Welcome to Physics Fast, your number one source for all things related to science. We're dedicated to giving you the very best of science with a focus on quality and real-world problem solution
P-N Junction Diode : It is a two-terminal device consisting of a P-N junction formed either in Ge or Si crystal.Its circuit symbol is shown in the below fig. 1(a)
Construction of P-N Junction Diode
The P-type and N-type regions are referred to as anode and cathode respectively.In fig. 1 (b), arrowhead indicates the conventional direction of current flow when forward biased. It is the same direction in which hole flow takes place.
Commercially available diodes usually have some means to indicate which lead is P and which lead is N. Standard notation consists of type numbers preceded by ‘IN’ such as IN 240 and IN 1250. Here, 240 and 1250 correspond to colour bands.
Fig. 2(a) shows typical diodes having a variety of physical structures where as fig. 2(b) illustrates terminal identifications.
Also refer to the picture of two commercial diodes shown in fig 1(c).
The low-current diodes whose body is about 3 mm long can carry a forward current of about 100 mA, have saturation current of 5 µA at room temperature (25°C) and can withstand a reverse voltage of 75 V without breaking down. The medium-current diodes can pass a forward current of about 500 mA and can withstand a reverse voltage of 250 V. The high-current diodes or power diodes can pass a forward current of many amperes and can survive several hundred volts of reverse voltage.
Low and medium-current diodes are usually mounted by soldering their leads to the connecting terminals. The heat generated by these diodes (when operating) is small enough to be carried away by air convection and conduction along the connecting leads. However. high-current stud-mounted diodes generate large amounts of heat for which air convection is totally inadequate. For coolong, they need heat sinks made of metals such as copper or aluminium which are good conductors of heat. The sink absorbs heat from the device and then transfers it to the surrounding air by convection and radiation since it has large surface area.
Working of P-N Junction Diode
A P-N junction diode is one-way device offering low resistance when forward-biased [Fig. 3 (a)) and behaving almost as an insulator when reverse-biased (Fig. 3 (b)]. Hence, such diodes are mostly used as rectifiers i.e. for converting alternating current into direct current.
V/I Characteristic P-N Junction Diode
Fig. 4 shows the static voltage-current characteristics for a low-power P-N junction diode.
When the diode is forward-biased and the applied voltage is increased from zero, hardly any current flows through the device in the beginning. It is so because the external voltage is being opposed by the internal barrier voltage VB , whose value is 0.7 V for Si and 0.3 V for Ge. As soon as VB is neutralized, current through the diode increases rapidly with increasing applied battery voltage. It is found that as little a voltage as 1.0 V produces a forward current of about 50 mA. A burnout is likely to occur if forward voltage is increased beyond a certain safe limit.
When the diode is reverse-biased, majority carriers are blocked and only a small current (due to minority carriers) flows through the diode. As the reverse voltage is increased from zero, the reverse current very quickly reaches its maximum or saturation value I, which is also known as leakage current. It is of the order of nanoamperes (nA) for Si and microamperes (uA) for Ge. The value of , (or 1,) is independent of the applied reverse voltage but depends on :
(b) Degree of doping
(c) Physical size of the junction
As seen from Fig. 52.4, when reverse voltage exceeds a certain value called break-down voltage Vn (or Zener voltage V.), the leakage current suddenly and sharply increases, the curve indicating zero resistance at this point. Any further increase in voltage is likely to produce burnout unless protected by a current-limiting resistor. When P-N junction diodes are employed primarily because of this breakdown property as voltage regulators, they are called Zener diodes (Art. 54.1).
Equation of the Static Characteristic of P-N Junction Diode
The volt-ampere characteristics described above are called static characteristics because they describe the d.c. behaviour of the diode. The forward and reverse characteristics have been combined into a single diagram of Fig. 4.
