NPN Transistor: What is it? (Symbol & Working Principle)

What is a NPN Transistor

An NPN bipolar junction transistor has a thin P-type semiconductor base between emitter and collector regions made from N-type semiconductors. Its three terminals are emitter, base and collector. The NPN transistor contains two PN junctions diodes, but two separate back-to-back diodes cannot reproduce transistor action because they do not share a thin base.

The two PN junctions, sometimes drawn as back-to-back diodes to show polarity, are the base-emitter and base-collector junctions. That drawing is not a functional equivalent circuit.

The heavily doped emitter injects electrons into the base. The thin, lightly doped base lets most injected electrons diffuse to the collector-base depletion region, where the electric field sweeps them into the collector. Base-emitter voltage controls the injection rate.

The figure below shows the polarity of the two junctions. It is a junction analogy, not an equivalent model for gain or collector current.

Donor doping makes electrons the majority carriers in an N-type semiconductor. A potential difference creates an electric field that causes carrier drift; concentration gradients also cause diffusion. Conventional current points opposite to electron motion.

Acceptor doping makes holes the majority carriers in a P-type semiconductor. Electrons are still present as minority carriers. Hole motion contributes conventional current in the same direction as the holes move.

Construction of NPN Transistor

An NPN transistor has two junctions and three terminals, but its regions are intentionally asymmetric. The figure shows the basic construction.

The base is much thinner than the emitter and collector regions. The emitter is heavily doped so that electron injection from emitter to base dominates hole injection in the opposite direction.

The base is lightly doped and thin compared with the other regions. Most injected electrons diffuse across it before recombining, which gives the transistor current gain.

The collector is more lightly doped than the emitter and is designed to withstand reverse voltage. Its junction area is often larger so it can collect carriers and dissipate heat.

NPN Transistor Symbol

The arrowhead does not directly show Collector current (I_C) or Base current (I_B). It is on the emitter terminal and shows the conventional direction of Emitter current (I_E) in forward-active operation. The NPN arrow points outwards.

How Does an NPN Transistor Work

In forward-active operation, the base-emitter junction is in a forward bias condition and the base-collector junction is reverse biased. The labelled supply voltage V_EE establishes the emitter-side bias, while V_CC holds the collector above the base in this NPN example.

With the shown supplies, the negative terminal of V_EE connects towards the N-type emitter semiconductor. The positive terminal of V_CC connects towards the N-type collector. The junction voltages, rather than supply names alone, determine the operating region.

The forward-biased emitter-base depletion region narrows, while the reverse-biased base-collector depletion region widens. A depletion region has very few mobile carriers and contains an electric field created by fixed ionised dopants.

Forward bias injects majority-carrier electrons from the N-type emitter into the P-type base. The terminal current associated with both electron and hole components is the emitter current I_E.

Injected electrons diffuse through the thin, lightly doped P-type base. A small fraction recombines with holes, while most reach the base-collector depletion region.

Base current I_B supplies charge lost through recombination and other junction-current components. It is smaller than emitter current in normal forward-active operation, but there is no universal 2-5% ratio. The ratio depends on device current gain, collector current, temperature and collector-emitter voltage.

Most injected electrons reach the base-collector depletion region and are swept into the collector. Their terminal-current contribution is collector current I_C, which is normally much larger than base current in forward-active operation.

NPN Transistor Circuit

The circuit of the NPN transistor is as shown in the below figure.

The collector connects to the positive terminal of V_CC through load resistance R_L. The supply, load and transistor voltage together set the collector current.

The base connects to positive base-supply voltage V_B through resistance R_B, which limits base current. A practical design checks the device’s specified gain range, required collector current and power dissipation.

In forward-active operation, a smaller base current accompanies a larger collector current. In saturation, the load circuit limits collector current and the forward-active current-gain relationship no longer applies.

Kirchhoff’s current law gives emitter current as the sum of base and collector currents.

   

Operating Mode of Transistor

The bias across the base-emitter and base-collector junctions defines the main operating regions used in circuit analysis.

• Cut-off mode
• Saturation mode
• Active mode

Cut-off Mode

In cut-off mode, the base-emitter junction is not forward biased enough to inject appreciable carriers. The transistor approximates an open switch, although small leakage current remains. A grounded-emitter switch can be in cut-off with nearly zero base-emitter bias rather than both junctions reverse biased.

Saturation Mode

In saturation, both the base-emitter and base-collector junctions are forward biased. Collector current is then set mainly by the external circuit. The transistor approximates a closed switch but retains a nonzero collector-emitter saturation voltage.

Active Mode

In forward-active mode, the base-emitter junction is forward biased and the base-collector junction is reverse biased. The transistor can provide current gain for analogue amplification.

A simple model uses collector current equal to DC current gain times base current. The gain varies widely with device, collector current, temperature and collector-emitter voltage, so precision circuits use biasing and feedback rather than assume one fixed value.

NPN Transistor Switch

A basic low-frequency NPN switch uses saturation for the ON state and cut-off for the OFF state. High-speed switching circuits may avoid deep saturation because stored charge slows turn-off.

Sufficient base drive can forward bias both junctions and put the transistor in saturation. Collector-emitter voltage then falls to a device- and current-dependent saturation voltage, not exactly zero.

Collector current is approximately the supply voltage minus collector-emitter saturation voltage, divided by the collector resistance. The protected simplified equation below assumes that saturation voltage is zero.

   

When the base-emitter junction is not forward biased, the transistor approximates an open OFF switch. In a grounded-emitter circuit this often occurs when base voltage is zero; both junctions need not be reverse biased.

With negligible collector current, almost the full Vcc supply appears across collector and emitter. Real devices still have specified leakage current, and collector-emitter is not a single PN junction that can be described as reverse biased.

The figure shows the idealised cut-off condition for this grounded-emitter switch.

NPN Transistor Pinout

A transistor has collector, emitter and base leads, but their package positions are not universal. The middle lead is not always the base.

A dot, bevel or package mark can identify pin 1, not a universal collector position. Use the exact manufacturer part number, package drawing and pin table.

Lead spacing also does not identify the terminals reliably. Check the datasheet before connecting a BJT because devices in similar packages can use different pin assignments.

NPN vs PNP Transistor

NPN and PNP transistors use opposite carrier types and voltage polarities. In forward-active mode, both types have a forward-biased base-emitter junction and a reverse-biased base-collector junction; the polarities reverse between NPN and PNP. The protected legacy table below states the PNP junction biases incorrectly and its blanket faster-versus-slower comparison is not universal. Switching speed depends on the specific device, circuit and drive conditions.