Semiconductor Devices Like Diodes Biology Essay


To study zone depletion first of all we should know about semiconductor devices like diodes because we deal with such devices in zone depletion.

A diode, also called semiconductor device because this is the simplest sort of semiconductor device. And further, a semiconductor is a material with a varying ability to conduct electrical current. Most semiconductors are made of a poor conductor that has had impurities added to it. The process of adding impurities is called doping.

In the case of LEDs, the conductor material is typically aluminum-gallium-arsenide In pure aluminum-gallium-arsenide, all of the atoms bond perfectly to their neighbors, leaving no free electrons (negatively-charged particles) to conduct electric current. In doped material, additional atoms change the balance, either adding free electrons or creating holes where electrons can go. Either of these additions make the material more conductive.

A semiconductor with extra electrons is called N-type material, since it has extra negatively-charged particles. In N-type material, free electrons move from a negatively-charged area to a positively charged area.

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A semiconductor with extra holes is called P-type material, since it effectively has extra positively-charged particles. Electrons can jump from hole to hole, moving from a negatively-charged area to a positively-charged area. As a result, the holes themselves appear to move from a positively-charged area to a negatively-charged area.

A diode comprises a section of N-type material bonded to a section of P-type material, with electrodes on each end. This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the N-type material fill holes from the P-type material along the junction between the layers, forming a depletion zone which we are going to study in detail now. In a depletion zone, the semiconductor material is returned to its original insulating state -- all of the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow. Now let us study following that is detail explanation of the zone depletion formation ,formation of depletion width and then make some analysis on the same as described below under the contentes of the topic to study and we can easily describe because now we are familiar with the what are depletion zone or region and where to study it and further on knowing this that we will study it in semiconductor devices we also are familiar with diode which is a semiconductor device and after that we also know about the p and n type semiconductor which has got its very much role to play in zone depletion formation and before going to the detail on depletion zone let us take some abstract to make the understanding of the topic more easier to understand .


In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the charge carriers have diffused away, or have been forced away by an electric field.

The depletion region is so named because it is formed from a conducting region by removal of all free charge carriers, leaving none to carry a current. Understanding the depletion region is key to explaining modern semiconductor electronics: diodes, bipolar junction transistors, field-effect transistors, and variable capacitance diodes all rely on depletion region phenomena. Now after introduction and abstract of the topic let us proceed further towards the basic contents of the topic that are very necessary for the one complete knowledge of the zone depletion or depletion region or simply the same in case of semiconductor devices:

Formation of depletion zone or region in a pn junction

A depletion region forms spontaneously across a P-N junction. It is most easily described when the junction is in thermal equilibrium or in a steady state: in both of these cases the properties of the system do not vary in time; they have been called dynamic equilibrium., Electrons and holes diffuse into regions with lower concentrations of electrons and holes, much as ink diffuses into water until it is uniformly distributed. By definition, N-type semiconductor has an excess of free electrons compared to the P-type region, and P-type has an excess of holes compared to the N-type region. Therefore when N-doped and P-doped pieces of semiconductor are placed together to form a junction, electrons diffuse into the P-side and holes diffuse into the N-side. Departure of an electron on the N-side for the P-side leaves a positive donor ion behind on the N-side, and likewise the hole leaves a negative acceptor ion on the P-side. Following transfer, the injected electrons come into contact with holes on the P-side and are eliminated by recombination. Likewise for the injected holes on the N-side. The net result is the injected electrons and holes are gone, leaving behind the charged ions adjacent to the interface in a region with no mobile carriers (called the depletion region). The uncompensated ions are positive on the N side and negative on the P side. This creates an electric field that provides a force opposing the continued diffusion of charge carriers. When the electric field is sufficient to arrest further transfer of holes and electrons, the depletion region has reached its equilibrium dimensions. Integrating the electric field across the depletion region determines what is called the built-in voltage (also called the junction voltage or barrier voltage or contact potential).

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Mathematically speaking, charge transfer in semiconductor devices is due both to conduction driven by the electric field (drift) and by diffusion. For a P-type region, where holes conduct with electrical conductivity σ and diffuse with diffusion constant D, the net current density is given by

j = σ E - D ∇qp

with q the elementary charge (1.6Ã-10−19 coulomb) and p the hole density (number per unit volume). Conduction forces the holes along the direction of the electric field. Diffusion moves the carriers in the direction of decreasing concentration, so for holes a negative current results for a positive density gradient. (If the carriers are electrons, we replace the hole density p by the negative of the electron density n; in some cases, both electrons and holes must be included.) When the two current components balance, as in the pn-junction depletion region at dynamic equilibrium, the current is zero due to the Einstein relation, which relates D to σ.

