A transistor can be represented as two diodes, with a junction in the middle shown for both polarities in the figure above. This figure only shows an analogy for better understanding. Hence, connecting two discrete diodes in this manner as shown in the diagram will not make a transistor, because the point where they meet must be a common junction on the same piece of silicon (or germanium) - hence (in part) the term Bipolar Junction Transistor. The "bipolar" term means that transistors use "charge carriers" of both polarities - positive and negative, and minority and majority.
Since the base to collector junction is reverse biased in normal operation, there will be no current flow. It is the action of injecting current into the base that causes current flow in the collector circuit.
Testing transistor with a multimeter
If your multimeter 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.
If a multimeter with a "diode check" function is used in to test transistor, it will be found that the emitter-base junction has a slightly greater forward voltage drop than the collector-base junction. This forward voltage difference is due to the imbalance in doping concentration between the emitter and collector regions of the transistor. The emitter region is heavily doped compared to collector region, causing its junction with the base to produce a higher forward voltage drop.
- Meter touching wire a (+) and b (-): "OL"
- Meter touching wire a (-) and b (+): "OL"
- Meter touching wire a (+) and c (-): 0.65 volts
- Meter touching wire a (-) and c (+): "OL"
- Meter touching wire b (+) and c (-): 0.63 volts
- Meter touching wire b (-) and c (+): "OL"
The only combinations of test points giving conducting meter readings are wires a and c (red test lead on a and black test lead on c), and wires b and c (red test lead on b and black test lead on c). These two readings must indicate forward biasing of the emitter-to-base junction (0.65 volts) and the collector-to-base junction (0.63 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 c, being common to both the a-c and the b-c test point combinations. In both those sets of meter readings, the black (-) meter test lead was touching wire c, 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 c, emitter on wire a and collector on wire b.
Use a multimeter, battery, resistor and LED to check each pair of leads for conduction. Set digital multimeter at diode test point and an analogue multimeter at low resistance range.
Test each pair of leads both ways (therefore total tests = six):
- The base-emitter (BE) junction should conduct like a diode and current is made to flow in one direction only.
- The base-collector (BC) junction should behave like a diode and conduct in one direction only.
- The collector-emitter (CE) should not conduct either way.
The above diagram shows how the junctions behave in a NPN transistor.
In case of a PNP transistor same test procedure can be used.