The two primary transistor types you will use on the bench are the Bipolar Junction Transistor (BJT) and the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). Use BJTs (like the 2N3904) for low-current signal switching, logic level translation, and amplification under 500mA. Use MOSFETs (like the IRLZ44N) for high-current power switching, PWM motor control, and low-side load driving where minimal voltage drop is critical.

While both devices act as electrically controlled valves for current, their control mechanisms differ fundamentally. A BJT is a current-controlled device; a small base current dictates a much larger collector current. A MOSFET is a voltage-controlled device; the voltage applied to its insulated gate creates an electric field that opens a conductive channel, drawing virtually zero steady-state gate current.

The Core Transistor Types and Their Safe Default Part Numbers

Before diving into complex power semiconductors, you need a reliable inventory of basic discrete transistors. When you are prototyping on a breadboard, reaching for these 'safe default' part numbers will cover 95% of your hobbyist and bench needs.

Pinout and Symbol Identification

  • BJT (NPN/PNP): Features three pins: Emitter (E), Base (B), and Collector (C). In schematic symbols, look for the arrow on the emitter leg. For an NPN transistor, the arrow points outward (Not Pointing iN). For a PNP, it points inward.
  • MOSFET (N-Channel/P-Channel): Features three main pins: Source (S), Gate (G), and Drain (D). The symbol shows a distinct gap between the gate and the channel, representing the silicon dioxide insulating layer. N-channel symbols have an arrow on the source pointing inward toward the channel.
Bench Tip: Always verify pinouts with the specific manufacturer's datasheet. While the TO-92 package for BJTs is often Emitter-Base-Collector (EBC) when viewing the flat face, some Asian-market variants swap the Base and Emitter. Never assume pinout based solely on package shape.
Safe Default Transistor Part Numbers and Ratings
Part Number Type Package Max Vce / Vds Max Ic / Id Power / Rds(on) Best Use Case
2N3904 NPN BJT TO-92 40V 200mA 625mW Logic switching, small signal amp
2N3906 PNP BJT TO-92 40V 200mA 625mW High-side switching (complementary to 2N3904)
2N2222 NPN BJT TO-18 / TO-92 40V 800mA 500mW Medium current switching, LED drivers
IRLZ44N N-Ch MOSFET TO-220 55V 47A 22mΩ @ 5Vgs 5V logic-level PWM motor control
IRF520 N-Ch MOSFET TO-220 100V 9.2A 270mΩ @ 10Vgs 12V/24V high-side/low-side power switching

Operation Regions: Where the Magic (and Heat) Happens

Understanding transistor and types behavior requires knowing their operating regions. A common trap for beginners is confusing the term 'saturation' between BJTs and MOSFETs. In a BJT, saturation means the switch is fully ON. In a MOSFET, the 'saturation' region actually refers to the constant-current amplifier mode, while the 'ohmic' or 'linear' region is where the switch is fully ON.

Terminology Trap: If you want a MOSFET to act as a closed switch with minimal heat, you must drive it into the Ohmic/Linear region, not the Saturation region. Consult All About Circuits' MOSFET guide for a deep dive into the characteristic curves.
Transistor Operation Regions and Typical Parameters
Region Name Device Function Typical Voltages & Currents
Cutoff BJT / MOSFET Switch OFF (Open) Vbe < 0.6V (BJT); Vgs < Vth (MOSFET). Ic/Id ≈ 0A.
Active / Linear BJT Amplifier Vce > 0.2V. Ic = β × Ib. High heat dissipation.
Saturation BJT Switch ON (Closed) Vce(sat) ≈ 0.2V. Ic is limited only by the external load.
Ohmic / Linear MOSFET Switch ON (Closed) Vds < (Vgs - Vth). Ids determined by Rds(on). Low heat.
Saturation / Active MOSFET Constant Current Source Vds > (Vgs - Vth). Ids is constant regardless of Vds.

Biasing and Application: A Complete 12V Motor Driver Circuit

Let's apply this theory to a real-world circuit. Suppose you need to drive a 12V DC motor drawing 2A using a 5V PWM signal from an Arduino Uno. A BJT like the 2N2222 will overheat and fail at 2A. You need a logic-level MOSFET like the IRLZ44N, which has a low Rds(on) of 22mΩ when the gate is driven at 5V.

Circuit Component Values and Biasing

  1. Load: 12V DC Motor connected between the +12V supply and the MOSFET Drain.
  2. Flyback Diode: 1N4007 rectifier diode placed in parallel with the motor (cathode to +12V, anode to Drain). This clamps inductive kickback voltage spikes that would otherwise punch through the MOSFET's drain-source junction.
  3. Gate Resistor (Rg): 100Ω resistor in series between the Arduino PWM pin and the MOSFET Gate. This limits the instantaneous inrush current into the gate capacitance, protecting the microcontroller's GPIO pin.
  4. Pull-down Resistor (Rpd): 10kΩ resistor between the MOSFET Gate and Ground. This is critical; it ensures the gate is held at 0V during microcontroller boot-up when GPIO pins are high-impedance (floating), preventing the motor from spinning out of control.
Biasing Math: The IRLZ44N has a typical Gate Threshold Voltage (Vgs(th)) of 1V to 2V. However, Vgs(th) is merely the voltage where the device begins to conduct (usually at 250µA). To fully turn it on and achieve the rated 22mΩ Rds(on), you must apply the Vgs specified in the datasheet tables, which is 5V. A 5V Arduino output perfectly satisfies this biasing requirement.