(1) Under reverse bias (P negative with respect to N), the potential drop (i.e., voltage) across the depletion region increases. This widens the depletion region, which increases the drift component of current and decreases the diffusion component. In this case the net current is leftward in the figure of the pn junction. The carrier density then is small and only a very small reverse saturation current flows.

(2) Forward bias (P positive with respect to N) narrows the depletion region and lowers the barrier to carrier injection. The diffusion component of the current greatly increases and the drift component decreases. In this case the net current is rightward in the figure of the pn junction. The carrier density is large (it varies exponentially with the applied bias voltage), making the junction conductive and allowing a large forward current. The mathematical description of the current is provided by the Shockley diode equation. The low current conducted under reverse bias and the large current under forward bias is an example of rectification.

. Now we will in this particular section will going to study about the formation of depletion zone in mos capacitor which is also an important part specially when we will discuss about the zone depletion

Formation of depletion zone in an MOS capacitor

Metal-oxide-semiconductor structure on P-type silicon

Another example of a depletion region occurs in the MOS capacitor. It is shown in the figure to the right, for a P-type substrate. Suppose that the semiconductor initially is charge neutral, with the charge due to holes exactly balanced by the negative charge due to acceptor doping impurities. If a positive voltage now is applied to the gate, which is done by introducing positive charge Q to the gate, then some positively charged holes in the semiconductor nearest the gate are repelled by the positive charge on the gate, and exit the device through the bottom contact. They leave behind a depleted region that is insulating because no mobile holes remain; only the immobile, negatively charged acceptor impurities. The greater the positive charge placed on the gate, the more positive the applied gate voltage, and the more holes that leave the semiconductor surface, enlarging the depletion region. (In this device there is a limit to how wide the depletion width may become. It is set by the onset of an inversion layer of carriers in a thin layer, or channel, near the surface. The above discussion applies for positive voltages low enough that an inversion layer does not form.)

If the gate material is polysilicon of opposite type to the bulk semiconductor, then a spontaneous depletion region forms if the gate is electrically shorted to the substrate, in much the same manner as described for the pn-junction above.

Depletion width formation

Depletion width describes the width of the depletion zone or region in a semiconductor, particularly in geometries that are one-dimensional, like the pn-junction and MOS capacitor. The width of the depletion region is governed by the principle of charge neutrality. Two examples follow:

Depletion width in case of pn-junction

The principle of charge neutrality in this case relates the depletion width wP in the p-region with acceptor doping NA to the depletion width wN in the n-region with donor doping ND:


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This condition insures that the net negative acceptor charge exactly balances the net positive donor charge. The total depletion width in this case is the sum w = wN + wP.

Depletion width in case of MOS capacitor

Again, the governing principle is charge neutrality. Let us assume a P-type substrate. If positive charge Q is placed on the gate, then holes are depleted to a depth w sufficient to expose sufficient negative acceptors to exactly balance the gate charge. Supposing the dopant density to be NA acceptors per unit volume, then charge neutrality requires the depletion width w to satisfy the relationship:

If the depletion width becomes wide enough, then electrons appear in a very thin layer at the semiconductor-oxide interface, called an inversion layer because they are oppositely charged to the holes that prevail in a P-type material. When an inversion layer forms the depletion width ceases to expand with increase in gate charge Q. In this case neutrality is achieved by attracting more electrons into the inversion layer. In the MOSFET this inversion layer is referred to as the channel.

Electric field in depletion width and band bending

Associated with the depletion layer is an effect known as band bending. This occurs because the electric field in the depletion layer varies linearly in space from its (maximum) value Em at the gate to zero at the edge of the depletion width

where A is the gate area, ε0 = 8.854Ã-10−12 F/m, F is the farad and m is the meter. This linearly-varying electric field leads to an electrical potential that varies quadratically in space. The energy levels, or energy bands, bend in response to this potential.

IMPORTANT NOTE : At the junction, free electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle of the diode called the depletion zone as shown in above diagram.


HENCE IN ORDER TO get rid of the depletion zone, WE have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction. To do this, you connect the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears, and charge moves across the diode.


