A diode-connected transistor MOSFET is simply a MOSFET with its gate and drain terminals shorted together. This physical wiring forces the device into the saturation (active) region, making it behave as a non-linear, voltage-dependent resistor rather than a digital switch. If you need an active load for an amplifier or a reference leg for a current mirror, you do not need to hunt for exotic components. For 90% of bench and hobbyist applications operating under 60V and 200mA, the 2N7000 (NMOS) or BS250 (PMOS) are the safe, default part numbers to use.

Below is the complete decision-forward guide to wiring, biasing, and testing these configurations, stripping away the abstract physics to focus on what actually works on the workbench.

The Diode-Connected Transistor MOSFET: Pinout and Wiring

To create this configuration, you tie the gate directly to the drain. The specific wiring depends on whether you are using an N-channel or P-channel device.

NMOS Configuration (e.g., 2N7000)

  • Drain & Gate: Shorted together and connected to the higher voltage node (e.g., VDD or the collector/drain of the driving transistor).
  • Source: Connected to the lower voltage node (usually Ground).
  • Pinout (TO-92 package, flat face toward you): Pin 1 = Source, Pin 2 = Gate, Pin 3 = Drain. You will physically bend Pin 2 (Gate) and Pin 3 (Drain) together or use a jumper wire on your breadboard.

PMOS Configuration (e.g., BS250)

  • Drain & Gate: Shorted together and connected to the lower voltage node (e.g., Ground or the pull-down network).
  • Source: Connected to the higher voltage node (VDD).
  • Pinout (TO-92 package, flat face toward you): Pin 1 = Drain, Pin 2 = Gate, Pin 3 = Source.
Bench Tip: When wiring TO-92 packages on a breadboard, the gate and drain pins are adjacent on the 2N7000 but separated by the source pin on some PMOS devices. Always verify the pinout with a multimeter's diode-check function before applying power to avoid shorting VDD to Ground through the body diode.

Operation Regions and Bias Characteristics

In a standard MOSFET, the triode (linear) region occurs when $V_{DS} < V_{GS} - V_{th}$. However, because a diode-connected MOSFET forces $V_{DS} = V_{GS}$, the condition for the triode region becomes $V_{GS} < V_{GS} - V_{th}$, which is mathematically impossible as long as the threshold voltage ($V_{th}$) is positive. Therefore, once the device turns on, it is always in the saturation region.

Table 1: Operation Regions and Typical Bias Parameters (NMOS 2N7000)
Device State Condition Typical $V_{GS}$ / $V_{DS}$ Typical $I_D$ (Drain Current) Effective Behavior
Cutoff $V_{GS} < V_{th}$ 0V to 1.5V 0 mA Open circuit (infinite resistance)
Saturation (Active) $V_{GS} \ge V_{th}$ 2.0V to 2.8V 1 mA to 10 mA Non-linear resistor / Voltage drop
High-Current Saturation $V_{GS} \gg V_{th}$ 3.5V to 5.0V 50 mA to 200 mA Low-value resistor (approaching $R_{DS(on)}$)

Notice that the voltage drop across a diode-connected 2N7000 at 1mA is roughly 2.0V to 2.5V, which is significantly higher than the 0.7V drop of a standard silicon BJT diode. This higher voltage headroom requirement is the primary trade-off when designing low-voltage circuits.

Decision Path: Selecting and Biasing for Your Circuit

Use this decision tree to select the right topology and bias point for your specific application. Do not overcomplicate the selection; follow the path to the concrete recommendation.

If your circuit requirement is... Then select this topology and default part...
Current Mirror Reference Leg
(Need a stable $V_{GS}$ to copy to a second transistor)		 NMOS: 2N7000
PMOS: BS250
Bias target: Set $I_{ref}$ between 1mA and 5mA for optimal thermal stability.
Active Load for High-Gain Amp
(Need high small-signal output resistance $r_o$)		 Matched Pair IC: ALD1101 (NMOS) or ALD1102 (PMOS)
Bias target: Keep $I_D$ low (100\mu A to 500\mu A) to maximize $r_o$.
Voltage Level Shifter / Clamp
(Need to drop a specific voltage from a logic rail)		 NMOS: 2N7000 stacked in series
Bias target: Stack two diode-connected 2N7000s to drop ~4.5V from a 5V rail to yield a ~0.5V output.
High-Side PMOS Current Source
(Need to source current from VDD to a load)		 PMOS: BS250
Bias target: Ensure VDD is at least $|V_{th}|$ + load voltage drop (usually VDD > 3.3V).

Complete Application Circuit: 1mA Active Load Current Mirror

The most common use for a diode-connected transistor MOSFET is the reference leg of a current mirror. Below is a complete, buildable circuit to generate a precise 1mA current source using two discrete 2N7000 transistors.

