Drain Characteristics of an n-Channel MOSFET (2N7000)

Adapted from the ExpEYES blog MOSFET drain-curve lab. Sweep $V_{DS}$ with PV1 at fixed $V_{GS}$ (PV2) and plot $I_D$ vs $V_{DS}$.

1. Aim

To obtain the output (drain) characteristics of a 2N7000 MOSFET for several gate–source voltages and to identify the cutoff / linear / saturation behaviour qualitatively.

2. Apparatus / Components Required

3. Theory & Principle

For an n-channel enhancement MOSFET, appreciable drain current appears only when $V_{GS}$ exceeds a threshold (for 2N7000, conduction in this setup is weak below about 1.65 V gate bias).

Indirect drain current (same method as diode/transistor labs):

\[I_D = \frac{V_{\text{PV1}} - V_{\text{A1}}}{R_D}, \qquad V_{DS} \approx V_{\text{A1}}\]

(with source grounded). Family of curves: fix $V_{GS}$ via PV2, sweep PV1, plot $I_D$ vs $V_{DS}$.

4. Circuit Diagram / Setup

  1. Source → GND.
  2. Gate → PV2.
  3. PV1 → $R_D = 1\text{ k}\Omega$ → Drain.
  4. A1 at the drain node ($V_{DS}$).

2N7000 MOSFET setup

5. Procedure

Part A — App / manual observation

  1. Set PV2 to a fixed gate voltage (start near 1.7 V).
  2. Sweep PV1 from 0 toward ~5 V; record A1 and compute $I_D$.
  3. Repeat for several PV2 values up to about 2.0 V.
  4. Plot $I_D$ vs $V_{DS}$ for each $V_{GS}$.

MOSFET drain family of curves

MOSFET drain plot

Part B — Python automation

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import time
import eyes17.eyes
from pylab import *

p = eyes17.eyes.open()
xlabel("Drain voltage V_DS (V)")
ylabel("Drain current I_D (A)")
ion()

pv2 = 1.65
while pv2 <= 2.0:
    vdsa, idra = [], []
    p.set_pv2(pv2)
    p.set_pv1(0)
    time.sleep(0.2)
    pv1 = 0.0
    while pv1 <= 4.8:
        p.set_pv1(pv1)
        a1 = p.get_voltage("A1")
        idr = (pv1 - a1) / 1000.0
        idra.append(idr)
        vdsa.append(a1)
        pv1 += 0.1
    plot(vdsa, idra, label=f"V_GS={pv2:.2f} V")
    pause(0.5)
    pv2 += 0.05

legend()
show()
p.set_pv1(0)
p.set_pv2(0)

6. Observation Table

$V_{GS}$ (PV2) (V) $I_D$ at $V_{DS}=1\text{ V}$ (mA) $I_D$ at $V_{DS}=3\text{ V}$ (mA) Remarks
1.65      
1.75      
1.85      
2.00      

7. Results and Discussion

8. Precautions

  1. Keep $R_D = 1\text{ k}\Omega$ in series with the drain — never short PV1 to drain.
  2. Observe 2N7000 pinout (D / G / S).
  3. Limit PV1/PV2 to safe SEELab ranges; zero outputs when finished.
  4. Static-sensitive device — handle with care.

9. Troubleshooting

Symptom Possible Cause Corrective Action
$I_D \approx 0$ for all PV1 $V_{GS}$ too low / gate open Raise PV2 above ~1.65 V; check gate wire
$V_{DS} \approx$ PV1 always MOSFET off or S not grounded Check source–GND and pinout
Excessive current Missing $R_D$ Insert 1 kΩ immediately

10. Viva-Voce Questions

Q1. How is $I_D$ measured without an ammeter?

Ans: From the drop on $R_D$: $I_D = (V_{\text{PV1}} - V_{\text{A1}})/R_D$ with A1 at the drain.

Q2. Why start $V_{GS}$ near 1.65 V for the 2N7000 in this lab?

Ans: Below that gate voltage the device barely conducts in this resistor-limited setup, so $I_D$–$V_{DS}$ curves are not useful.

Q3. What is the role of PV1 vs PV2?

Ans: PV2 sets gate–source bias ($V_{GS}$); PV1 sweeps the drain supply so $V_{DS}$ and $I_D$ can be traced for each gate setting.