IV Characteristics

Part of Semiconductors

Plotting current versus voltage curves to understand and verify semiconductor device behavior.

Why This Matters

The I-V characteristic curve — current plotted against applied voltage — is the complete behavioral fingerprint of a semiconductor device. Every diode, transistor, and junction has a characteristic curve that captures its operating limits, switching behavior, and quality. Reading and plotting I-V curves is how you verify a device works, identify its parameters, detect degradation, and compare devices.

For a rebuilding civilization, I-V measurement is the primary quality control tool for semiconductor fabrication. A batch of diodes or transistors is characterized by plotting their curves. Devices that fall outside acceptable ranges are rejected or used in less critical applications. Devices with anomalous curves reveal fabrication problems: contamination shows up as excessive leakage; insufficient doping shows up as shifted threshold voltage; crystal defects show up as soft breakdown.

The measurement equipment needed is minimal: a variable power supply, a voltmeter, and a milliammeter. An oscilloscope with a curve tracer circuit can display the full curve automatically, but manual point-by-point measurement and graph plotting works equally well and requires only basic instruments.

Diode I-V Curve

The diode has the simplest characteristic: exponential current increase in forward bias, negligible current in reverse bias, and abrupt breakdown at high reverse voltage.

Ideal diode equation: I = I_s × (e^(V/nV_T) - 1)

where I_s is the reverse saturation current (leakage), V is applied voltage, V_T = kT/q ≈ 0.026 V at room temperature, and n is the ideality factor (1 for ideal diffusion current, 2 for recombination-dominated current, between 1 and 2 for most real diodes).

Measuring the forward curve:

1. Connect the diode with a current-limiting resistor in series (1 kΩ for signal diodes, 100 Ω for power diodes).
2. Apply voltage from 0 to 1.5V in steps of 0.1V using an adjustable supply.
3. At each voltage step, record the voltage across the diode (with voltmeter) and the current (from voltmeter across series resistor divided by resistance, or directly with milliammeter).
4. Plot current (y-axis, milliamps) versus diode voltage (x-axis, volts).

Expected result for silicon diode: near-zero current below 0.5V, rising steeply above 0.6V, reaching 1-50 mA by 0.7-0.8V. For germanium: current begins rising above 0.2V, reaches significant values by 0.3-0.4V.

Measuring the reverse curve:

1. Reverse the diode in the circuit.
2. Apply voltage from 0 to (just below) rated breakdown voltage.
3. Current should remain in the microamp range (leakage) until breakdown.
4. At breakdown (avalanche or Zener), current rises sharply — do not exceed this point without current limiting or the diode may be destroyed.

Key parameters extracted from diode I-V:

• Forward voltage at 1 mA (V_F): threshold parameter, distinguishes Si from Ge
• Reverse leakage at rated voltage: purity/quality indicator
• Breakdown voltage: sets maximum reverse voltage in circuit design
• Ideality factor n: measure of recombination quality; n>2 indicates defects

Transistor I-V Curves: Output Characteristics

For a BJT, the most important set of curves is the output characteristic: collector current (IC) versus collector-emitter voltage (VCE) for several fixed base currents (IB). This family of curves reveals the operating regions and parameters.

Measurement setup:

1. Base circuit: variable resistor from fixed 5V supply to base, with milliammeter to measure IB.
2. Collector circuit: variable supply from 0-20V with milliammeter in series to measure IC.
3. Emitter: connected to ground.

Measurement procedure:

1. Set IB to a small fixed value (e.g., 10 µA) by adjusting the base resistor.
2. Sweep VCE from 0 to 20V in steps of 0.5V, recording IC at each step.
3. Repeat for IB = 20, 40, 60, 80, 100 µA.
4. Plot all curves on same graph: IC (y-axis, mA) vs VCE (x-axis, V) with IB as the curve parameter.

Reading the output characteristics:

Saturation region (VCE < ~0.2V for Si, ~0.1V for Ge): Both junctions are forward biased. IC is low and increases steeply with VCE. The transistor behaves like a small resistor. This is the “switch ON” state.

Active region (VCE from ~0.3V to near breakdown): IC is nearly independent of VCE and proportional to IB. Curves are nearly horizontal lines. This is the amplifier operating region. The spacing between curves reveals hFE = ΔIC / ΔIB. If curves for IB = 10 to 100 µA span IC = 1 to 10 mA, hFE = 100.

Breakdown region: At high VCE, IC rises sharply regardless of IB — avalanche breakdown. The voltage where this begins is BVCEO. Never operate in this region.

Cutoff region (IB = 0): Very small collector current (ICEO leakage). Ideally zero for a switch-off state.

Transistor Transfer Characteristics

The transfer characteristic plots IC versus VBE (base-emitter voltage) at fixed VCE. This reveals threshold voltage and the transistor’s transconductance (gm = ΔIC / ΔVBE).

For a silicon NPN transistor:

• IC near zero below VBE ≈ 0.5V
• IC rises exponentially from 0.5 to 0.7V
• For VBE above 0.7V, IC determined by external circuit

This curve confirms the transistor is silicon (0.6-0.7V threshold) or germanium (0.2-0.3V threshold), verifies the junction is healthy (smooth exponential), and detects contamination (excess subthreshold current from surface states).

Practical Curve Tracer

A basic oscilloscope-based curve tracer displays the output family automatically:

Circuit: A sawtooth generator sweeps VCE at ~50-100 Hz. A step generator increments IB once per sweep period. The oscilloscope’s X-input connects to VCE; Y-input connects to a current-sense resistor in the collector line. The result is the family of curves displayed in real time.

Simple implementation: use the AC line (rectified) for VCE sweep; use a step-switched resistor bank for base current. Synchronize via a relay or transistor switch triggered at line frequency.

Without an oscilloscope, the same information comes from manual point-by-point measurement — slower but requiring only a voltmeter and milliammeter. For a small batch of 10 transistors, an hour of careful measurement produces complete characterization.

Interpreting Defects from Curves

Abnormal I-V curves diagnose specific fabrication problems:

High leakage in reverse bias: Contamination with metallic impurities (Fe, Cu, Ni) creating recombination centers. Indicates insufficient purification.

Soft breakdown (gradual instead of sharp): Surface contamination or crystal damage at junction perimeter. Etch junction surface and re-measure.

Low hFE with correct Q-point: Short minority carrier lifetime in base — recombination before carriers cross. Base too thick, or contamination reducing lifetime.

hFE that varies strongly with IC: Base resistance too high (voltage drop in base changes effective VBE); or device entering high injection at low currents (lightly doped base).

Negative resistance region: Oscillation or measurement instability. Check for parasitic inductance in test leads; the transistor may be oscillating at high frequency during measurement. Add 100 Ω resistors in base and collector leads to suppress.

Keep I-V curves on file for each fabricated batch. Comparing curves over time reveals process drift and lets you correlate curve characteristics with device performance in actual circuits.