
multimeter's "resistance" or "diode check" functions:
Here I'm assuming the use of a multimeter
with only a single continuity range (resistance) function to check the
PN junctions. Some multimeters are equipped with two separate continuity
check functions: resistance and "diode check," each with its own
purpose. If your meter has a designated "diode check" function, use that
rather than the "resistance" range, and the meter will display the
actual forward voltage of the PN junction and not just whether or not it
conducts current.
Meter readings will be exactly opposite,
of course, for an NPN transistor, with both PN junctions facing the
other way. If a multimeter with a "diode check" function is used in this
test, it will be found that the emitter-base junction possesses a
slightly greater forward voltage drop than the collector-base junction.
This forward voltage difference is due to the disparity in doping
concentration between the emitter and collector regions of the
transistor: the emitter is a much more heavily doped piece of
semiconductor material than the collector, causing its junction with the
base to produce a higher forward voltage drop.
Knowing this, it becomes possible to
determine which wire is which on an unmarked transistor. This is
important because transistor packaging, unfortunately, is not
standardized. All bipolar transistors have three wires, of course, but
the positions of the three wires on the actual physical package are not
arranged in any universal, standardized order.
Suppose a technician finds a bipolar
transistor and proceeds to measure continuity with a multimeter set in
the "diode check" mode. Measuring between pairs of wires and recording
the values displayed by the meter, the technician obtains the following
data:
- Meter touching wire 1 (+) and 2 (-): "OL"
- Meter touching wire 1 (-) and 2 (+): "OL"
- Meter touching wire 1 (+) and 3 (-):
0.655 volts
- Meter touching wire 1 (-) and 3 (+): "OL"
- Meter touching wire 2 (+) and 3 (-):
0.621 volts
- Meter touching wire 2 (-) and 3 (+): "OL"
The only combinations of test points
giving conducting meter readings are wires 1 and 3 (red test lead on 1
and black test lead on 3), and wires 2 and 3 (red test lead on 2 and
black test lead on 3). These two readings must indicate forward
biasing of the emitter-to-base junction (0.655 volts) and the
collector-to-base junction (0.621 volts).
Now we look for the one wire common to
both sets of conductive readings. It must be the base connection of the
transistor, because the base is the only layer of the three-layer device
common to both sets of PN junctions (emitter-base and collector-base).
In this example, that wire is number 3, being common to both the 1-3 and
the 2-3 test point combinations. In both those sets of meter readings,
the black (-) meter test lead was touching wire 3, which tells us
that the base of this transistor is made of N-type semiconductor
material (black = negative). Thus, the transistor is an PNP type with
base on wire 3, emitter on wire 1 and collector on wire 2:
Please note that the base wire in this
example is not the middle lead of the transistor, as one might
expect from the three-layer "sandwich" model of a bipolar transistor.
This is quite often the case, and tends to confuse new students of
electronics. The only way to be sure which lead is which is by a meter
check, or by referencing the manufacturer's "data sheet" documentation
on that particular part number of transistor.
Knowing that a bipolar transistor behaves
as two back-to-back diodes when tested with a conductivity meter is
helpful for identifying an unknown transistor purely by meter readings.
It is also helpful for a quick functional check of the transistor. If
the technician were to measure continuity in any more than two or any
less than two of the six test lead combinations, he or she would
immediately know that the transistor was defective (or else that it
wasn't a bipolar transistor but rather something else -- a distinct
possibility if no part numbers can be referenced for sure
identification!). However, the "two diode" model of the transistor fails
to explain how or why it acts as an amplifying device.
To better illustrate this paradox, let's
examine one of the transistor switch circuits using the physical diagram
rather than the schematic symbol to represent the transistor. This way
the two PN junctions will be easier to see:
A grey-colored diagonal arrow shows the
direction of electron flow through the emitter-base junction. This part
makes sense, since the electrons are flowing from the N-type emitter to
the P-type base: the junction is obviously forward-biased. However, the
base-collector junction is another matter entirely. Notice how the
grey-colored thick arrow is pointing in the direction of electron flow
(upwards) from base to collector. With the base made of P-type material
and the collector of N-type material, this direction of electron flow is
clearly backwards to the direction normally associated with a PN
junction! A normal PN junction wouldn't permit this "backward" direction
of flow, at least not without offering significant opposition. However,
when the transistor is saturated, there is very little opposition to
electrons all the way from emitter to collector, as evidenced by the
lamp's illumination!
Clearly then, something is going on here
that defies the simple "two-diode" explanatory model of the bipolar
transistor. When I was first learning about transistor operation, I
tried to construct my own transistor from two back-to-back diodes, like
this:
My circuit didn't work, and I was
mystified. However useful the "two diode" description of a transistor
might be for testing purposes, it doesn't explain how a transistor can
behave as a controlled switch.
What happens in a transistor is this: the
reverse bias of the base-collector junction prevents collector current
when the transistor is in cutoff mode (that is, when there is no base
current). However, when the base-emitter junction is forward biased by
the controlling signal, the normally-blocking action of the
base-collector junction is overridden and current is permitted through
the collector, despite the fact that electrons are going the "wrong way"
through that PN junction. This action is dependent on the quantum
physics of semiconductor junctions, and can only take place when the two
junctions are properly spaced and the doping concentrations of the three
layers are properly proportioned. Two diodes wired in series fail to
meet these criteria, and so the top diode can never "turn on" when it is
reversed biased, no matter how much current goes through the bottom
diode in the base wire loop.
That doping concentrations play a crucial
part in the special abilities of the transistor is further evidenced by
the fact that collector and emitter are not interchangeable. If the
transistor is merely viewed as two back-to-back PN junctions, or merely
as a plain N-P-N or P-N-P sandwich of materials, it may seem as though
either end of the transistor could serve as collector or emitter. This,
however, is not true. If connected "backwards" in a circuit, a
base-collector current will fail to control current between collector
and emitter. Despite the fact that both the emitter and collector layers
of a bipolar transistor are of the same doping type (either N or
P), they are definitely not identical!
So, current through the emitter-base
junction allows current through the reverse-biased base-collector
junction. The action of base current can be thought of as "opening a
gate" for current through the collector. More specifically, any given
amount of emitter-to-base current permits a limited amount of
base-to-collector current. For every electron that passes through the
emitter-base junction and on through the base wire, there is allowed a
certain, restricted number of electrons to pass through the
base-collector junction and no more.
In the next section, this
current-limiting behavior of the transistor will be investigated in more
detail.
- REVIEW:
- Tested with a multimeter in the
"resistance" or "diode check" modes, a transistor behaves like two
back-to-back PN (diode) junctions.
- The emitter-base PN junction has a
slightly greater forward voltage drop than the collector-base PN
junction, due to more concentrated doping of the emitter semiconductor
layer.
- The reverse-biased base-collector
junction normally blocks any current from going through the transistor
between emitter and collector. However, that junction begins to
conduct if current is drawn through the base wire. Base current can be
thought of as "opening a gate" for a certain, limited amount of
current through the collector.
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