Parts

2N3906 PNP transistor: pinout, ratings and a high-side switch

The 2N3906 is E-B-C: with the flat face toward you and the leads down, the emitter is on the left and the collector on the right. That is the same order as the 2N3904, its NPN partner, and the reverse of the BC557, the PNP often sold beside it.

It is rated 40 V and 200 mA. As a switch it goes on the positive side of the load: emitter to the supply, load from the collector to ground, base pulled low to turn it on. Both datasheets used here, Diotec and MCC, print the same voltage, current and gain limits.

Datasheet: Diotec Semiconductor 2N3906 General Purpose PNP Transistors (PDF)Ask Datasheet Guru about the 2N3906

Pinout by package

2N3906 TO-92 · Diotec1=E 2=B 3=C

Flat face toward you, leads pointing down: E, B, C from left to right.

The same order as the 2N3904, its NPN partner, and the reverse of the BC557 (C-B-E).

Source: Diotec Semiconductor 2N3906 General Purpose PNP Transistors, p. 1 (TO-92 drawing with leads 1, 2, 3 and PNP symbol: E (1), B (2), C (3))

2N3906 TO-92 · MCC1=E 2=B 3=C

Flat face toward you, leads pointing down: pins 1, 2, 3 from left to right.

Source: Micro Commercial Components 2N3906 PNP General Purpose Amplifier (TO-92), p. 1 (TO-92 outline numbered 1 2 3; 1 emitter, 2 base, 3 collector)

Which one do you have?

Read the print on the flat face: 2N3906, sometimes with a maker's logo and a date code. Unlike the BC557 there are no A, B or C gain groups; every 2N3906 meets one gain table. The SOT-23 version is the MMBT3906. All of them are PNP, so a meter on diode test reads the two junctions with the black probe, not the red, on the base.

Absolute maximum ratings

Both makers rate the 2N3906 for 40 V collector to emitter, 5 V emitter to base and 200 mA. Diotec allows 625 mW, MCC 600 mW; design to 600 mW, and less in a closed box. A PNP datasheet prints every current and voltage with a minus sign, because they flow and point the other way from an NPN's. The tables here give magnitudes.

Absolute maximum ratings at 25 °C, from each maker's datasheet
Parameter	2N3906TO-92	2N3906TO-92
Collector-emitter voltage, base open VCEO	40 V	40 V
Collector-base voltage VCBO	40 V	40 V
Emitter-base voltage VEBO	5 V	5 V
Collector current, continuous IC	200 mA	200 mA
Power dissipation at 25 °C ambient Ptot	625 mW (leads at ambient 2 mm from the body)	600 mW
Thermal resistance, junction to air RθJA	under 200 K/W (3 mm² copper pads)	208 °C/W
Junction temperature TJ	−55 to 150 °C	−55 to 150 °C

Gain and saturation

Limits at 25 °C (min/max unless marked typical)
Parameter	2N3906TO-92	2N3906TO-92
DC current gain hFEIC = 0.1 mA, VCE = 1 V	60 min	60 min
DC current gain hFEIC = 1 mA, VCE = 1 V	80 min	80 min
DC current gain hFEIC = 10 mA, VCE = 1 V	100 to 300	100 to 300
DC current gain hFEIC = 50 mA, VCE = 1 V	60 min	60 min
DC current gain hFEIC = 100 mA, VCE = 1 V	30 min	30 min
Saturation voltage VCE(sat)IC = 10 mA, IB = 1 mA	0.25 V max	0.25 V max
Saturation voltage VCE(sat)IC = 50 mA, IB = 5 mA	0.4 V max	0.4 V max
Base-emitter voltage in saturation VBE(sat)IC = 10 mA, IB = 1 mA	0.65 to 0.85 V	0.65 to 0.85 V
Base-emitter voltage in saturation VBE(sat)IC = 50 mA, IB = 5 mA	0.95 V max	0.95 V max
Transition frequency fTIC = 10 mA, VCE = 20 V, f = 100 MHz	250 MHz min	250 MHz min
Switching: storage and fall time ts, tfVCC = 3 V, IC = 10 mA, IB1 = IB2 = 1 mA	225 ns and 75 ns max	225 ns and 75 ns max
Noise figure FIC = 100 µA, VCE = 5 V, RG = 1 kΩ, 1 kHz	4 dB max	not given

