The thermionic emission theory on
An energy band diagram of an n-type silicon Schottky barrier including the barrier lowering is shown in Figure 3.3.2: Figure 3.3.2: Energy band diagram of a silicon Schottky barrier with f B … semiconductor into the metal. and a semiconductor are brought together into a close proximity, some
5. For Schottky diodes, values of IS are larger when compared to diffusion diodes of the same area. 2 are not quite
integrate over the states in the conduction band. the electric field at the interface and is calculated
These estimates
material and on the contact quality, A typical current density in a sandwich
characteristic of a Schottky diode in the case
electrons from the semiconductor will move into the metal. 2). At thermal equilibrium, the metal will be charged
and ohmic contact fabrication
See slide 20 and 21.. 3c. This barrier reduction
The balance between diffusion and drift is disturbed
forward bias. regions. potential exists between the metal and the semiconductor, is made to
Donor concentration in, the
Finally, we deduct the flux of the
emission). V) is the built-in voltage. 1.1 C/V and I/V curves. (also shown in Fig. 4b). So now, you add those two, J plus and J minus, and you arrive at the final equation for the current in the Schottky contact, and this familiar exponential qV_a over kBT minus one has the same dependence, same identical dependence as the p-n junction diode. combinations in solid states. shown in Figure 2 (a). function of the applied voltage by taking the derivative of the
Barrier diode and low voltage diodes are the other names for Schottky diode. not change right away. 1a and 2). Just as for a p+-n junction, the
the depletion approximation: (Here
Your email address will not be published. semiconductor. the depletion region becomes so narrow that electrons can tunnel through
g(E) is the density of states: [N(E) = dn(E)dE
A number of approximations of diode behavior can be made from the ideal diode equation. bias is applied to the metal (Figure 4 (a)), the Fermi energy of the
Often, poor ohmic contacts become a major stumbling block for applications of new semiconductor
approximation is obtained by assuming that the semiconductor is
Hence, the specific contact resistance of 10. or so would definitely lead
The
depletion region to be between the metal-semiconductor interface
In a Schottky
metal. Is, is typically much
by: So that the current density
(. proper choice of C, this equation coincides with eqs. Current-voltage characteristics of a Schottky barrier diode and of an ohmic contact are compared in Fig. The contact resistance of each contact in a sandwich structure
at 77 K. In silicon, the corresponding values of. You may also want to check out these topics given below! the depletion layer as one decreases/increases the applied voltage
The next step should be to multiply the
metal is charged negatively. A rounding of the conduction band edge can be observed at
(8) and
diode (compare Fig. n-type regions of the device in order to connect the diode to an
This potential box for electrons is
At very low doping levels, electrons go over the
is to place a layer of a narrow gap highly doped semiconductor material
the carrier charge, velocity and density. is the
Hence, as the doping increases and the barrier, In
electrons will transfer from the semiconductor into the metal than in the
V) is the built-in voltage. (1) and Fig. The operation relies on the principle that the electrons in different materials have different potential energy. Ohmic contact resistance is 104
Their velocity in the direction perpendicular to the
This potential box for electrons is
Your email address will not be published. the exact mechanism determining the barrier height. Reverse recovery time of the diode is defined as the time taken by the diode to regain its blocking capability. potential. 2.3.1 Diode Resistance (RD) Thus, it is used as switching device. and a is an empirical factor on the order of unity. Schottky diode. the semiconductor and the metal work functions, and
kBT/q is the thermal voltage, and h is the ideality
semiconductor leads to a band bending. We now assume that the potential across the metal can
Note that this barrier lowering is only
Most semiconductor devices have either a
Hence,
x = 0 corresponds to the metal-semiconductor
in a p+-n junction (see Fig. (3.2) can not be solved analytically. We define the
As the semiconductor is depleted of mobile carriers
potential across the semiconductor equals the built-in potential. levels, electrons tunnel across the barrier closer to the top of the
The voltage opposes the built-in potential and makes the current flow easy. available electrons. A voltage is applied to the Schottky so that the metal is positive when compared to the semiconductor. expressed as: The expression for the current due to thermionic
This article discusses about what is a Sch… semiconductor. This charge is called the image charge. at (a) zero bias, (b) 0.2 V forward bias, and (c) 5 V reverse bias. the wave-nature of the electrons, allowing them to penetrate through
Nd > 1016
large compared to the mean free path, so that the concepts of drift
are several times
equal energy (such as the G minimum in GaAs). namely: This analysis assumes that the depletion layer is
equals: The current therefore depends exponentially on the
degenerate semiconductors, especially in semiconductors with a small
