Special-Purpose Diodes

Objectives

Describe the characteristics of a zener diode and analyze its operation.
Explain how a zener is used in voltage regulation and limiting.
Describe the varactor diode and its variable capacitance characteristics.
Discuss the operation and characteristics of LEDs and photodiodes.
Discuss the basic characteristics of the current regulator diode, the PIN diode, the step-recovery diode, the tunnel diode, and the laser diode.

Introduction

The primary function of a zener diode is to maintain a specific voltage across its terminals within given limits of line or load change.

Typically used for providing a stable reference voltage for use in power supplies and other equipment.

Zener Diodes – Operating Range

A zener diode is similar to a normal diode, but it is placed in the circuit in reverse bias and operates in reverse breakdown.

Its forward characteristics are like a normal diode.

Zener Diodes – Regulation Ranges

The zener diode’s breakdown characteristics are determined by the doping process.

Low voltage zeners (>5V) operate in the zener breakdown range.
Zeners designed to operate <5 V operate mostly in avalanche breakdown range.
Zeners are available with voltage breakdowns of 1.8 V to 200 V.
The curve illustrates the minimum and maximum ranges of current operation that the zener can effectively maintain its voltage.

Zener Diodes – Breakdown Characteristics

Note the very small reverse current before the knee.
Breakdown occurs @ knee.
Breakdown Characteristics:
- $V_Z$ remains near constant.
- $V_Z$ provides:
  - Reference voltage
  - Voltage regulation
- $I_Z$ escalates rapidly.
- $I_{ZMAX}$ is achieved quickly.
- Exceeding $I_{ZMAX}$ is fatal.

Zener Diodes - Voltage Regulation

$VZ$ @ $I{ZT}$
$V_R$
$V_{ZK}$
$I_{zk}$ (zener knee current)
Regulation occurs between:
- $V_{zk}$ - knee voltage
- $I{ZT}$ (zener test current) to $V{ZM}$ - $I_{max}$
$I_R$
$I_{ZM}$ (zener maximum current)

Zener Diodes – Equivalent Circuit

Ideal Zener exhibits a constant voltage, regardless of current draw.
Ideal Zener exhibits no resistance characteristics.
Zener exhibits a near-constant voltage, varied by current draw through the series resistance $Z_Z$ .
As $Iz$ increases, $Vz$ also increases.

Zener Diodes – Characteristic Curve

$\Delta Vz$ results from $\Delta Iz$ .
$\Delta Iz$ thru $Zz$ produce this.

Zener Diode – Data Sheet

Power ratings
Temperature ratings
$V_z$ nominal
Impedance
Power derating curves
Temperature coefficients
$\Delta Z_z$ - Zener impedance

Zener Diode - Applications Regulation

In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages.

Zener current will increase or decrease directly with voltage input changes.
The zener current, $Iz$ , will vary to maintain a constant $Vz$ .
Note: The zener has a finite range of current operation.
$V_{Zener}$ remains constant.

Zener Diode - Applications Regulation

In this simple illustration of the zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads ( $R_L$ ) to be able to maintain its designated zener voltage.

Zener current will increase or decrease directly with voltage input changes.
The zener current will increase or decrease inversely with varying loads.
Again, the zener has a finite range of operation.
$V_{Zener}$ remains constant.

Zener Diode - Applications

Calculate $V_{ZRegulate}:$
$V{inMIN} = VR + V_Z = 55mV + 10V = 10.055V$
$VR = IZR = (100mA)(220) = 22V$
$V_{in(max)} = 22V + 10V = 32V$
$\therefore V_{Reg}$ is ≈10V to 32V.
1N4740 $P_{DMAX} = 1W$
$V_Z = 10V$
$I{ZK} = 0.25mA$ to $I{ZM} = 100mA$
$V{Rmin} = I{ZKR}= .25mA \times 220 = 55mV$
$V{Rmax} = I{ZM} = 100mA \times 220 = 22V$

Zener Limiting

Zener diodes can be used for limiting just as normal diodes.

The difference to consider for a zener limiter is its zener breakdown characteristics.

