A Darlington transistor (or Darlington pair) is a compound semiconductor structure consisting of two bipolar junction transistors (BJTs) connected in cascade. The emitter of the first transistor feeds directly into the base of the second, while their collectors are tied together. This configuration multiplies their individual current gains ($h_{FE}$), resulting in a single device with massive current amplification—often exceeding 1,000 to 10,000. You can drive high-current loads like motors and solenoids directly from low-current microcontroller GPIO pins without needing intermediate driver stages.

However, this extreme gain comes with strict tradeoffs: a high base-emitter turn-on voltage (typically 1.4V to 2.0V), a high collector-emitter saturation voltage ($V_{CE(sat)}$ usually between 0.8V and 1.5V), and significantly slower switching speeds compared to single BJTs or MOSFETs. If you are designing a switching circuit, you must account for the heat generated by that high saturation voltage. Below is a practical, bench-tested guide to selecting, biasing, and testing Darlington transistors.

The Data-Dense Spec Sheet: Safe Default Darlington Part Numbers

When sourcing components for a prototype or production run, stick to industry-standard, second-sourced part numbers. The TIP-series (originally Texas Instruments, now widely manufactured by ON Semi and STMicroelectronics) remains the undisputed safe default for discrete power Darlingtons. For low-power logic interfacing, Darlington arrays are the standard.

Part Number Polarity Max $V_{CEO}$ Max $I_C$ (Cont.) Min $h_{FE}$ (Gain) Package
TIP120 NPN 60V 5A 1000 (at 3A) TO-220
TIP122 NPN 100V 5A 1000 (at 3A) TO-220
TIP125 PNP 60V 5A 1000 (at 3A) TO-220
TIP142 NPN 100V 10A 1000 (at 5A) TO-3P
MJ11015 PNP 60V 30A 500 (at 10A) TO-204 (TO-3)
Bench Note on Internal Resistors: Almost all modern power Darlingtons (like the TIP120 series) include integrated base-emitter bleed resistors (typically 8k$\Omega$ for Q1 and 120$\Omega$ for Q2). These are built into the silicon die to shunt leakage current away from the base at high temperatures, preventing thermal runaway. You do not need to add external pull-down resistors for basic switching, though adding a 10k$\Omega$ external pull-down on the main base pin is still good practice for noisy environments.

Symbol, Pinout, and Operation Regions

On a schematic, the Darlington symbol looks like a single transistor with an extra internal loop, or explicitly as two BJTs with Q1's emitter tied to Q2's base and both collectors joined. The three external terminals remain Base (B), Collector (C), and Emitter (E). For the standard TO-220 package (TIP120/122/125), holding the device with the metal tab facing away from you and the pins pointing down, the pinout from left to right is: Pin 1 = Base, Pin 2 = Collector, Pin 3 = Emitter. Note that the metal tab is internally connected to the Collector.

Because there are two base-emitter junctions in series, the voltage requirements to push the Darlington through its operating regions are roughly double that of a standard BJT.

Operation Region Base-Emitter Voltage ($V_{BE}$) Collector-Emitter Voltage ($V_{CE}$) Behavior & Current Flow
Cutoff < 1.2V $\approx V_{CC}$ (Supply) Both transistors are off. $I_C$ is near zero (only minor leakage).
Active (Linear) 1.2V to 1.4V > 1.5V Device acts as a current amplifier. $I_C = h_{FE} \times I_B$. High heat dissipation.
Saturation 1.5V to 2.5V 0.8V to 1.5V ($V_{CE(sat)}$) Fully turned ON. $I_C$ is limited by the external load, not the transistor gain.

How to Bias and Select a Darlington for Your Circuit

Selecting and biasing a Darlington requires calculating the base resistor to ensure the device reaches hard saturation without exceeding the current limits of your driving source (like a microcontroller). Let's walk through a complete application circuit.

Application Circuit: Driving a 12V, 1A DC Motor from a 5V ESP32/Arduino

The Scenario: You need to switch a 12V DC motor that draws 1A under normal load using a 5V GPIO pin from an ESP32 or Arduino. The ESP32 GPIO can safely source up to 12mA (absolute max is higher, but 12mA is the recommended continuous limit for stable 3.3V/5V logic levels depending on the board variant).