These characteristics can be described by the analytical equation called Boltzmann diode equation given below :
I0 = diode reverse saturation current
V = voltage across junction – positive for forward bias and negative for reverse bias.
k = Boltzmann constant =
T = crystal temperature in
= 1 (for germanium)
= 2 (for silicon)
hence, the above diode equation becomes
Now,= 11600 and putting , the above equation may be written as
Now at room temperature of (273+20)=, . Substituting the value of , we have
We may also write the above diode equation as under
The diode parameters of greatest interest are as under :
Bulk resistance (rg)
It is the sum of the resistance values of the P-and N-type semiconductor materials of which the diode is made of.
Usually, it is very small. It is given by
It is the resistance offered by the diode well above the barrier voltage i.e. when current is large. Obviously, this resistance is offered in the forward direction.
Junction resistance (r.)
Its value for forward-biased junction depends on the magnitude of forward de current.
Obviously, it is a variable resistance.
Dynamic or ac resistance
For large values öf forward current,, is negligible. Hence, . For small values of , is negligible as compared to , Hence .
Forward voltage drop
It is given by the relation
Reverse saturation current (I)
It has already been discussed before.
Reverse breakdown voltage (Vun)
It is discussed before. reverse voltage
Reverse de resistance
The main applications of semiconductor diodes in modern electronic circuitry are as under:
- As power or rectifier diodes. They convert ac current into de current for de power supplies of electronic circuits.
- As signal diodes in communication circuits for modulation and demodulation of small signals.
- As Zener diodes in voltage stabilizing circuits.
- As varactor diodes-for use in voltage-controlled tuning circuits as may be found in radio and TV receivers. For this purpose, the diode is deliberately made to have a certain range of junction capacitance. The capacitance of the reverse-biased diode is given by . Whwre is the reverse voltage.
Derivation of Junction Resistance of P-N Junction Diode
Junction resistance is also known as incremental or dynamic resistance and is an important parameter particularly in connection with small-signal operations of the diode.
(a) Reverse bias of P-N Junction Diode
When reverse bias is greater than a few tenths of a volt i.e. when, then , is extremely small so that is very large. That high value is also represented by .
(b) Forward bias of P-N Junction Diode
Again, for a forward bias greater than a few tenths of a volt,
Junction Breakdown of P-N Junction Diode
If the reverse bias applied to a P-N junction is increased, a point is reached when the junction breaks down and reverse current rises sharply to a value limited only by the external resistance connected in series with the junetion (Fig. 7). This critical value of the voltage is known as breakdown voltage (VR. Is found that once breakdown bas occured, very little further increase in voltage is required to increase the current to rela- tively high values. The junction itself offers almost zero resistance at this point. 2066 The breakdown voltage depends on the width of the depletion region which, in turn, depends on the doping levei. The following two mechanism are responsible for breakdown under increasing reverse voltage:
Zener Breakdown of P-N Junction Diode
This form of breakdown occurs in junctions which, being heavily doped, have narrow depletion layers. The breakdown voltage sets up a very strong electrie field (about 10* V/m) across this narrow layer. This field is strong enough to break or rupture the covalent bonds thereby generating electron-hole pairs. Even a small further increase in reverse voltage is capable of producing large number of current carriers. That is why the junction has very low resistance in the break-down region.
Avalanche Breakdown of P-N Junction Diode
This form of breakdown occurs in junctions which being lightly-doped, have wide depletion layers where the electric field is not strong enough to produce Zener breakdown. Instead, the minority carriers (accelerated by this field) collide with the semiconductor atoms in the depletion region. Upon collision with valence electrons, covalent bonds are broken and electron-hole pairs are generated. These newly-generated charge carriers are also accelerated by the electric field resulting in more collisions and hence further production of charge cariers. This leads to an avalanche (or flood) of charge carriers and, consequently, to a very low reverse resistance. The two breakdown phenomena are shown in Fig. 7
Junction Capacitance of P-N Junction Diode
Capacitive effects are exhibited by P-N junctions when they are either forward-biased or reverse- biased.