The gain is guaranteed at five currents. It is best, 100 to 300, around 10 mA, and falls to 60 at 50 mA and 30 at 100 mA. At small currents the 2N3906 is guaranteed a little more gain than the 2N3904: 60 against 40 at 0.1 mA.

Figure 1. The 2N3906 and its NPN partner share the top of the curve. Below 10 mA the PNP is guaranteed a little more gain.

For switching, both makers give saturation at a base current of one tenth of the collector current, up to 50 mA: at most 0.4 V across the transistor and 0.95 V from base to emitter. Above 50 mA no maker guarantees anything about saturation, even though the part is rated for 200 mA.

Figure 2. The 2N3906 is rated for four times the current its saturation is specified at. Size a switch by the blue bar.

Worked example: switching 50 mA on the high side

A 50 mA load on a 5 V rail, switched from a 5 V microcontroller pin: emitter to +5 V, load from the collector to ground, a resistor from the pin to the base. The pin goes low to turn the load on and high to turn it off. 50 mA is the top of the saturation table, so every limit below is printed in the datasheet.

1. Pick the base current. 50 mA / 10 = 5 mA, the 1:10 ratio the saturation figures were measured at.

2. Take the worst-case base-emitter voltage. V_BE(sat) is 0.95 V at most. With the pin at 0 V, R ≤ (5 V − 0.95 V) / 5 mA = 810 Ω.

3. Round down to a standard value. The next E12 value is 680 Ω, giving 5.96 to 6.40 mA of base current (the upper figure assumes V_BE as low as 0.65 V). Check the pin can source that much.

4. Check the heat. At most 50 mA × 0.4 V plus 6.4 mA × 0.95 V = 26 mW, about 5.4 °C above the air at 208 °C/W.

Figure 3. The high-side switch, drawn as ngspice checked it. With the pin at 0 V its base current lands inside the worked range of 5.96 to 6.40 mA, and the load gets more than 46 mA of its 50 mA.

On a 3.3 V board with a 3.3 V rail the same sum gives R ≤ 470 Ω, so 470 Ω and 5.00 to 5.64 mA of base current.

Figure 4. Base resistor and base current for the 50 mA load, with the pin and the rail at the same voltage.

Worked example in numbers

Base resistor: 680 Ω
Base current: 5.96 to 6.40 mA
Transistor heat: 26 mW
Junction rise: 5.4 °C

Why a 3.3 V pin cannot turn it off on 5 V

A PNP switch is off only when its base is pulled up to the emitter. Wire the example to an ESP32 or any other 3.3 V chip while the load stays on 5 V, and the pin's "high" is 1.7 V short of the rail. That 1.7 V still sits across the 680 Ω and the emitter-base junction.

With the junction at 0.65 to 0.85 V, 1.25 to 1.54 mA keeps flowing into the base, a quarter to a third of the drive the switch was sized for. With a gain of 60 or more that is still enough for the whole load. The ngspice run behind Figure 3 shows it: with the pin at 3.3 V the load current is within 1 % of the current with the pin at 0 V.

Figure 5. "Off" from a 3.3 V pin on a 5 V rail leaves 1.7 V across the base path, so the transistor never turns off.

The usual fix is a second transistor: a 2N3904 pulls the 2N3906's base low through the same 680 Ω, and a resistor from base to emitter holds the 2N3906 off when the 2N3904 lets go. The 3.3 V pin drives the 2N3904's base through its own resistor and never connects to the 5 V side. Our 3.3 V to 5 V level shifter guide covers the same mismatch for logic signals.