Varactor Diodes

A varactor diode is best explained as a variable capacitor.

Think of the depletion region as a variable dielectric.
The diode is placed in reverse bias.
The dielectric is “adjusted” by reverse bias voltage changes.

Varactor Diodes

The varactor diode can be useful in filter circuits as the adjustable component for resonance frequency selection.

Resonant Band-pass Filter w/ Varactor Diode

Series Resonant Tank
Parallel Resonant Tank
Varactor
Varactor Bias $V_{BIAS} = 2.9V$ to 29V
$C_{Varactor} = 17pF$ to 55pF
Resonant Frequency Range: $f_r = 679kHz$ to 1.22MHz.
31.6V
$V_R = 2.85$ to 28.7V

Optical Diodes

The light-emitting diode (LED) emits photons as visible light.

Its purpose is for indication and other intelligible displays.
Various impurities are added during the doping process to vary the color output.

Optical Diodes

Electroluminescence, the process of emitting photons from a parent material (substrate), is the basis for LEDs.

Colors result from the choice of substrate material and the resulting wavelength.
Today's LEDs (green, red, yellow) are based on indium gallium aluminum phosphide
Blue uses silicon carbide or gallium nitride
IR (infrared) – GaAs (gallium arsenide)
LED Biasing: 1.2V to 3.2V is typical.
Some newer LEDs run at higher voltages and emit immense light energy.
- Applications:
  - Traffic signals
  - Outdoor video screens
  - Runway markers
A strong +bias encourages conduction-band electrons in the N-material to leap the junction and recombine with available holes releasing light and heat.

LED – Spectral Curves

Note the wavelengths of the various colors and infrared.
Note lead designations to the right.

LED Datasheet - MLED81 Infrared LED

Maximum Ratings

Rating	Symbol	Value	Unit
Reverse voltage	$V_R$	5	Volts
Forward current-continuous	$I_F$	100	mA
Forward current-peak pulse	$I_F$	1	A
Total power dissipation @ TA = 25°C	$P_p$	100	mW
Derate above 25°C		2.2	mW/°C
Ambient operating temperature range	$T_A$	-30 to +70	°C
Storage temperature	$T_{stg}$	-30 to +80	°C
Lead soldering temperature, 5 seconds max, 1/16 inch from case		260	°C

Electrical Characteristics (TA = 25°C unless otherwise noted)

Characteristic	Symbol	Min	Typ	Max	Unit
Reverse leakage current ( $V_R$ = 3 V)	$I_R$		1	10	nA
Reverse leakage current ( $V_R$ = 5 V)	$I_R$		1	10	ΜA
Forward voltage (I = 100 mA)	$V_E$	1.35	1.7		V
Temperature coefficient of forward voltage	$\Alpha V_E$	-1.6			mV/K
Capacitance (f= 1 MHz)	C		25		pF

Optical Characteristics (TA = 25°C unless otherwise noted)

Characteristic	Symbol	Min	Typ	Max	Unit
Peak wavelength (I = 100 mA)	$\Chi_p$		940		nm
Spectral half-power bandwidth	$\Alpha\lambda$		50		nm
Total power output (I = 100 mA)	$\Phi_e$	16			mW
Temperature coefficient of total power output	$\Delta\varepsilon$			-0.25	%/K
Axial radiant intensity (I = 100 mA)	I	10			mW/sr
Temperature coefficient of axial radiant intensity	$\Delta I$			-0.25	%/K
Power half-angle	\		15	+30	0

Optical Diodes

The seven-segment display is an example of LEDs used for the display of decimal digits.

Photodiodes

Unlike LEDs, photodiodes receive light rather than produce light.

The photodiode varies its current in response to the amount of light that strikes it.
It is placed in the circuit in reverse bias.
As with most diodes, no current flows when in reverse bias, but when light strikes the exposed junction through a tiny window, reverse current increases proportionally to light intensity (irradiance).
Photodiodes exhibit a “reverse leakage current” which appears as an inverse variable resistance.
Irradiance causes the device to exhibit a reduction in the variable resistance characteristic.

