I-V characteristic curves · Drag load line to move Q-point · Switch BJT / MOSFET
Visualise BJT and MOSFET transistor I-V characteristic curves. Drag the load line to set the quiescent point (Q-point). Switch between NPN BJT and N-channel MOSFET models.
A transistor's I-V curves map the relationship between collector/drain current and voltage for different base/gate inputs. The Q-point — where the load line intersects the active characteristic — determines the operating region: cutoff, active, or saturation.
Switch between BJT and MOSFET modes. Adjust the base current (BJT) or gate voltage (MOSFET) to see different I-V curves. Drag the load line to move the Q-point between regions.
The MOSFET is the most manufactured device in human history — over 13 sextillion (1.3 × 10²²) have been produced since 1960. A modern CPU contains billions of MOSFETs, each switching billions of times per second.
This simulation plots the output characteristic curves of two transistor types: the NPN bipolar junction transistor (BJT) and the N-channel MOSFET. For the BJT it draws a family of collector-current curves I_C versus V_CE for five base-current levels (10–50 μA), using I_C = β·I_B with β = 100 plus a small Early-effect slope. For the MOSFET it plots drain current I_D against V_DS using the square-law model with V_th = 1.5 V.
You pick the transistor type, then set the base current (BJT) or gate voltage V_GS (MOSFET), the supply voltage V_CC/V_DD, and the collector/drain resistor R_C/R_D. A dashed load line runs from the supply voltage on the V axis to V/R on the I axis; its intersection with the active curve gives the Q-point, reported as V_CE/V_DS, I_C/I_D and the region. This is the cornerstone of analogue amplifier biasing and digital switching.
What does this transistor simulation actually show?
It shows the output I-V characteristic curves of a transistor: collector current versus collector-emitter voltage for a BJT, or drain current versus drain-source voltage for a MOSFET. A dashed load line and a highlighted Q-point reveal which operating region the device sits in for the chosen circuit values.
What is the load line and the Q-point?
The load line represents the external circuit constraint I = (V_supply − V) / R imposed by the supply and the collector/drain resistor. It is a straight line from V_supply on the horizontal axis to V_supply/R on the vertical axis. The Q-point (quiescent point) is where this line crosses the transistor's active characteristic curve, fixing the DC operating current and voltage.
What do the three operating regions mean?
For a BJT: cutoff means essentially no collector current; the active region gives a nearly constant current set by the base current, used for linear amplification; and saturation (low V_CE) is when the transistor is fully on, used as a closed switch. The MOSFET has the analogous cutoff, triode (linear) and saturation regions.
The model uses I_C = β·I_B with β = 100, so each 10 μA step of base current scales to roughly 1 mA of collector current. Below V_CE ≈ 0.3 V the current is ramped down linearly to represent saturation, and in the active region a factor of (1 + 0.02·V_CE) adds a slight upward slope to approximate the Early effect.
It uses the standard square-law model with threshold V_th = 1.5 V and transconductance parameter k = 0.5 mA/V². In saturation I_D = ½·k·(V_GS − V_th)²·(1 + 0.02·V_DS), and in the triode region I_D = k·[(V_GS − V_th)·V_DS − ½·V_DS²]. The boundary between the two, V_DS = V_GS − V_th, is the pinch-off locus shown as a dashed line.
The tabs switch between NPN BJT and N-MOS models. For the BJT you select one of five base-current levels (10–50 μA), the supply V_CC (5–20 V) and the resistor R_C (0.5–5 kΩ). For the MOSFET you set the gate voltage V_GS (1–5 V), the supply V_DD and the drain resistor R_D. Changing any of these moves the load line and so the Q-point.
The load line's vertical intercept is V_supply/R, so a larger R_C or R_D lowers that intercept and makes the line shallower. This pulls the Q-point toward lower current. A smaller resistor raises the current intercept, steepens the line, and pushes the device closer to saturation for a given base current or gate voltage.
It captures the correct qualitative behaviour and the standard textbook equations, but it is a simplified large-signal model. Real transistors have temperature dependence, more complex saturation behaviour, base-width modulation, channel-length modulation and parasitic capacitances that are only approximated here by the small 0.02 slope terms. The values are illustrative rather than tied to a specific datasheet part.
A BJT is current-controlled: its collector current is set by the base current through the gain β. A MOSFET is voltage-controlled: its drain current depends on the gate-source voltage above threshold, and its gate draws almost no steady current. MOSFETs dominate digital integrated circuits for their high input impedance and low static power, while BJTs are still common in precision analogue and high-current roles.
For linear amplification you place the Q-point in the middle of the active (BJT) or saturation (MOSFET) region, well clear of cutoff and saturation, so the signal can swing in both directions without clipping. For switching you drive the device hard between cutoff (off) and saturation/triode (fully on), where the voltage drop across it is minimal. Try moving the controls to see the Q-point shift between these zones.
They underpin almost all electronics design: amplifier biasing, logic-gate switching thresholds, power-stage efficiency, current mirrors and voltage references. Engineers read these curves from datasheets to choose operating points, estimate gain and power dissipation, and ensure a device stays within its safe operating area. The MOSFET in particular is the building block of every modern processor and memory chip.