Bipolar Junction Transistors - A Challenger for MOSFETs

03/24/2021 Know-How

Digital switches are usually created using MOSFETs, but bipolar junction transistors have become an alternative to be taken seriously when it comes to models with low saturation voltages. For applications with low voltages and currents, they provide not only superior current amplification but also cost benefits.

In load switch applications, the transistor needs to amplify the base current precisely enough for the output voltage to be close to zero or so that only the saturation voltage of the transistor is measurable. MOSFETs are usually used for this purpose because they do not require any underlying controller as voltage-controlled components. On the other hand, bipolar junction transistors (BJTs) are current-controlled components that require an underlying controller capable of continuously carrying current.

Bipolar junction transistors with a much higher current gain (hFE) and much lower saturation voltage (VCEsat) can get by on a much lower base current though. Their higher current gain reduces the base current enough to allow it to be directly switched by the microcontroller. For example, if a transistor needs to conduct a 1A current and has an hFE of 100, the base current needs to be at least 10mA to ensure that the transistor is saturated. If the transistor provides a current gain of 500, 2mA is sufficient.

It also significantly reduces the losses via the base bias resistor and the base-emitter voltage (VBE). If the transistor is operated as a low-frequency switch, the low saturation voltage drops reduce the collector-emitter power dissipation and enable higher collector currents (IC) on a standardized chip surface.

For a fully-on state, the low-VCEsat BJTs therefore require a low base-emitter voltage of just 0.3 to 0.9V, making them ideal for low-voltage switching applications. This control voltage applies across the entire temperature range.

If bipolar junction transistors are used as saturated switches, they can also affect the conductivity of the collector region, thus reducing the collector-emitter resistance considerably while saturated (RCE(sat)). MOSFETs do not offer this conductivity. However, this does increase the reverse recovery time of the base, which means longer switching cycles.

Due to their transit frequency, these transistors can only be used for applications involving up to several hundred kHz. Dividing the transit frequency by the current gain factor produces the cut-off frequency. This is defined as the threshold at which the current gain falls to -3dB (which is 0.707). It is important to maintain some distance from this cut-off frequency.

Longer Service Life for Mobile Applications

Due to their high gain performance, low-VCEsat BJTs are also more efficient than conventional BJTs and MOSFETs, so when combined with a base resistor they can replace a MOSFET and a Schottky diode. This provides benefits in the form of longer battery charge life and reduced component costs, especially with mobile and/or battery-operated applications such as electric toothbrushes, shavers or hand mixers. Bipolar junction transistors are also much less sensitive to ESD (electrostatic discharge) compared to MOSFETs with an ESD tolerance of over 8000V, and also offer internal protection against voltage spikes.

The gain of the transistors increases even further with higher temperatures. At the same time, the share of the base-emitter voltage relative to the forward voltage (UBE(sat)) present at the maximum permitted base current is reduced. As a result, the collector-emitter resistance at saturation (RCE(sat)) is lower for BJTs than the on resistance (RDS(on)) of a comparable MOSFET. BJTs also generate less heat at higher current densities and/or under continuous current than MOSFETs with the same chip surface area.

Also, the saturation voltage remains proportional to power loss at a given load current. Low-VCEsat BJTs therefore offer lower power loss, which in turn results in a reduced need for heatsinking. Considering the total power loss, however, it is also important to consider the losses incurred in controlling the base. When using low­VCEsat BJTs with higher gain, these too are lower.

Another advantage of bipolar junction transistors is that they can block in both directions, eliminating the need for an additional anti-parallel MOSFET. They are also cheaper, thus offering a significant cost advantage over MOSFETs.

High Switching Performance

BJTs can provide switching performance many times higher than their maximum permitted power loss, because a transistor operating as a switch has two stationary operating points. If enough base current flows into the first, this results in a collector current that closes the switch, across which there is only a residual voltage drop. As the base current at the second operating point is therefore zero, the transistor at which the full operating voltage is present serves as a block. The transition between the two operating points is very fast. This allows the load line to be positioned such that it cuts the power loss hyperbola when the transition from conducting to blocked transistor and vice versa is fast enough and does not occur too frequently. The stationary operating points only need to be located below the hyperbola.

Because BJTs enable very fast switching in the linear range and offer a high pulsed current with a high current density, they are suitable for use as drivers for controlling MOSFETs. This allows for reduced dimensions and lower costs compared to specialized IC driver solutions.

Small Components, Great Performance

Low-VCEsat BJTs are typically available with a maximum collector-emitter voltage (VCEO) of 12 to 100V and collector currents of up to several ampere in SOT packages. The smallest bipolar junction transistors in the world are currently delivered in DFN0606-3 ultra-small packages from Diodes. With a footprint of 0.36mm2 and a height of just 0.4mm, the 45V NPN small-signal bipolar junction transistor BC847BFZ is 40% smaller than comparable DFN1006, SOT883 and SOT1123 components and still offers greater performance than comparable transistors with much larger form factors, because its leadless package allows higher power densities with a heat resistance of just 135°C/W. Diodes' models allow low-voltage applications to be switched with less than 1V, enabling mobile applications to be fully switched on with minimal power. With a collector current of 100mA and power loss of 925mW, they are especially well-suited to wearables such as smartwatches, health & fitness gadgets and other consumer devices such as smartphones and tablets. The corresponding PNP transistor is the BC857BZ (Figure 2).


For many circuit applications, BJTs with low saturation voltages are not only an adequate substitute for MOSFETS but also provide a number of advantages, with a low on resistance, working with a control voltage of less than 1V, offering excellent temperature stability and being non-sensitive to ESD. As they block current in both directions, they can make a second MOSFET superfluous. Their power loss and the resultant heat output are lower, as is their price.


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