Transistors & MOSFETs

Transistors let a small signal control a larger current. de:volt ships four editable generic devices: the NPN BJT, the PNP BJT, the N-Channel MOSFET, and the P-Channel MOSFET. They are compact teaching models, not guaranteed vendor fits for a particular 2N2222, 2N2907, 2N7000, or BS250.

BJT vs. MOSFET

The two families control current in fundamentally different ways, and that difference decides how you drive them:

• BJTs are current-controlled. A small base current sets a much larger collector current (roughly β times bigger). To turn one on you push current into the base, which is why BJT switches always want a series base resistor.
• MOSFETs are voltage-controlled. A gate voltage opens or closes the channel. The model omits DC gate leakage, but its Cgs and Cgd capacitances draw displacement current during transitions. A floating gate therefore retains charge and needs an explicit pull-up or pull-down when you want a defined idle state.

de:volt models BJTs with an Ebers-Moll large-signal model and MOSFETs with a Shichman-Hodges level 1 model. Authored Inspector parameters feed those models directly. Exact catalog identity supplies the representative defaults listed below; a legacy or mismatched identity uses a labelled fallback instead of guessing a device.

NPN BJT

A 2N2222-class small-signal NPN: the default low-side switch. Conducts collector-to-emitter when you push current into the base; the base-emitter junction sits at ~0.7 V (Vbe) when on, and a saturated NPN drops only ~0.2 V (Vce) across collector-emitter.

Pinout

Specs

The Ebers-Moll params (Is, betaF, betaR, nF, nR) are editable in the Inspector. earlyVoltage adds a fixed forward Early-effect slope; set it to 0 to disable that effect. Ambient temperature changes the junction thermal voltage, but the model does not heat itself.

Low-side switch

The emitter goes to GND, the load sits between the collector and +V, and the microcontroller drives the base through a series resistor:

+V ──── Load ──── C
                  │
   GPIO ─ 1kΩ ─── B    (NPN)
                  │
                  E ──── GND
   10kΩ from B to GND (pull-down)

Drive the base through ~1 kΩ to set the base current; add a 10 kΩ base pull-down so the transistor stays firmly off when the GPIO floats (during reset or before the pin is configured). Use this for motors, relays, and high-current LEDs that a GPIO can’t drive directly: see /parts/diodes for the freewheeling diode an inductive load needs.

PNP BJT

The complement of the NPN, used as a high-side switch. Everything is mirrored: the emitter goes to +V, the load hangs off the collector to GND, and you turn it on by pulling the base low (so current flows out of the base).

Pinout

Specs

The PNP uses the complementary Ebers-Moll model with its own 100 V representative Early value. Wire the emitter to +V and the collector to the load; pull the base low through a suitable resistor to switch the load on. Because the controlling signal has to swing below +V, a PNP high-side switch is commonly paired with a small NPN driving its base.

N-Channel MOSFET

A logic-level enhancement NMOS, 2N7000 / IRLZ44N class: the voltage-driven equivalent of the NPN low-side switch, and the better choice for higher currents because it has no continuous base-drive loss.

Pinout

Specs

The Shichman-Hodges params (vto, k, lambda) and fixed gate capacitances are editable in the Inspector. vto marks the start of conduction, not a guarantee of low on-resistance. Use a real device’s transfer and on-resistance curves when choosing hardware.

Low-side switch

+V ──── Load ──── D
                  │
   GPIO ───────── G    (NMOS)
                  │
                  S ──── GND
   10kΩ from G to GND (pull-down)

Source to GND, drain to the load. Add a 10 kΩ pull-down from gate to GND for clean turn-off when nothing drives it. The fixed Cgs/Cgd model makes edge current and Miller coupling visible, but it omits gate resistance and a complete voltage-dependent gate-charge curve. The 5 A catalog field is a reference limit for diagnostics, not a simulated current clamp or proof of safe hardware operation.

P-Channel MOSFET

The complementary high-side MOSFET: source to +V, drain to the load, and the gate driven low (relative to the source) to turn it on. The voltage-driven counterpart to the PNP.

Pinout

Specs

The PMOS uses the complementary level-1 model. Wire the source to +V and the drain to the load; pull the gate low relative to the source to switch the channel on, and return it to the source voltage to switch off. A small NMOS commonly provides the level shift.

Model limits

BJTs include Ebers-Moll DC transport, gain, ambient-temperature dependence, and optional forward Early effect. They omit junction capacitance, charge storage, reverse Early effect, high-injection beta roll-off, breakdown, noise, and self-heating.

MOSFETs include cutoff, triode and saturation regions, a body diode, optional channel-length modulation, and fixed Cgs/Cgd Miller coupling. They omit voltage-dependent capacitance and full gate-charge curves, DC gate leakage, gate resistance, subthreshold conduction, avalanche, mobility-temperature drift, switching-energy failure, and self-heating.

The catalog current fields are not enforced safe-operating-area curves. Select the part to see exact identity, signed terminal-current information, and Included / Not included before relying on a result.