Photodiodes

General graph of reverse current versus irradiance
Example of a graph of reverse current versus reverse voltage for several values of irradiance

Photodiodes - MRD821

Maximum Ratings

Rating	Symbol	Value	Unit
Reverse voltage	$V_R$	35	Volts
Forward current-continuous	$I_F$	100	mA
Total power dissipation @ TA = 25°C	$P_D$	150	mW
Derate above 25°C		3.3	mW/°C
Ambient operating temperature range	$T_A$	-30 to +70	°C
Storage temperature	$Tag$	-40 to +80	°C
Lead soldering temperature, 5 seconds max, 1/16 inch from case		260	°C

Electrical Characteristics (TA = 25°C unless otherwise noted)

Characteristic	Symbol	Min	Typ	Max	Unit
Dark current ( $V_R$ = 10 V)	$I_D$		3	30	nA
Capacitance (f= 1 MHz, V = 0)	$C_i$		175		pF

Optical Characteristics (TA = 25°C unless otherwise noted)

Characteristic	Symbol	Typ	Max	Unit
Wavelength of maximum sensitivity	$\lambda_{max}$	940		nm
Spectral range	$\Alpha\lambda$	170		nm
Sensitivity (=940 nm. $V_R$ = 20 V)	S	50		µA/mW/cm²
Temperature coefficient of sensitivity	$\Alpha S$		0.18	%/K
Acceptance half-angle			±70
Short circuit current (Ev = 1000 lux)	$I_s$	50		ΜA
Open circuit voltage (Ev = 1000 lux)	$V_L$	0.3		V

Other Diode Types

Current regulator diodes (constant current diodes) keep a constant current value over a specified range of forward bias voltages ranging from about 1.5 V to 6 V.
This device exhibits very high impedances.

Other Diode Types

The Schottky diode’s (hot-carrier diodes) significant characteristic is its fast switching speed.

This is useful for high frequencies and digital applications.
It is not a typical diode in that it does not have a p-n junction.
Instead, it consists of a lightly-doped n-material and heavily-doped (conduction-band electrons) metal bounded together.

Other Diode Types

The PIN diode is also used in mostly microwave frequency applications.

Its variable forward series resistance characteristic is used for attenuation, modulation, and switching.
In reverse bias, it exhibits a nearly constant capacitance.

Other Diode Types

The step-recovery diode is also used for fast switching applications.

This is achieved by reduced doping near the junction.
The diode recovers very quickly, making it useful in high-frequency (VHF) applications.

Other Diode Types

The tunnel diode exhibits negative resistance.

It will actually conduct well with low forward bias.
With further increases in bias, it reaches the negative resistance range where current will actually go down.
This is achieved by heavily-doped p and n materials that create a very thin depletion region which permits electrons to “tunnel” thru the barrier region.
Germanium or Gallium Tank circuits oscillate but “die out” due to the internal resistance.

Tunnel Diodes

Tank circuits oscillate but “die out” due to the internal resistance.

A tunnel diode will provide “negative resistance” that overcomes the loses and maintains the oscillations.

Other Diode Types

The laser diode (light amplification by stimulated emission of radiation) produces monochromatic (single color) “coherent” light.

Laser diodes in conjunction with photodiodes are used to retrieve data from compact discs.
Forward bias the diode and electrons move thru the junction, recombination occurs (as ordinary).
Recombinations result in photon release, causing a chain reaction of releases and avalanching photons which form an intense laser beam.

Summary

The zener diode operates in reverse breakdown.
A zener diode maintains a nearly constant voltage across its terminals over a specified range of currents.
Line regulation is the maintenance of a specific voltage with changing input voltages.
Load regulation is the maintenance of a specific voltage for different loads.
There are other diode types used for specific RF purposes such as varactor diodes (variable capacitance), Schottky diodes (high speed switching), and PIN diodes (microwave attenuation and switching).
Light emitting diodes (LED) emit either infrared or visible light when forward-biased.
Photodiodes exhibit an increase in reverse current with light intensity.
The laser diode emits a monochromatic light.