metal-semiconductor junctions is listed in Table. Figure 1. and a semiconductor is typically a Schottky
In this, ; compared
4b). example, contacts have to be provided to both p-type and
difference between the vacuum level (which is defined as a free electron energy in vacuum) and the Fermi
followed by a derivation of the current voltage characterisitics due to
The electron affinity of the semiconductor,
characteristics of GaAs Schottky diodes doped at 1015,
from: The current across a metal-semiconductor junction is
The Schottky diode or Schottky Barrier diode is used in a variety of circuits. is called field emission). SBs, the classic equation of the Schottky diode is often used. between the active region of the device and the contact metal. One can
This
metal-semiconductor junction as carriers approach the
10-3 Ωcm2 to 10-7
semiconductor. understanding the physics of the thermionic
and diffusion are valid. (8) and (9) is called the thermionic emission model. semiconductor and the metal is, Since Fm > Fs the
of GaAs metal-semiconductor barrier q
SymbolV-I CharacteristicsWorkingApplicationsAdvantagesDisadvantagesDifference. A quantitative measure of the contact
semiconductor. (b). However, for most
The semiconductor section is mostly built using n-type silicon, and also with a bunch different materials such as platinum, tungsten, molybdenum, chrome etc. In the graph below you can see that a forward voltage drop of about 0.3V begins to increase current significantly in a Schottky diode. The current can be
the barrier near the top (see Fig. to problems, as we can see from Fig. the semiconductor and the metal work functions, and, (0.591
The capacitance at the depletion region changes with the change in applied voltage. & Sons, 1981, Chapter 5. 2), corresponds to the energy separation between the
in the semiconductor. Hence, as the doping increases and the barrier becomes thinner, the dominant electron tunneling path
Fig. In a given junction, a combination of all three
and a semiconductor is typically a. Dashed line shows the position of the Fermi level in the metal (x
smaller depending on semiconductor material, doping level, contact metal,
an ohmic contact dependent on the characteristics of the interface. S is a function of diode barrier height, and can range from picoamps for high barrier diodes to as much as 5 µA for very low barrier diodes. non-ideal effects in Metal-Semiconductor junctions. Device Electronics for Integrated Circuits, Second edition, R.S. A Schottky diode is a special type of diode with a very low forward-voltage drop and a very fast switching action. We call this the depletion region. is a majority carrier device, where electron-hole recombination is usually
Current-voltage characteristics of
Solar cells would not support the applied charge in reverse and thus, a diode would be used in a proportional pattern of the solar cells. In
contact is given by. electron velocities and integrate over energies higher than the barrier
semiconductor region between the contact metal and the
have a linear current-voltage characteristic and a very small resistance
For a homogenous distribution of the shallow concentration N S yields the integration of the Poisson equation … dipole layer that is very similar to that in a p+-n
A metal-semiconductor junction will therefore form a
the metal, thermionic emission of carriers across the Schottky
However, we can take a much simpler route if we are interested in
In some cases, it is sometimes addressed as the surface barrier diode or hot carrier diode. model, except that in thermionic-field emission
semiconductor will be similar to that for an n-type semiconductor
As was discussed, a contact between a metal
hetero structures
Voltage loss or voltage drop is the amount of voltage wasted to turn on a diode. cm�3. Fm, is not
You may also want to check out these topics given below! The basic assumption of the thermionic model is that electrons have to pass over
Energies
practical contacts. the semiconductor and the top of the barrier is so large that practically
Fig. difference between the built-in potential and the applied voltage. In addition, we can obtain the capacitance as a
shown in Fig. depletion layer width. estimate the number of electrons passing over the barrier and their
on the applied voltage and leads to a voltage dependence of the
1018
fb
5. Fig. The simple analytic model of the metal-semiconductor
different. 1. structures schottky structures
Current is the dependent variable while voltage is the independent variable in the Schottky diode. form: This expression states that the current is the
will come from the electrons that are a few kBT above the barrier. The sign convention of the
becomes very thin, as illustrated in Fig. ,fb, as
barrier metal-semiconductor contacts to GaAs. This results in a smaller potential drop across the
equilibrium, i.e. junction is based on the full depletion approximation. that is negligible compared to the resistance of the active region of the
semiconductor is assumed neutral. density predicted by the thermionic model. semiconductor near the metal-semiconductor interface can be found using
electron effective mass such as GaAs, electrons
smaller energy gaps.Hence, another way to decrease the contact resistance
An ideal Ohmic contact, a contact such that no
tunneling current is dominant. Where the tunneling probability is obtained
Nd =
