Experiment 6Bench 6MZU-PE / CVT-6Ready

The p-n Junction and Diode Characteristics

Semiconductor Curve Tracer

Socket a diode into the curve tracer, sweep the bias and record the rectifying characteristic.

Duration: 3 hours
Kittel: Chapter 8
Topics: Depletion regionBand bendingRectification

Curve tracer

Silicon diode (1N4148) · 300 K · 100 Ω limit

ONOFF

Log I

Standby

No characteristic — press SWEEP

CharacteristicOperating pointKnee at 0.827 V (1 mA)

Junction band diagram

Live view of the depletion region under the applied bias

Bench meters

Digital multimeter across the device, milliammeter in series

Supply voltage

Junction voltage

Current

µAOC

Chamber temperature

Thermal voltage k_BT/q

Barrier height

Device data

Band gap

1.12eV

Saturation current

2.50e-12A

Ideality η

1.60

Depletion width

1.000 W₀

The standard rectifier. Turn-on near 0.6 V and reverse leakage far too small to see on this range.

Tracer control

MZU-PE / CVT-6

Ready

Device socket

Supply

Bias voltage0.000 V

Current limit R100 Ω

Chamber temp300 K

Sweep range

Start-6.00 V

Stop1.20 V

ForwardReverseOver-current

Key Equations

I = I_0\left[\exp\!\left(\frac{qV}{\eta k_BT}\right)-1\right]

The saturation current carries the temperature dependence of the minority carriers,

I_0 \propto T^{3}e^{-E_g/k_BT}

which is why the whole characteristic shifts left when the chamber is warmed. The depletion region scales as

W \propto \sqrt{V_{bi}-V}

Procedure

0/4

ABand bending under biasSocket the silicon diode and set the bias to zero, then to +0.6 V and −4 V. At each setting record the barrier height and depletion width, and describe which way the bands bend.
BRecord the characteristicPress SWEEP and let the tracer draw the whole curve. Then step the bias by hand through the knee and record I at 0.4, 0.5, 0.55, 0.6, 0.65 and 0.7 V.
CTemperature dependenceHold the bias at 0.6 V and take the chamber from 250 K to 400 K in 25 K steps, recording the current each time. Note how far the knee moves per kelvin.
DCompare the devicesSweep Ge, Si, GaAs and the Schottky in turn at 300 K and record each knee voltage. Then socket the Zener, set the start of the sweep to −8 V, and find its breakdown voltage.

Table 6 — diode characteristics

0 entries

Read the junction voltage, not the supply voltage: the difference is the drop across the limiting resistor.

#	Device	E_g/ eV	T/ K	Supply/ V	V junction/ V	I/ mA	Knee at 1 mA/ V	Barrier/ eV	W/W₀
No readings yet — set the controls, then press “Record reading”.

Analysis & Reflection

Analysis questions

Switch the tracer to a logarithmic current axis. Over the exponential region, plot $\ln I$ against $V$ using your Task B data. The gradient is $q/\eta k_BT$ — extract $\eta$ and compare it with the panel.
Above about 1 mA the log plot stops being straight. Which circuit element causes that, and how could you confirm it experimentally?
From Task C, how many millivolts does the knee move per kelvin? Explain the sign using the temperature dependence of I₀.
Rank Ge, Si and GaAs by band gap and by knee voltage. Are the two orders the same? Why should they be?
Describe what happens to the depletion width and the barrier height in reverse bias, and why that makes the junction a rectifier.

Physics problems

Show that at 300 K the thermal voltage $k_BT/q$ is about 25.9 mV, and confirm it against the panel readout.
A silicon diode passes 1 mA at 0.6 V. Estimate the current at 0.7 V assuming η = 1.6 and ignoring series resistance. Compare with the tracer.
Why is a Zener diode operated deliberately in reverse breakdown, while an ordinary rectifier must never be?
Explain why the Schottky diode has an ideality factor close to 1 while the p-n junctions do not. What does η actually measure?