(a) Transition Capacitance (C) or Space-charge Capacitance
When a P-N junction is reverse-biased, the depletion region acts like an insulator or as a dielectric material essential for making a capacitor. The P- and N-type regions on either side have low resistance and act as the plates. We, therefore, have all the components necessary for making a parallel-plate capacitor. This junction capacitance is called transition or space charge capacitance (C or C). may be calculated by the usual formula C = EAld. Its typical value is 40 pE. Since thickness of depletion (or transition) layer depends on the amount of reverse bias, capacitance Cy can be controlled with the help of applied bias. This property of variable capacitance possessed by a reverse- biased P-N junction is used in the construction of a device known as varicap or varactor This capacitance is voltage dependent as given by the relation K (V +VR)” V. = knee voltage: K = constant depending on semiconductor material where V = applied reverse voltage 5- for alloy junction and = -for diffused junction The voltage-variance capacitance of a reverse-biased P-N junction is used in many circuits one of which is automatic frequency control (AFC) in an FM tuner. Other applications include self-balancing bridge circuits, special type of amplifiers known as parametric amplifiers and electronic tuners in TV.
When used in such a role, the diodes are called varactors, varicaps or voltacups. The symbol of these diodes when used in this role is shown in Fig.8 along with its equivalent circuit. When used in a resonant circuit, varactor acts as a variable capacitor and allows the resonant frequency to be adjusted by a variable voltage level. In Fig. 8, two caractors have been used to provide total variable capacitance in a parallel resonant circuit. Here V, is a variable de voltage that controls the reverse bias and hence the capacitance of the diodes.
(b) Diffusion or Storage Capacitance (C,)
This capacitive effect is present when the junction is forward-biased. It is called diffusion capacitance to account for the time delay in moving charges across the junction by diffusion process. Due to this fact, this capacitance cannot be identified in terms of a dielectric and plates. It varies directly with the magnitude of forward current as explained below in more details. Consider a forward-biased junction which is carrying a forward current I Suppose the applied voltage is suddently reversed, then /, ceases suddenly but leaves lot of majority charge carriers in the depletion region. These charge carriers must get out of the region which, to their bad luck, becomes wider under the reverse bias. Hence, it is seen that when a forward-biased P-N junction is suddenly reverse-biased, a reverse current flows which is large initially but gradually decreases to the level of saturation current I. This effect can be likened to the discharging current of a capacitor and is, therefore, rightly represented by a capacitance called diffusion capacitance Cp. Since the number of charge carriers left in depletion layer is proportional to forward current, C, is direcly proportional to Ip Its typical value is 0.02 uF which is 5000 times Cy The capacitance assumes great significance in the operation of devices which are required to switch rapidly from forward to reverse bias. If C, is large, this switchover cannot be rapid. It will delay both the switch-on and the switch-off. This effect of C, is variously known as recovery time or carrier storage. In the case of forward bias, the diode current is almost entirely due to diffusion (drift current being negligible).
Equivalent Circuit of a of P-N Junction Diode
We have seen from above that a forward-biased junction offers ac resistance rac and possesses diffusion capacitance C, (which comes into the picture only when frequency of the applied voltage is very high). Hence, it can be represented by the equivalent circuit of Fig. 10 (a). An opposing battery has been connected in series with r to account for the junction barrier potential. As seen from Fig. 10 (b), a reverse-biased junction can be simply represented by a reverse resistance Rp connected in parallel with a capacitance C, or Cp
The electrical characteristics of a semiconductor diode depend on two factors (i) the material employed and (ii) the type of P-N junction used. The two most commonly-used materials are germanium (Ge) and silicon (Si). Since Ge has higher electrical conduc- tion than Si, it is often used in low- and medium-power di- odes. On the other hand, Si has been found more suitable for high-power applications because it can be operated at higher temperatures than Ge. A new material called gallium-arsenide (GaAs) is found to combine desirable features of both Ge and Si and is finding ever-increasing use in many new applica- tions. The P-N junction may be produced by any one of the following methods :
1. grown junction
2. alloy junction
3. epitaxial growth
4. diffused junction
5. point contact junction.
