Source-measure unit (SMU)

A source-measure unit (SMU) combines a programmable source of voltage or current with a precision meter for the other quantity, so it can force a current through a device and read the voltage across it, or apply a voltage and read the current, through one connection. It works in all four quadrants of the current-voltage plane: it delivers power in the first and third, and absorbs it in the second and fourth, as when it holds an illuminated photodiode or solar cell at a forward voltage below its open-circuit voltage. A limit on the quantity it is not sourcing, the compliance, protects the device, and built-in sweeps and triggers let it step through a curve in time with other instruments. In a photonics lab it drives laser diodes while recording their voltage for the LIV curve, measures the dark current and IV curves of photodiodes, and reads the photocurrent from which optical power is derived.

Source, measure and compliance

In the source-current mode the SMU regulates the current and measures the voltage; in the source-voltage mode it does the reverse. The compliance is a limit on the measured quantity. If a device would need more than the compliance voltage to carry the set current, for example because a probe has lifted and the circuit is open, the SMU holds the voltage at the limit and the current falls short of its set value. For a laser diode the compliance voltage is therefore set just above the highest voltage the device should reach, so that an open contact cannot raise the voltage far and closing it again does not discharge a large voltage through the diode. The example laser of the LIV curve entry, whose voltage follows 0.95 V + 5 Ω × $I$ above threshold, reaches 1.45 V at 100 mA, and a limit 0.3 V higher, 1.75 V, is the margin used in the bench script of Setting Up a Temperature-Controlled Laser Characterization Bench. In the source-voltage mode the compliance limits the current instead, which protects a photodiode swept toward breakdown or into forward conduction.

Each quantity is measured on a set of ranges, and resolution and accuracy are specified per range, usually as a percentage of the reading plus a fixed offset, so a small current read on a large range carries a large relative error. Autoranging picks the range reading by reading, at some cost in speed, and can leave small steps in a curve where the range changes; a sweep over a known span is often run on a fixed range for that reason.

Four-wire sensing

With two wires, the voltage the SMU reads includes the drop in its leads and contacts. A 50 mΩ contact resistance in the current path adds 5 mV at 100 mA, and in the example laser it adds 1% to the 5 Ω series resistance read from the slope of the curve. Four-wire (Kelvin) sensing separates the two functions: one pair of leads, the force leads, carries the current, and a second pair, the sense leads, measures the voltage at the device through a high impedance that draws almost no current, so the drops in the force leads and their contacts do not appear. The sense leads are joined to the force leads as close to the device as possible, at the probe tips or the mount, and in the source-voltage mode the SMU uses them to regulate the voltage at the device rather than at its own terminals.

Low currents: guarding, settling and integration time

Photodiode dark currents run from nanoamperes down to picoamperes, where the currents leaking through cable insulation, fixtures and humidity films become comparable to the signal: 10 V across 1 TΩ of insulation drives 10 pA, 1% of a 1 nA dark current. A guard removes that path. It is a conductor, usually the inner shield of a triaxial cable, that the SMU drives at the same potential as the signal line, so there is almost no voltage across the insulation between them and almost no leakage. Low-current fixtures carry the guard to the device and keep it separate from the grounded outer shield (Keithley).

A sweep also charges the capacitance of the device and cables, and the charging current adds to the reading: 100 pF swept at 1 V/s draws 100 pA. Low-current sweeps therefore step slowly or wait after each step until the reading settles. The integration time, often set in power-line cycles (1 PLC is 20 ms on a 50 Hz supply and 16.7 ms on a 60 Hz supply), trades speed for noise, and integrating over whole cycles rejects pickup at the mains frequency. The device must be in the dark, since room light on a photodiode produces a photocurrent far above its dark current.

Measuring an LIV curve

An SMU measures the current and voltage of a laser diode but not its light. A complete LIV setup uses two channels, or two SMUs triggered together: the first sources current into the laser in four-wire mode with the compliance voltage set as above, and the second holds a large-area photodiode, or the detector of an integrating sphere, at zero or a small reverse bias and measures its photocurrent. The photocurrent divided by the detector's responsivity at the laser's wavelength gives the optical power: an InGaAs photodiode with a quantum efficiency of 0.8 has a responsivity of 1.00 A/W at 1550 nm, so 1 mW of light gives 1.00 mA. At each step of the current sweep both readings are taken after the laser and detector have settled, and the recorded curve gives the threshold current, the slope efficiency and the series resistance (Threshold Extraction Methods).

The drive is ramped up from zero and back down to zero rather than switched at a high current, the output is turned on and off only at zero current, and the laser is handled with the precautions against electrostatic discharge described in the bench article. For high-power lasers, self-heating during continuous operation biases the measurement, and short pulses at low duty cycle (for example 1 µs at 0.1%) largely remove this bias, which can be checked by confirming that halving and doubling the pulse width leave the result unchanged. The pulse modes of SMUs typically reach tens of microseconds or longer, depending on the model, so pulses that short usually need a dedicated pulsed laser driver (Pulsed vs Continuous-Wave LIV Measurement).

Photodiode measurements

• IV curve and dark current. The SMU sources the bias voltage, from forward through zero into reverse, and measures the current in the dark, with guarding and settling as above. The dark current at the operating bias is the figure datasheets quote.
• Responsivity. With a known optical power on the detector, the photocurrent at the operating bias, less the dark current, divided by the power is the responsivity.
• Photovoltaic mode. Held at 0 V, the SMU acts as an ammeter with no burden voltage, and the photodiode operates without bias, where its dark current is smallest.
• Avalanche photodiodes. The breakdown voltage is found by sweeping the reverse bias with a current compliance of microamperes, or by sourcing a small reverse current and reading the voltage, so that the diode is never driven hard into breakdown.

Several channels

Multichannel SMUs, or several SMUs sharing a trigger line, bias the sections of a multi-section device at once: the laser and the electro-absorption modulator of an EML, where the modulator section is swept in reverse bias while its photocurrent, a measure of the light it absorbs, is recorded; the gain, phase and grating sections of a tunable laser; or a laser together with its back-facet monitor photodiode. Synchronized triggering puts every reading of a step at the same moment, which matters when the device is still settling thermally.

Common questions

What is a source-measure unit?

An instrument that sources a voltage or a current and measures the other quantity at the same time, with a programmable limit on the measured quantity. It works in all four quadrants of the current-voltage plane, so it can deliver or absorb power, and it is the usual instrument for measuring the electrical characteristics of laser diodes, LEDs and photodiodes.

How is an SMU different from a power supply?

A laboratory power supply usually delivers power in one quadrant and regulates it with modest precision. An SMU also absorbs power, measures what it sources with high resolution, switches between sourcing voltage and current, and runs fast, triggered sweeps, at the cost of lower output power.

What does four-quadrant operation mean?

That the SMU can source both positive and negative voltages and currents. In the first and third quadrants, where voltage and current have the same sign, it delivers power, as to a forward- or reverse-biased diode; in the second and fourth it absorbs power, as from a solar cell or an illuminated photodiode operating in forward bias.

Why is 4-wire sensing used for laser diodes?

Because the current through a laser diode is large enough that the voltage drop in the leads and contacts distorts the measured voltage and the series resistance derived from it. Separate sense leads measure the voltage at the device itself and exclude those drops.

What compliance voltage should be set for a laser diode?

A little above the highest voltage the laser should reach at the top of the sweep, such as 0.3 V above it. A limit close to the operating voltage keeps an open or intermittent contact from raising the voltage far and then discharging it through the diode.

References: Keithley Instruments, Low Level Measurements Handbook, 7th ed. (Keithley Instruments, 2014). L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012).