PN Junction Diodes

Current Flow

Some Diodes

Power Rectifiers Surface mount diodes Small signal silicon diodes

Miscellaneous diodes from the junk boxA Canadian band from the 1980’s

Doping of Semiconductors

The addition of a small percentage of foreign atoms in the regular crystal lattice of

silicon or germanium produces dramatic changes in their electrical properties,

producing n-type and p-type semiconductors.

Pentavalent

Trivalent

P and N Type Semiconductors

N- type P-type

Formation of Depletion Region

cathodeanode

Reverse Bias

Depletion region widens

No appreciable current flow

Forward Bias

Depletion region narrows

Substantial current flows

Current-versus-Voltage (IV) Characteristic Curve

Avalanche Breakdown

Thermally excited electrons cause a leakage current to flow in the reverse bias state.

These electrons accelerate in the bias field, picking up energy, and collide with the lattice, releasing more electrons.

At sufficiently high field strength the process achieves a runaway condition called avalanche.

Zener Diodes

Zener diodes have well-controlled and specified avalanche voltages.

The avalanche or zener voltage is often used to establish stable reference potentials in circuits.

Schottky Diodes

Schottky diodes are formed with metal/semiconductor junctions

They exhibit lower forward voltage drops and higher switching speeds between forward and reverse states. Reverse leakage currents tend to be higher than with PN junction diodes.

Varicap Tuning Diodes

In reverse bias, a diode exhibits capacitance between its terminals.

As the reverse bias potential increases, the depletion region grows wider and capacitance decreases.

As the reverse bias potential decreases, the depletion region narrows and the capacitance increases.

Diodes that are designed specifically to be voltage-variable capacitors are called varicaps.

Virtually all modern radio and television receivers, as well as cell-phones, handy-talkies, etc. are tuned by varicap diodes.

Next Steps

Explore diode properties with curve tracer in lab.

Investigate use of diode as half-wave rectifier.

AC

0-40 v rms

RED

BLACK

R1=1000 OHMS

R2=1 OHM

RED BLACK

HORIZONTAL SIGNAL

Voltage across DUT

VERTICAL SIGNAL

Current through DUT

TO OSCILLOSCOPE INPUTS

Z

DEVICE UNDER TESTSIMPLE CURVE TRACER

Variac Transformer

Half Wave Rectifier

AC

C1 330UF

R1200 OHMS

10W

+

-

TO SCOPE

DIODE

VARIABLE

20V MAX

RED

BLACK

RED

BLACK

FIGURE 1HALF WAVE RECTIFIER DEMO

S1 S2

VOLTAGE

TIME

FIGURE 2HALF WAVE RECTIFIED SINE WAVE

VOLTAGE

TIME

FIGURE 3OUTPUT VOLTAGE WITH CAPACITOR AND

LOAD

CHARGE DISCHARGE

Capacitor charges and discharges along this path