Basic Guide To Transistor Selection

Step 2 – Electrical Characteristics

Once the transistor technology and general type have been chosen, you can start applying actual electrical requirements. This is the point where the datasheet stops becoming optional reading material and starts becoming a mandatory document. The goal here is not to find a "perfect" transistor, but to ensure the device you choose can survive electrically and behave predictably in your circuit.

Operating Voltage / Current

The first parameters to consider are the operating voltage and current. Every transistor has hard limits on how much voltage it can block and how much current it can conduct. For BJTs this appears as collector current and collector-emitter voltage ratings, and for FETs, it shows up as drain current and drain-source voltage. These numbers define what the device can tolerate without entering breakdown or overheating, and exceeding them, even briefly, will result in failure.

Control Requirements (Gate-Source Voltage / Base Current)

Just as equally important as the operating voltage and current are the control requirements, as a transistor that can handle the load is but cannot be driven correctly is useless. In a BJT, the base-emitter junction must be forward biased before the device conducts, typically around 0.6 to 0.7 V for silicon (as this region is a PN diode junction). More importantly, however, is that the base current must be sufficient to support the required collector current.

In FETs, conduction begins once the gate-source voltage exceeds the threshold voltage. Now, this threshold is often misunderstood, as it does not indicate full conduction, but only the point at which the conductive channel starts to form. To switch a load efficiently, the gate must be driven well beyond this minimum.

Transistor Gain

At this stage, you should have confirmed two things. Firstly, the transistor can switch the voltage and current present in the circuit, and secondly, the available control signal can reliably turn it on and off under all operating conditions. Many marginal designs fail not because the transistor is under-rated, but because it is under-driven.

Gain is the next parameter that deserves attention, and it is a common source of mistakes. If the transistor is being used as a simple on-off switch, the gain only needs to be high enough that the device enters full saturation (its conductive channel is at its lowest possible resistance). To ensure this, designers often add generous margins here, because gain varies significantly with temperature, current, and even identical device parts (for example, the gain of the 2N3904 can vary anywhere from 150 to 600).

However, if the transistor is used in its linear region for amplification, the gain becomes far more critical. A device with insufficient gain may never achieve the desired output swing, no matter how carefully it is biased. Worse still, transistor gain is not a tightly controlled parameter (due to the manufacturing process), meaning that any circuit relying on a specific gain value without feedback is extremely fragile by design. This is why practical amplifier circuits almost always use negative feedback to define behaviour, rather than trusting the transistor to behave consistently.

Power Dissipation

Power dissipation is the next major constraint, and ties directly into a transistors reliability. Every transistor has a maximum power rating, which represents how much heat it can safely dissipate. This limit is reached not only in high-current switching applications, but also in linear operation where voltage and current are present simultaneously. A transistor switching a large load may meet all voltage and current ratings, and yet, still fail simply because it could not get rid of the generated heat fast enough.

Power dissipation also interacts strongly with temperature. As the device heats up, many of its parameters shift, sometimes in ways that increase dissipation further. This feedback loop is how perfectly reasonable-looking designs end up destroying themselves under real-world conditions (these scenarios are often referred to as thermal runaway).

At this point, it should be clear why transistor datasheets are long and tedious; there are many interdependent electrical parameters, none of them existing in isolation. As designs become more demanding, additional characteristics start to matter, such as on-resistance, leakage currents, safe operating area, switching speed, and minimum drive requirements. While you do not need to optimize every parameter, you do need to know which ones matter for your specific application.

Electrical characteristics are where theoretical understanding meets practical reality. Getting this step right is the difference between a circuit that works on the bench and one that survives in the field.