In
electronicsElectronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, a
step recovery diode (
SRD) is a semiconductor junction diode having the ability to generate extremely short pulses. It is also called
snap-off diode or
charge-storage diode or
memory varactor, and has a variety of uses in
microwaveMicrowaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
electronics as
pulse generatorA pulse generator is either an electronic circuit or a piece of electronic test equipment used to generate rectangular pulses. This article describes the test equipment.-Bench pulse generators:...
or parametric amplifier.
When diodes switch from forward conduction to reverse cut-off, a reverse current flows briefly as stored charge is removed. It is the abruptness with which this reverse current ceases which characterises the step recovery diode.
Historical note
The first published paper on the SRD is : the authors start the brief survey stating that "the recovery characteristics of certain types of pn-junction diodes exhibit a discontinuity which may be used to advantage for the generation of harmonics or for the production of
millimicrosecondA nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
pulses". They also refer that they first observed this phenomenon in February, 1959.
Physical principles
The main phenomenon used in SRDs is the storage of
electric chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
during forward conduction, which is present in all semiconductor junction diodes and is due to finite lifetime of minority carriers in
semiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s. Assume that the SRD is forward biased and in
steady stateA system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...
i.e. the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
bias current does not change with
timeTime is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
: since
chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
transport in a junction diode is mainly due to
diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
, i.e. to a non constant spatial
chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
carrier
densityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
caused by bias voltage, a
chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
is stored in the device. This
stored charge depends on
- Intensity of the forward anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flowing in the device during its steady stateA system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...
.
- Minority carrier lifetime
, i.e. the mean time a free charge carrierIn physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
moves inside a semiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
region before recombiningIn the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as...
.
Quantitatively, if the
steady stateA system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...
of forward conduction lasts for a time much greater than
, the stored
chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
has the following approximate expression
Now suppose that the voltage bias abruptly changes, switching from its stationary positive value to a higher
magnitudeThe magnitude of an object in mathematics is its size: a property by which it can be compared as larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which it belongs....
constant negative value: then, since a certain amount of charge has been stored during forward conduction, diode
resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
is still low (
i.e. the anode-to-cathode voltage 
has nearly the same forward conduction value). Anode
currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
does not cease but reverses its polarity (i.e. the direction of its flow) and stored
chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
starts to flow out of the device at an almost constant rate
. All the stored charge is thus removed in a certain amount of time: this time is the
storage time 
and its approximate expression is
When all stored charge has been removed,
diodeIn electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
suddenly changes, rising to its
cut-offIn electronics, the term Cut-off identifies a state of negligible electrical conduction which is proper of several types of electronic components when a control parameter , is lowered or increased past a value called conduction threshold or simply...
value at reverse bias within a time
, the
transition time. This can be exploited to produce a pulse with a rise time of
.
See also
- Minority carrier.
- P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
.
- Pulse generator
A pulse generator is either an electronic circuit or a piece of electronic test equipment used to generate rectangular pulses. This article describes the test equipment.-Bench pulse generators:...
- Semiconductor diode.
External links
. An interesting paper describing the construction and reporting the measured performance of an extremely fast
heterojunctionA heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...
SRD.. It is a PhD thesis in which an SRD is a key element. Chapter 5 is particularly relevant.