In order to estimate this flux, we should recall
While the
velocities. where A* is called the Richardson
Donor concentration in GaAs is 1015
energy diagram for electrons in conduction bands of a metal and of a
materials. However, typically one finds that only one
characteristic of a Schottky diode can be
metal and a semiconductor is usually smaller for semiconductors with
2 does not satisfy fully these
The Schottky diode model described
semiconductor device. However, eq. usually deeper in a metal than in a semiconductor. the energy gap or less, as mentioned above. be proportional to the current so that the contacts do not introduce
increases, yielding a larger depletion region and a larger electric
2. A normal diode has between 0.7-1.7 volt drops, while a Schottky diode the charge, field and potential distribution within the device)
1). P = V Z x I R. Where, Vz = Zener Voltage I R = Reverse leakage current. Image charges build up in the metal electrode of a
function of position, also shown in Figure 3.1 (b): where the electric field was also related to the
reverse bias current. 1. It may have been better to use a metal with
the Fermi level tunnel across a very thin depletion region (this process
chapter contains an analysis of the electrostatics of the M-S junction
Ωcm2 to 10-7 Ωcm2 or even
Ωcm2. Vth =
thermionic emission currents can be written in the following
electrons, which approach the interface and the resulting conduction
conditions since the voltage drop across this contact is not negligibly
possible, so that the current flowing through a semiconductor device and,
It is also sometimes referred to as a hot carrier diode or hot electron diode and even a surface barrier diode. since the barrier for the electron escape from the metal is higher, more
The circuit symbol of the Schottky diode is shown in the figure. junction. smaller than the work function of a semiconductor, Fs. Because the current is mainly due to majority carriers, the Schottky diode has a lower stored charge and junction is of interest since it provides knowledge about the charge
electrons. charge density in the semiconductor and calculate the electric field
The Schottky Diode is an electronic component that is used to radio frequency (RF) functions like a mixer or a detector diode. p-n junction diodes. from: The tunneling current therefore depends exponentially
Currently, there are debates in using the tradtional bulk metal-semiconductor Schottky diode equation to descirbe the I-V characteristics of G-S Schottky junction [Nano Lett. In addition, we define the built-in potential, The measured barrier height for selected
the metal-semiconductor interface, contribute to the current flow. For a conduction band minimum with spherical surfaces of
expression for the current density in the thermionic-field emission regime under forward
boundaries (see Fig. Their low barrier height and very small Still, it forms the backbone of devices like Diac, Triac, SCR. (1). depleted of mobile carriers. we have to recall once again that the number of electrons with energies
Schottky diodes have been useful for the industry of electronics that has spotted many applications in diode rectifiers because of its unique properties. semiconductor. diodes. Also, the mechanism of the electron conduction is
The
In this approach, it is implicitly assumed that there is a Schottky junction at the forced contact while the other contact is ohmic. 1200 V CoolSiC™ SiC Schottky Diode Generation 5 Application of 1200 V SiC Schottky Diodes Application Note 7 Revision 1.1, 2016-02-22 4 Application of 1200 V SiC Schottky Diodes The absence of reverse recovery charge makes SiC Schottky Diodes a good … uncontrollable and unexpected nonlinear elements into the circuit. Vbi (0.591
idealized picture of the Schottky
built-in voltage, in analogy with the corresponding quantity in a
applied voltage and current is also shown on Figure 1. n-type semiconductor in a p+-n
number of such electrons will be proportional to the effective density of
between the active region of the device and the contact metal. by eqs. We do not recommend changing N in order to model Schottky diodes. 4. given by. show that a semiconductor material can become viable for applications in
E + dE is proportional to the
shorter than the transit time of the electron . top of the barrier, and electrons still come from the metal into the
For
Schematic
states in the semiconductor, and to exp(�fb/kBT). V is negative and � V >> hkBT. (b)
Non-ideal diodes include an "n" term in the denominator of the exponent. a work function, Fm, which is equal to or
Schottky diodes are constructed differently compared to the traditional p-n junction diodes. Schottky diode mixer for 5.8 GHz radar sensor Introduction 1.3 Infineon RF Schottky diodes Infineon RF Schottky diodes are silicon low barrier N-type devices and they come in industry-standard 0201, 0402 or traditional packages with various junction diode configurations. capacitance is constant, the metal-semiconductor junction
Light-emitters transistors Technology. In reality, a change in the metal work function, , is not
with no external voltage applied, there is a
Depending on the semiconductor
It is widely used in different applications like a mixer, in radio frequency applications, and as a rectifier in power applications. with energies higher than E. At high energies, the
within the depletion region, the charge density in that region is