Component Selection & Values:

  • Transistor: TIP120 (NPN, 60V, 5A). Safe margin for a 1A load.
  • Base Resistor ($R_B$): The TIP120 datasheet specifies a minimum $h_{FE}$ of 1000 at 3A. At 1A, gain is even higher, but we will use a conservative $h_{FE}$ of 500 to guarantee saturation. Required base current: $I_B = I_C / h_{FE} = 1A / 500 = 2mA$. To ensure hard saturation, we'll overdrive it slightly to 5mA.
    Calculation: $R_B = (V_{GPIO} - V_{BE(sat)}) / I_B$. Assuming $V_{GPIO} = 5V$ and $V_{BE(sat)} \approx 1.5V$.
    $R_B = (5V - 1.5V) / 0.005A = 700\Omega$.
    Selected Value: 680$\Omega$ (standard E12 value) or 470$\Omega$ for an even firmer 7.4mA drive.
  • Flyback Diode: 1N4007 or 1N5819 (Schottky) placed in reverse bias across the motor terminals (cathode to 12V, anode to Collector). Mandatory to absorb inductive kickback.
  • Base Pull-down Resistor: 10k$\Omega$ from Base to Ground. Prevents the motor from twitching if the microcontroller pin floats during boot-up.
Thermal Warning: In saturation, the TIP120 will drop about $V_{CE(sat)} = 1.0V$ at 1A. Power dissipation is $P = V \times I = 1.0V \times 1A = 1.0W$. A bare TO-220 package can dissipate roughly 1.5W to 2.0W in free air at 25°C ambient, but the case temperature will exceed 80°C. For continuous duty, mount the TIP120 to a small extruded aluminum heatsink (e.g., 15x15x10mm) using thermal paste.

Failure Modes and Multimeter Testing

Darlington transistors generally fail in three ways: thermal runaway (if the heatsink is undersized and leakage current multiplies with heat), secondary breakdown (exceeding the Safe Operating Area (SOA) when switching high voltage and high current simultaneously), and inductive spike puncture (when a flyback diode is omitted, causing $V_{CE}$ to exceed the breakdown voltage, punching through the silicon die).

You can verify the health of a suspected Darlington using a standard digital multimeter (DMM) in Diode Test Mode. Because of the dual junctions and internal resistors, the readings differ from a standard BJT.

Numbered Steps for DMM Testing (NPN Darlington like TIP120)

  1. Identify the Pins: Confirm Base (1), Collector (2), and Emitter (3).
  2. Test Base to Emitter (Forward): Place the red probe on the Base and the black probe on the Emitter. You are measuring two diode drops in series. The DMM should read between 1.100V and 1.400V. (A standard BJT would read ~0.600V).
  3. Test Base to Emitter (Reverse): Swap probes (black on Base, red on Emitter). The meter should read OL (Overload/Open). If it reads near 0V, the base-emitter junction is shorted.
  4. Test Base to Collector (Forward): Red probe on Base, black on Collector. You are measuring only the first transistor's base-collector junction. The DMM should read a standard single diode drop: 0.500V to 0.700V.
  5. Test Base to Collector (Reverse): Swap probes. The meter should read OL.
  6. Test Collector to Emitter: Place probes across Collector and Emitter in both directions. Both readings must be OL. If you read a short (near 0.000V) or a low resistance in either direction, the device has suffered secondary breakdown or thermal melt and is dead.

Darlington Transistors vs. Logic-Level MOSFETs

While Darlington transistors are excellent for high-gain, low-frequency switching, the modern workbench heavily favors logic-level N-channel MOSFETs (like the IRLZ44N) for most power switching tasks. Understanding when to use which is critical for efficient circuit design. For a deeper look at the underlying semiconductor physics of cascaded pairs, refer to the Darlington Pair chapter in the All About Circuits textbook.

Criteria Darlington BJT (e.g., TIP120) Logic-Level MOSFET (e.g., IRLZ44N)
Drive Voltage Required High ($V_{BE} \approx 1.5V$ to turn on) Low ($V_{GS(th)} \approx 1.0V$ to 2.0V)
On-State Voltage Drop High ($V_{CE(sat)} \approx 0.8V$ to 1.5V) Negligible ($V_{DS} = I \times R_{DS(on)}$, often < 0.1V)
Switching Speed Slow (kHz range, limited by charge storage) Fast (MHz range, limited by gate capacitance)
Input Impedance Moderate (Draws continuous base current) Extremely High (Draws almost zero steady-state current)
Thermal Stability Prone to thermal runaway (requires heatsinking) Self-limiting at high temps (positive tempco of $R_{DS(on)}$)

When to Choose Which

Choose the Darlington Transistor when: You are building linear amplifiers (like audio output stages or linear voltage regulators) where the smooth, continuous current gain is necessary. They are also ideal when driving heavy loads from extremely weak, high-impedance current sources where the gate charge of a large MOSFET would cause unacceptable turn-on delays. For detailed electrical characteristics and Safe Operating Area (SOA) graphs, always consult the manufacturer's official TIP120 datasheet.

Choose the Logic-Level MOSFET when: You are building PWM motor controllers, switching power supplies, or any high-frequency digital switching circuit. The near-zero $R_{DS(on)}$ of a MOSFET eliminates the 1V+ saturation drop of a Darlington, drastically reducing heat and removing the need for bulky heatsinks in high-current applications.