When the negative end of the circuit is hooked up to the N-type layer and the positive end is hooked up to P-type layer, electrons and holes start moving and the depletion zone disappears.

If you try to run current the other way, with the P-type side connected to the negative end of the circuit and the N-type side connected to the positive end, current will not flow. The negative electrons in the N-type material are attracted to the positive electrode. The positive holes in the P-type material are attracted to the negative electrode. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. The depletion zone increases.


When the positive end of the circuit is hooked up to the N-type layer and the negative end is hooked up to the P-type layer, free electrons collect on one end of the diode and holes collect on the other. The depletion zone gets bigger.

The interaction between electrons and holes in this setup has an interesting side effect -- it generates light! In the next section, we'll find out exactly why this is.


When P-type and N-type silicon are placed in contact with one another it forms a PN junction. At this junction an interesting phenomenon occurs, one that is the foundation of solid-state electronics.

A basic PN junction creates a diode that allows electricity to flow in one direction but not the other. We can see in the diagram of a diode that the N type material has free electrons shown as black dots and the P type material has holes shown as white dots.

Near the PN junction the electrons diffuse into the vacant holes in the P material causing a depletion zone. This depletion zone acts like an insulator preventing other free electrons in the N-type silicon and holes in the P-type silicon from combining.

In addition this leaves a small electrical imbalance inside the crystal. Since the N region is missing some electrons it has obtained a positive charge. And the extra electrons that filled the holes in the P region, have given it a negative charge. Unfortunately one cannot generate power from this electrical imbalance. However the stage is set to see how the PN junction functions as a diode.

In the next diagram we have connected an external power source; a battery with a light and current meter that indicate current flow. The negative terminal of the battery is connected to the N-type silicon. Like charges repel, so the free electrons are pushed toward the PN junction. Similarly the hole are repelled by the positive terminal of the battery toward the PN junction. If the voltage pushing the electrons and holes has sufficient strength to overcome the depletion zone (approximately 0.7 V for typical silicon diode) the electrons and holes combine at the junction and current passes through the diode. When a diode is arranged this way with a power supply it is said to be forward-biased.

In the last diagram the battery is connect to the diode so that the negative terminal of the battery connects to the P-type silicon and the positive terminal of the battery connects to the N-type silicon. The negative terminal attracts the positive holes in the P-type silicon and the positive terminal of the battery attracts the free electrons in the N-type silicon. All the charge carriers are pulled away from the PN junction which essentially creates a larger depletion region and no current flows. When a diode is arranged this way with a power supply it is said to be reverse-biased.


Once the two types of silicon are fabricated the free electrons in the N-type silicon are attracted by the positive charge in the P-type silicon.

At first there is little opposition to the movement of charge in the silicon.

When an electron & hole meet, they recombine (holes can move, but that gets complicated)

This results in a region either side of the junction (imagine a line down the middle of the silicon, inbetween the two regions) that is depleted of (moving) charges.

The region is usually called the Depletion Zone!

Once the depletion zone reaches a certain width something else happens;

1.Now when an electron enters the depletion zone the electric field within is sufficiently 'strong' enough to prevent the electron from crossing the junction.

2.In simple terms, the electron is 'pushed' back towards the side it approached from (the N-type side).

3.Now, this means that in order for an electron to cross the depletion zone it requires a certain amount of energy.

4.It is this requirement that gives rise to the forward voltage drop.

5.The electron needs to 'use up' 0.6V worth of energy. In order to do so it must already have at least 0.6V worth of energy.

6.This of course, applies to forward bias only.


Reverse Bias

1.In reverse bias the electrons are not driven towards the depletion zone, but pulled away (as are the holes).

2.This results in the widening of the depletion zone.

3.This makes it even harder for electrons to cross the zone.

4.If the reverse voltage is high enough the depletion zone 'breaks-down' & current flows. (in very simple terms)

5.If you're interested in this then the primary cause is the quantum TUNNELLING of electrons through the bandgap.


P-n junctions are formed by joining n-type and p-type semiconductor materials, as shown below. Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side. If the electrons and holes were not charged, this diffusion process would continue until the concentration of electrons and holes on the two sides were the same, as happens if two gasses come into contact with each other. However, in a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On the n-type side, positive ion cores are exposed. On the p-type side, negative ion cores are exposed. An electric field Ê forms between the positive ion cores in the n-type material and negative ion cores in the p-type material. This region is called the DEPLETION REGION since the electric field quickly sweeps free carriers out, hence the region is depleted of free carriers. A "built in" potential Vbi due to Ê is formed at the junction.

