The Transistor: NPN Structure and Switching

CSC-114 · Semester I · Introduction to Information Technology

beginner20 minAvailable

Bipolar junction transistorNPN · switch & amplifier

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Lab brief

Objective

Inspect the three-dimensional anatomy of a bipolar junction transistor, then drive it as a switch through cut-off, active and saturation by sweeping the base supply voltage and reading the base and collector currents.

Theory & concepts

A bipolar junction transistor (BJT) is a sandwich of three doped semiconductor layers: N-P-N for an NPN transistor. The middle layer, the base, is made very thin and lightly doped so that charge carriers can cross it.

Rendering diagram…

NPN transistor: layers, biasing and switching regions

When the base-emitter junction is forward biased past its 0.7 V knee, a small current IB flows into the base. Because the base is thin, almost all the injected electrons get pulled across into the collector, so the collector current equals beta times the base current: IC = beta x IB while the transistor stays active.

The transistor is a switch in three regions. In cut-off, IB = 0 and no collector current flows - off. In the active region, IC = beta x IB, so a small base signal controls a large collector current - amplification. In saturation, the load resistor caps IC and V_CE collapses to about 0.2 V - fully on.

The two-bit engine

Every digital gate in a CPU is built from transistors used as switches that slam between cut-off and saturation. Billions of these on/off states per second are how a computer computes.

Prerequisites

Lesson: Evolution of Computers: The Generations
A basic idea that a transistor is an electronic switch

Instructions

1Rotate the model to view the transistor from every side and find the flat face, the epoxy body and the three leads.
2Identify the leads by their coloured labels: E (emitter), B (base) and C (collector).
3Switch to the NPN cutaway to see the three doped layers and the depletion regions between them.
4Watch the blue electrons move from the emitter through the thin base into the collector while red holes drift in the base.
5Move to Switching mode and sweep Vbb from 0 V to 5 V using the slider.
6Find the 0.7 V knee, then the saturation point where IC stops rising even though Vbb keeps climbing.

Tasks

Find the three terminals
Rotate the model and point out emitter, base and collector on the real body.
Read the cutaway
In the cutaway, name the doping type of each of the three regions.
Walk the operating regions
Sweep Vbb and record the approximate Vbb values where cut-off, active and saturation begin.
Verify beta
Pick a Vbb in the active region, divide the collector current by the base current, and confirm it equals beta (100).

Self-check

The student can label emitter, base and collector on the model.
The student can state the layer order and doping of an NPN transistor.
The student can explain the transistor as both an amplifier and a switch.
The student can identify the three operating regions and the 0.7 V knee from the readouts.

Environment

interactive 3D transistorNPN cutaway with moving carriersbase voltage sliderIB, IC, beta and V_CE readoutscut-off / active / saturation indicator