Darlington Transistor vs. MOSFET: Selection Guidelines

Key Takeaways

• A Darlington transistor combines at least two BJTs to multiply the forward-operation current gain of the individual transistors.

• Darlington transistors are excellent for operating servos and other low-power off low-input currents but are inefficient and sometimes unstable.

• A MOSFET is a family of active devices most well-known for digital circuit applications due to low power consumption.

A Darlington transistor vs. MOSFET selection comes down to the power needs of the circuit.

Early transistors suffered from significant drift in the beta value (β), indicating quality and forward-operation current gain capabilities. Shortly after the first transistors’ development, Sidney Darlington realized he could greatly improve the stability of early semiconductor manufacturing results by tying together two (or more) transistors within a single package. This solution was a groundbreaking development at the onset of transistor manufacturing, but maturing manufacturing processes and new transistor technology have lessened the impact of Darlington’s contributions. Today, the dominant digital transistor is the metal-oxide-semiconductor field-effect transistor (MOSFET) due to its high level of performance and excellent miniaturization capabilities. Depending on circuit applications, the choice between Darlington transistor vs. MOSFET is not as cut-and-dry as selecting the latest and greatest component.

The Design and Performance of the Darlington Transistor

A Darlington transistor configuration combines multiple (most often two) bipolar junction transistors (BJTs) for a multi-stage amplification effect. By tying the emitter of the first transistor (for an NPN–note that PNP Darlington transistors are also a valid design) to the second transistor's base, the combined transistor network effectively acts as a single transistor. The performance improvement arises from the multiplicative effects of the β from the individual transistors: as the current gain of a single transistor already has a significant forward-operation current gain, the product of multiple transistors increases even further. Therefore, even a small base current can rapidly grow by three magnitudes or sometimes more. 

The Darlington transistor pair is readily implementable from discrete components or in a unified package format. In this sense, it offers a high amount of flexibility during circuit prototyping; if layout space is ample, designers can employ two lesser (and potentially cheaper) transistors in the place of a more robust IC or realize unparalleled forward-operating current gain values with two high-performing BJTs. A Darlington transistor also offers excellent impedance capabilities on both ends, with a high input impedance to minimize voltage drop (BJTs are, by nature, voltage-controlled devices) and a low output impedance to minimize power loss before reaching the load.

Of course, there’s no such thing as a free lunch: the improved current gain comes at the cost of a higher turn-on voltage at the combined base-emitter junction, doubling the individual voltage requirements on a single BJT. A Darlington transistor functions best at low frequencies, as the phase shift of the pair is greater than the sum of its parts. Despite its original formulation as a method to facilitate negative feedback, a Darlington transistor offers poor stability; in these cases, a common-emitter transistor pair provides a better high-frequency response. Additionally, a Darlington transistor increases the saturation voltage drop across the collector-emitter junction of the second transistor by a factor greater than four (for silicon-based semiconductors), which can become a significant source of power loss and cause a mismatch between low-output levels and driven TTL logic. Finally, the turn-off responsiveness decreases as the second transistor lags behind the shutoff of the first; a resistor across the base-emitter junction of the second transistor helps expedite this process by providing a low-impedance dissipation pathway.

Darlington Transistor vs. MOSFET: Capabilities and Applications

The most immediate difference between a MOSFET and a Darlington transistor is the power efficiency: MOSFETs offer minute power consumption, making them the premier transistor type for digital circuits (precisely, the combined MOSFET or CMOS variety). A CMOS uses an n-channel and p-channel MOSFET pairing to minimize the power losses during state switches. Unlike a Darlington transistor that is at least two or more daisy-chained BJTs, a MOSFET can exist singularly (although some components may combine billions of transistors for greater computational ability). 

In general, many differences boil down to the inherent advantages or disadvantages of BJTs and MOSFETs, typically with MOSFETs dominating digital circuitry design and BJT configurations having a slight edge in some high-frequency and analog applications. MOSFETs do not require an input current to control the load current. In contrast, a Darlington transistor can multiply a small input current to produce an appreciable load current for low-power applications. In this sense, there is some overlap between the functionality of the two components, as they both operate where the input current is significantly lower than the requirement for other active devices.

More explicitly, a Darlington transistor is an excellent candidate for push-pull outputs of audio amplifiers, including symmetrical push-pull circuits that utilize a pair of Darlington transistors to drive output from the positive and negative rail. This configuration uses a PNP Darlington transistor on the positive rail and an NPN transistor on the negative rail. Additionally, a Darlington transistor with its current gain capabilities is an excellent candidate for driving servomechanical devices off of limited current outputs (for example, those of a microcontroller). The MOSFET, meanwhile, finds further usage as an analog switch by passing signals in the on state and blocking conduction in the off state. Because MOSFETs are a large class of semiconductor devices, their applicability dwarfs that of the more specific Darlington transistor.

Cadence Powers Transistor Selection With Thorough Modeling

Darlington transistor vs. MOSFET has no definitive answer; circuit form follows function, and particular characteristics of a certain active device may be more or less suitable for the task. At some stage, designers must confirm performance through simulation before confirmation on a prototype board. Circuit modeling must provide a library of new and legacy components and numerous perturbation abilities to encompass many permutations fully. Cadence’s PCB Design and Analysis Software suite gives electronic development teams an all-encompassing ECAD environment that can quickly and easily translate simulation results into actionable design. Alongside OrCAD PCB Designer, PCB layout for simulation and DFM has never been simpler.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.