Carrier Movement in Equilibrium

A p-n junction with no external inputs represents an equilibrium between carrier generation, recombination, diffusion and drift in the presence of the electric field in the depletion region. Despite the presence of the electric field, which creates an impediment to the diffusion of carriers across the electric field, some carriers still cross the junction by diffusion. In the animation below, most majority carriers which enter the depletion region move back towards the region from which they originated. However, statistically some carriers will have a high velocity and travel in a sufficient net direction such that they cross the junction. Once a majority carrier crosses the junction, it becomes a minority carrier. It will continue to diffuse away from the junction and can travel a distance on average equal to the diffusion length before it recombines. The current caused by the diffusion of carriers across the junction is called a diffusion current. Remember that in an actual p-n junction the number and velocity of the carriers is much greater and that the number of carriers crossing the junction are much larger.

Minority carriers which reach the edge of the diffusion region are swept across it by the electric field in the depletion region. This current is called the drift current. In equilibrium the drift current is limited by the number of minority carriers which are thermally generated within a diffusion length of the junction.

In equilibrium, the net current from the device is zero. The electron drift current and the electron diffusion current exactly balance out (if they did not there would be a net buildup of electrons on either one side or the other of the device). Similarly, the hole drift current and the hole diffusion current also balance each other out.

Zone depletion in case of diode under forward biase

Forward bias refers to the application of voltage across the device such that the electric field at the junction is reduced. By applying a positive voltage to the p-type material and a negative voltage to the n-type material, an electric field with opposite direction to that in the depletion region is applied across the device. Since the resistivity of the depletion region is much higher than that in the remainder of the device (due to the limited number of carriers in the depletion region), nearly all of the applied electric field is dropped across the depletion region. The net electric field is the difference between the existing field in the depletion region and the applied field (for realistic devices, the built-in field is always larger than the applied field), thus reducing the net electric field in the depletion region. Reducing the electric field disturbs the equilibrium existing at the junction, reducing the barrier to the diffusion of carriers from one side of the junction to the other and increasing the diffusion current. While the diffusion current increases, the drift current remains essentially unchanged since it depends on the number of carriers generated within a diffusion length of the depletion region or in the depletion region itself. Since the depletion region is only reduced in width by a minor amount, the number of minority carriers swept across the junction is essentially unchanged.

Carrier Injection and Forward Bias Current Flow

The increased diffusion from one side of the junction to the other causes minority carrier injection at the edge of the depletion region. These carriers move away from the junction due to diffusion and will eventually recombine with a majority carrier. The majority carrier is supplied from the external circuit and hence a net current flows under forward bias. In the absence of recombination, the minority carrier concentration would reach a new, higher equilibrium concentration and the diffusion of carriers from one side of the junction to the other would cease, much the same as when two different gasses are introduced. Initially, gas molecules have a net movement from the high carrier concentration to the low carrier concentration region, but when a uniform concentration is reached, there is no longer a net gas molecule movement. In a semiconductor however, the injected minority carriers recombine and thus more carriers can diffuse across the junction. Consequently, the diffusion current which flows in forward bias is a recombination current. The higher the rate of recombination events, the greater the current which flows across the junction.

The "dark saturation current" (I0) is an extremely important parameter which differentiates one diode from another. I0 is a measure of the recombination in a device. A diode with a larger recombination will have a larger I0.

Zone depletion in case of diode under Reverse Biased

In reverse bias a voltage is applied across the device such that the electric field at the junction increases. The higher electric field in the depletion region decreases the probability that carriers can diffuse from one side of the junction to the other, hence the diffusion current decreases. As in forward bias, the drift current is limited by the number of minority carriers on either side of the p-n junction and is relatively unchanged by the increased electric field. A small increase in the drift current is experienced due to the small increase in the width of the depletion region


Semiconductors essentially serve as the basic building materials which are used to construct some very important electronic components. These semiconductor components are in turn used to construct electronic circuits and equipment. The three most commonly used semiconductor devices are Diodes, Transistors, and Integrated Circuits (IC's) however, other special components are also available.


Semiconductors essentially serve as the basic building materials which are used to construct some very important electronic components. These semiconductor components are in turn used to construct electronic circuits and equipment. The three most commonly used semiconductor devices are Diodes, Transistors, and Integrated Circuits (IC's) however, other special components are also available.