For a foundational understanding of how base and gate biasing networks interact with load lines, refer to the BJT and biasing chapter on All About Circuits.

Failure Modes and Multimeter Diagnostics

Transistors rarely fail without a reason. BJTs typically fail due to thermal runaway (as they heat up, they conduct more, generating more heat) or secondary breakdown from exceeding the Safe Operating Area (SOA) with high simultaneous voltage and current. MOSFETs usually fail from gate oxide punch-through (exceeding the ±20V Vgs max rating, often via electrostatic discharge) or avalanche breakdown from inductive spikes exceeding the Vds rating.

When a transistor fails, it almost always fails shorted (all three pins read as a dead short) rather than open. Here is how to test them on your bench using a standard digital multimeter (DMM) in Diode Test mode.

Testing an NPN BJT (e.g., 2N3904)

  1. Set your DMM to Diode Test mode (the diode symbol).
  2. Place the Red probe on the Base and the Black probe on the Emitter. You should read a forward voltage drop between 0.600V and 0.750V.
  3. Move the Black probe to the Collector (Red still on Base). You should read a similar 0.600V to 0.750V drop.
  4. Reverse the probes (Black on Base, Red on Emitter, then Red on Collector). The meter should read 'OL' (Open Loop / Over Limit).
  5. Test Collector to Emitter in both directions. Both should read 'OL'. If any of these readings are 0.00V or shorted, the BJT is dead.

Testing an N-Channel MOSFET (e.g., IRLZ44N)

  1. Short all three pins together with a piece of wire or your finger to discharge any residual gate capacitance.
  2. Set DMM to Diode Test. Place Red on Source and Black on Drain. You should read the forward voltage of the internal body diode (typically 0.4V to 0.6V).
  3. Reverse probes (Black on Source, Red on Drain). It should read 'OL'.
  4. While keeping the Black probe on the Source, briefly touch the Red probe to the Gate to charge the gate capacitance with the DMM's internal ~3V battery.
  5. Move the Red probe back to the Drain. The reading should now drop to near 0.00V (or a very low resistance), indicating the channel has turned on.
  6. Short Gate to Source again to discharge. The Drain-Source reading should revert to 'OL'.

Frequently Asked Questions About Transistors

What is the difference between a transistor and a mechanical relay?

A mechanical relay uses an electromagnet to physically close metal contacts, providing total galvanic isolation between the control circuit and the load. Transistors are solid-state; they switch silently, infinitely faster (enabling PWM), and have no moving parts to wear out. However, standard transistors do not provide galvanic isolation, and they always exhibit a small voltage drop (Vce(sat) or Rds(on)) that generates heat, whereas closed relay contacts have near-zero resistance.

Can I swap a PNP transistor for an NPN in my existing PCB layout?

No. NPN and PNP transistors are complementary, but they require opposite circuit topologies. An NPN transistor is typically used for low-side switching (placed between the load and ground) and requires a positive base current to turn on. A PNP transistor is used for high-side switching (placed between the positive supply and the load) and requires the base voltage to be pulled lower than the emitter to turn on. Swapping them without redesigning the surrounding biasing network and load placement will result in a non-functional circuit or a shorted component.

Why did my MOSFET fail shorted even though the gate voltage was correct?

If your Vgs was within spec (e.g., 5V for a logic-level MOSFET) and the load current was below the rated Id, the most likely culprit is inductive kickback or operating in the linear region. If you are switching an inductive load (motor, solenoid, relay coil) without a flyback diode, the collapsing magnetic field generates a massive voltage spike that exceeds the MOSFET's Vds rating, causing avalanche breakdown. Alternatively, if you are using the MOSFET to linearly regulate current (acting as a variable resistor) rather than switching it fully ON or OFF, it is likely exceeding its Safe Operating Area (SOA) and suffering from localized thermal hot-spotting.

How do I calculate the exact base resistor value for a 2N2222?

To use a 2N2222 as a switch, you must drive it into deep saturation. First, determine your load current (Ic). Let's say Ic = 200mA. Assume a conservative forced Beta (β) of 10 to guarantee saturation (rather than the datasheet's linear β of 100+). Your required Base current (Ib) is Ic / 10 = 20mA. If driving from a 5V Arduino pin, and assuming a Base-Emitter voltage drop (Vbe) of 0.7V, the resistor voltage is 5V - 0.7V = 4.3V. Using Ohm's Law: R = V / I = 4.3V / 0.020A = 215Ω. The closest standard E12 value is 220Ω. Always verify that your microcontroller GPIO can safely source the calculated base current.