What it is good for

Switching the positive side of a load up to 50 mA, where saturation is still guaranteed, with the load's negative side on ground.
The PNP half of a complementary pair with a 2N3904: push-pull drivers, current mirrors and the two-transistor level shifter above.
Fast small-signal switching: storage time is 225 ns and fall time 75 ns at most at 10 mA.

Tips and checks

Before soldering a 2N3906 where a BC557 was, or the other way round, check the pin order on the board: one is E-B-C, the other C-B-E.
Put a resistor from base to emitter (10 kΩ is common) so the switch stays off while the microcontroller is starting up and its pins float.
Measure a suspect switch from the rail, not from ground: on, the emitter-collector drop should be a few hundred millivolts at most.
On a meter's hFE socket, use the PNP side. In the NPN side a good 2N3906 reads near zero and looks dead.
For loads above 50 mA, drive harder than 1:10 or pick a larger transistor; the datasheets say nothing about saturation up there.

Substitutes and the NPN partner

Pin order read with the flat face toward you, leads down, left to right
Part	Pins	Verdict
2N3904 (NPN partner)	E-B-C	The NPN complement Diotec names on the 2N3906 sheet. Same pins, 40 V and 200 mA, opposite polarity, so it fits only where the whole circuit is mirrored.Diotec Semiconductor 2N3906 General Purpose PNP Transistors, p. 1
BC557	C-B-E	Works electrically in a 5 V or 12 V circuit up to 100 mA, but its pins run the other way. Turn it round so the flat face points the other way, and keep the load under half of 100 mA.Diotec Semiconductor BC556 ... BC559 General Purpose PNP Transistors, p. 1

Finding the pins with a multimeter

Set the meter to diode test. Put the black probe on each lead in turn until you find the one that conducts to both others, about 0.6 to 0.7 V each. That is the base of a PNP; on a 2N3906 it is the middle lead.
Reverse the probes: both junctions should read open, and the outer leads open both ways.
With the flat face toward you, the emitter is the left lead and the collector the right.

The transistor testing lesson goes through it step by step, and continuity and diode test explains what the meter is doing.

What we'd do

Use a 2N3906 for high-side loads up to 50 mA when the pin and the load share one supply, with 680 Ω for 1:10 drive at 5 V. On a 3.3 V microcontroller switching a 5 V load, put a 2N3904 in front of it. Keep a few 2N3906s and 2N3904s together in one drawer, apart from the BC557s, so the pin orders never get mixed.

Questions people ask

What is the 2N3906 pinout?

Emitter, base, collector from left to right, with the flat face toward you and the leads pointing down. It is the same as the 2N3904.

Is the 2N3906 NPN or PNP?

PNP. Its NPN partner is the 2N3904, which Diotec names as the complementary part.

How much current can a 2N3906 handle?

200 mA continuous, and 600 to 625 mW of dissipation at 25 °C. Saturation is specified only up to 50 mA, so keep a switch there unless you drive the base harder.

Can I use a BC557 instead of a 2N3906?

Up to about 50 mA, yes, but turn it round: the BC557 is C-B-E and the 2N3906 is E-B-C. The BC557 is rated for 100 mA, half the 2N3906's current.

How do you turn a 2N3906 on and off?

Pull the base low through a resistor to turn it on; bring the base up to the emitter's voltage to turn it off. If the driving pin cannot reach the emitter's supply, it cannot turn the transistor off.

Buying 2N3906 transistors in India

The 2N3906 is sold loose, often next to the 2N3904 and the BC557, so check the print before you fit it. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.

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Used in these lessons

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Datasheets this page was checked against

Diotec Semiconductor, 2N3906 General Purpose PNP Transistors (Version 2026-03-30)
Micro Commercial Components, 2N3906 PNP General Purpose Amplifier (TO-92) (Rev.3-2-12012020)
Diotec Semiconductor, BC556 ... BC559 General Purpose PNP Transistors (Version 2026-03-30)

Makers revise datasheets. If the copy you download has a newer revision and a number here disagrees with it, the datasheet wins, and we would like to hear about it.