It is used most commonly in processors

also used in memory cards

used in digital analog systems also

batteries sensors, communication system ,ics in computer, radios ,tv ,video and almost everywhere in big or small electronics or electrical related industries found its applications or deal with the use of semiconductor devices only

hence we can say that semiconductor devices has almost or atmost found its application everywhere

It's where electronics begin. we cannot have any electronic device without having a chip inside it, and without the heros of this industry we would be far behind where we are in the digital world of today which has much use of semiconductor devices therefore semiconductor devices plays a very important role in our daily day to day life


Now here after whole discussion one question arises in our mind whether there are any advantage of using these semiconductor devices or not

although it is clear from the uses of semiconductor devices but yet let us discuss in detail also

Components which are made of semiconductor materials are often referred to as solid-state components because they are made from solid materials. Because of this solid-state construction, these components are more rugged than vacuum tubes which are made of glass, metal, and ceramic materials. Because of this ruggedness, semiconductor devices are able to operate under extremely hazardous environmental condi­tions. This ruggedness is responsible for the reliability of solid-state devices.


The solid-state construction also eliminates the need for filaments or heaters as found in all vacuum tubes. This means that additional power is not required to operate the filaments and component operation is cooler and more efficient. By eliminating the filaments, a prime source of trouble is also avoided because the filaments generally have a limited life expectancy. The absence of filaments also means that a warm-up period is not required before the device can operate properly. In other words, the solid-state component operates the instant it receives electrical power.


Solid state components are also able to operate with very low voltages (between 1 and 25 volts) while vacuum tubes usually require an operating voltage of 100 volts or more. This means that solid state components generally use less power than vacuum tubes and are, therefore, more suitable for use in portable equipment which obtains its power from batteries. The lower voltages are also much safer to work with. Pocket-size radios, hand held computers, TV's, DVD and MP3 players, and many other small battery operated devices are typical examples which take advantage of highly efficient, power saving components.


The small size of the solid-state component also makes it suitable for use in portable electronic equipment. Although equipment of this type can be constructed with vacuum tubes, such equipment would be much larger and heavier. A typical transistor is only a fraction of an inch high and wide while a vacuum tube of comparable performance may be an inch or more wide and several inches high. The small size also means a significant weight savings.


Solid-state components are much less expensive than comparable vac­uum tube components. The very nature of a solid-state component makes it suitable for production in mass quantities which brings about a high cost saving. In fact, a large number of solid-state components can be constructed as easily and quickly as a single component.


The most sophisticated semiconductor devices are Integrated Circuits. These are complete circuits where all of the components are constructed with semiconductor materials in a single micro-miniature package. These devices not only replace individual electronic circuits but also complete pieces of equipment or entire systems. Entire computers and radio receivers can be constructed as a single device no larger than a typical transistor. Integrated circuits have taken us one step farther in improving electronic equipment through the use of semiconductor materials. All electronic equipment has benefited from solid state components and particularly from the development of integrated circuits.

Now if we are talking about advantage than we cannot ignore disadvantage also


Although solid-state components have many advantages over the vacuum tubes that were once widely used, they also have several inherent disadvantages. First, solid-state components are highly sus­ceptible to changes in temperature and can be damaged if they are operated at extremely high temperatures. Additional components are often required simply for the purpose of stabilizing solid-state circuits so that they will operate over a wide temperature range. Solid-state components may be easily damaged by exceeding their power dissipa­tion limits and they may also be occasionally damaged when their normal operating voltages are reversed. In comparison, vacuum tube components are not nearly as sensitive to temperature changes or improper operating voltages.


There are still a few areas where semiconductor devices cannot replace tubes. This is particularly true in extremely high power and some ultra high radio frequency applications. However, as semiconductor technology develops, these limitations are gradually being overcome.


Despite the several disadvantages just mentioned, solid-state compo­nents are still the most efficient and reliable devices to be found. They are used in all new equipment designs and new applications are constantly being found for these devices in the military, industrial, and consumer fields. The continued use of semiconductor materials to construct new and better solid-state components is almost assured because the techniques used are constantly being refined thus making it possible to obtain even superior components at less cost.


Semiconductors have had a profound effect on the design and application of electronic equipment. Not only have they greatly improved existing equipment and techniques by making them better and cheaper, but also they have permitted us to do things that were not previously possible. Semiconductors have revolutionized the electronic industry and they continue to show their even greater potential. Your work in electronics will always involve semiconductor devices.