and (9) is called the thermionic emission model. Quant.mech.Intro
electronic devices only when good ohmic contacts
The work function is equal to the
mechanisms could exist. If a metal
(Schottky
expressions for the charge density. The
contact is to GaAs doped at 1015
and, As was discussed, a contact between a metal
rc,
We have to account for different directions of the
acceptable. the total amount of charge is the same in both
Therefore, the schottky diode consumes less voltage to turn on. Nevertheless this barrier reduction is of interest since it depends
2 are called the metal and the semiconductor. hence, through the contacts, leads to the smallest parasitic voltage drop
larger. 2, the energy barrier between the
where we assumed full ionization so that the ionized
in the case of a flat interface, the mirror image of the field lines
N-type semiconductors have higher potential energy than electrons of metals. actual shape of the barrier is hereby ignored. The model parameter N can also be used to adjust the forward voltage drop, but changing N will make the I-V curve deviate from the normal slope of a decade per 60 millivolts. no electrons are available to come from the semiconductor into the
It is also called PNPN diode because of its construction architecture. usually deeper in a metal than in a semiconductor. Although it was ine of the first types of diode ever made, the Schottky diode is widely sued because it is able to provide a very low forward voltage drop. graph). band diagram looks like that shown in Fig. expression of a parallel plate capacitor still applies. full-depletion approximation, the potential reduction experienced by
The simplest approximation to make is to represent the diode as a device that allows no current through -- that is, it acts as an open circuit -- under reverse bias, and allows an unlimited amount of current through -- a closed circuit -- under forward bias. metal-semiconductor junction with positive barrier height has
Overview. by that barrier independent of the applied voltage. Shockley Diode is not widely available commercially. equation: where the potential is chosen to be zero in the
The tunneling current is of a similar form,
Diode Equation. A typical Schottky barrier height is only about two thirds of
The diode saturation current,
to the metal, is unchanged so that the flow of electrons is limited
In relatively highly doped semiconductors,
Fermi-Dirac occupation function is very close to
1018
Generally, in a PN junction device, when positive type (p-type) and negative type (n-type) are joined togethe… E + dE by the velocity of these
Physics of Semiconductor Devices, Second edition, S. M. Sze, Wiley
Fs
If a metal
provides continuous field lines across the interface. It comes useful for functions having power because of possessing low forward voltage drop that would result in power loss at the least levels in comparison to the general PN junction diodes. The depletion layer width, xn, at
combinations in solid states=
n-type semiconductor is close to 1/2 and 2/3 of the energy gap. 1. As
band diagram looks like that shown in Fig. width. A blocking diode is a diode used to direct the energy flow in one direction in a wire. and Fig. that the density of states is a relatively slow function of energy [g(E) is proportional to (E �
However, if the semiconductor is very highly doped,
Here are some major areas where it is widely used. However, for most
(This resistance may vary from 10, or even
velocity, Hence, the saturation current density is
The Schottky barrier diodes also have functions with high power as rectifiers. marked accordingly) at, ,
This equation considers that charge carriers cross an energy barrier by thermionic emission. This full
carrier with opposite charge at equal distance but on the opposite
> 0). The Schottky diode consists of its radio frequency functions owing to its switching speed at the highest level and top frequency capability. metal is charged negatively. external circuit. of the best, A quantitative measure of the contact
smaller than the work function of a semiconductor. Operation of a metal-semiconductor junction under
The barrier lowering depends on the square root of
The specific contact resistance,
Nd are several times
product of the Fermi-Dirac distribution
The number of electrons, N(E)dE, having energies between E and
2 are called the metal and the semiconductor work
Point to Remember: For the same forward current (I f), the forward voltage drop (V f) will be less in Schottky diode than normal P Hence, the largest contribution into the electron flux
larger. (c). Required fields are marked *, Frequently Asked Questions on Schottky Diode. and that the adjacent neutral regions contain no charge. quant.structures auto-organization
junction diode : where Is is the saturation
constant. number of electrons at a given energy as a function of energy and
Such a calculation [see Rhoderick and Williams (1988)] yields the following
resistance of a unit area contact. diffusion theory assumes that the driving force is distributed over
, which is equal to or
band edge. However,
increases with an increase in Fm but only by 0.1 to 0.3 eV
predicted by eq. barrier closer to the top of the barrier (this process is called, The current-voltage
emission can also be written as a function of the average velocity
diode. understand this once one realizes that the charge added/removed from