Component List

  • Q1: 2N7000 (Diode-connected reference)
  • Q2: 2N7000 (Mirror output)
  • R_ref: 2.4 k\Omega (1/4W, 1% tolerance)
  • C_bypass: 100 nF ceramic capacitor
  • VDD: 5.0V DC regulated supply

Wiring Steps

  1. Insert Q1 and Q2 into the breadboard. Critical: Place them physically touching each other so they share the same ambient temperature. Thermal mismatch will cause current mirroring errors.
  2. Wire Q1 as a diode: Connect Q1 Gate (Pin 2) directly to Q1 Drain (Pin 3).
  3. Connect Q1 Gate/Drain node to Q2 Gate (Pin 2).
  4. Connect Q1 Source (Pin 1) and Q2 Source (Pin 1) to Ground.
  5. Connect one end of R_ref to VDD (5V) and the other end to the Q1 Gate/Drain node.
  6. Connect Q2 Drain (Pin 3) to your load. The load will now sink exactly 1mA (assuming Q1 and Q2 are matched and at the same temperature).
  7. Place C_bypass (100nF) between VDD and Ground to filter high-frequency noise from the power rail.

The Math Behind the Bias

According to the ON Semiconductor 2N7000 datasheet, the typical threshold voltage ($V_{th}$) is 2.1V, and the forward transconductance parameter ($k_n$) yields a $V_{GS}$ of approximately 2.6V at 1mA.

To set 1mA through Q1, we calculate R_ref using Ohm's Law:

$R_{ref} = \frac{V_{DD} - V_{GS}}{I_{ref}} = \frac{5.0V - 2.6V}{0.001A} = 2400 \Omega$

A standard 1% 2.4 k\Omega resistor is the exact required value. Because Q2 shares the same $V_{GS}$ (2.6V) and is the same part number, it will also conduct 1mA through its drain, acting as an active load for whatever circuit you attach to it.

Failure Modes and Multimeter Diode Testing

Discrete MOSFETs fail in specific, predictable ways. The most common failure mode in diode-connected configurations is gate oxide punch-through caused by Electrostatic Discharge (ESD) during breadboarding, which permanently shorts the gate to the source. The second is thermal runaway in current mirrors, where one transistor heats up, lowers its $V_{th}$, hogs the current, and burns out.

How to Test a Diode-Connected MOSFET with a DMM

Use your multimeter's Diode Test mode to verify the health of a 2N7000 before and after soldering. Follow these exact numbered steps:

  1. Discharge the Gate: Briefly short all three pins together with a piece of bare wire or your fingers to drain any stored gate capacitance.
  2. Test the Body Diode (Source to Drain): Place the red probe on the Source (Pin 1) and the black probe on the Drain (Pin 3). You should read a standard diode drop (typically 0.5V to 0.7V). Reverse the probes (Red on Drain, Black on Source); the meter should read "OL" (Open Loop).
  3. Verify Gate Isolation: Place one probe on the Gate (Pin 2) and the other on the Source (Pin 1). The meter must read "OL" in both directions. If you read any continuity or a voltage drop here, the gate oxide is punctured. Throw the part away.
  4. Verify the Diode Connection: Since you wired Gate to Drain, place the red probe on the Gate/Drain shorted node and the black probe on Source. You should read the body diode drop (~0.6V). If you read 0.0V, your jumper wire is shorting to ground or the device is internally shorted.

Safe Default Part Numbers for the Workbench

Stop guessing which MOSFET to pull from your parts bin. When designing with diode-connected topologies, device matching and threshold voltage predictability are more important than raw current handling. Stock these three specific part numbers to cover every scenario.

Table 2: Recommended Default MOSFETs for Diode-Connected Circuits
Part Number Type Max $V_{DS}$ Max $I_D$ Typical $V_{th}$ Best Use Case Approx. Price (2026)
2N7000 NMOS 60V 200mA 2.1V General purpose active loads, 5V current mirrors. $0.12 / ea
BS250 PMOS -45V -230mA -1.8V High-side current sources, PMOS active loads. $0.18 / ea
ALD1101 NMOS Pair 10.6V 30mA 0.7V Precision current mirrors, low-voltage differential pairs. $1.45 / IC
Expert Insight on Matched Pairs: If your current mirror requires better than 5% accuracy, do not use two discrete 2N7000s. Discrete parts have a $V_{th}$ variance of up to \pm 0.5V from the same manufacturing batch. Instead, use a monolithic matched pair IC like the ALD1101. Because both transistors are fabricated on the exact same silicon die, their $V_{th}$ and thermal characteristics are inherently matched, virtually eliminating mirroring errors without needing external trimming potentiometers.

By standardizing on the 2N7000 for N-channel and BS250 for P-channel discrete work, and stepping up to the ALD1101 when precision matters, you eliminate the trial-and-error of threshold voltage hunting. Wire the gate to the drain, calculate your reference resistor using the $V_{GS}$ datasheet value, and your active load will perform predictably on the first power-up.