The first step in the manufacture of any semiconductor device is to obtain the semiconductor material in an extremely pure form. The accepted impurity level is less than one part of impurity in one billion (10) parts of the semiconductor material!. To begin with, the raw material is subjected to a series of chemical reactions and then to a zone refining process which employs induction heating to reduce the impurity level of the polycrystalline structure. Next, the Czochralski or floating zone technique is used to form single crystals of Ge or Si for fabrication of diodes. These crystals are then cut into wafers as thin as 0.025 mm (nearly one fourth the thickness of this paper). Now, we will briefly discuss the four basic processes commonly used in the manufacture of semiconductor diodes.
Such junctions are produced by employing either the Czochralski or floating zone technique. The apparatus used for Czochralski technique is shown in Fig.12. A single crystal seed of the desired impurity level is immersed in the molten semiconductor material contained in a crucible. Then, it is gradually withdrawn while the shaft holding the seed is slowly turning. When crystal is being pulled out, impurities of P- and N-type are alternately added to produce a P-N junction. This large area crystal is then cut into a large number of smaller-area diodes.
The alloy process produces junction diodes that have high PIV and current ratings but which have large junction capacitance due to their large junction area. In this process, a tiny dot (or pellet) of indium (or any other P-type impurity) is placed on the surface of an N-type silicon wafer and the two are heated well above the melting temperature of indium (about 150°C) as shown in Fig. 12 (a). Consequently, indium melts and dissolves some of the silicon. The temperature is then lowered and silicon refreezes to form a single crystal having a P-N junction as shown in Fig. 12. (b).
The diffusion process employs either solid or gas- cous diffusion. This process takes more time than al- loy process but is relatively cheaper and more accu- rately controllable. In this process, particles from an area of high concentration drift to surrounding region of lesser concentration.
The solid diffusion process starts with the ‘painting’ of a P-type impurity (say, indium) on an N- type substrate and heating the two until the impurity (say, indium) on an N-type substrate and heating the two until the impurity diffuses a short distance into the substrate to form P-type layer (Fig. 14).
In the gaseous diffusion process, an N-type material is heated in a chamber containing a high concentration of an acceptor impurity in vapour form (Fig. 15). Some of the acceptor atoms are diffused (or absorbed) into the N-type substrate to form the P-type layer thus creating a P-N junction By exposing only part of the N-type material during the diffusion process (the remainder being covered by a thin coating of SiO2,), the size of the P-region can be controlled. Finally, metal contacts are electroplated on the surface of each region for connecting the leads.
The diffusion technique enables simultaneous fabrication of many hundreds of diodes on one small dise of a semiconductor material. That is why it is the most frequently-used technique not only for the manufacture of semi-conductor diodes but also for the production of transistors and integrated circuits etc.
This junction differs from the diffusion junction only in the manner in which the junction is fabricated. Such junctions are grown on top of an N-type wafer in a high temperature chamber. The growth proceeds atom by atom and hence is exactly similar to the crystal lattice of the wafer on which it is grown. Such junctions have the advantage of low resistance.
Point Contact Junction
It consists of an N-type germanium or silicon wafer about 1.25 mm square by 0.5 mm thick, one face of which is soldered to a metal base by radio-frequency heating as shown in Fig. 16 (a). The other face has a phosphor bronze (or tungsten) spring (called a cat’s whisker) pressed against it. The P-N junction is formed by passing a large current for a second or two through the wire while the crystal face with wire point is kept positive. The heat so produced drives away some of the electrons from the atoms in the small region around the point of contact thereby leaving holes behind. This small region of the N-type material is, consequently, converted into P-type material as shown in Fig. 16 (a). The small area of the P-N junction results in very low junction capacitance as mentioned earlier.
The ldeal Diode
There is no such thing as an ideal diode or perfect diode. The existence of such a diode is visualized simply as an aid in analysing the diode circuits. An ideal diode may be defined as a two-terminal device which (a) conducts with zero resistance when forward-biased, and (b) appears as an infinite resistance when reverse-biased. In other words, such a device acts as a short-circuit in the forward direction and as an open-circuit in the reverse direction as shown in Fig.17. Also, in the forward direction, there is no voltage drop (even though current is there) since a short has zero resistance, On the other hand, there is no reverse current because reverse resistance is infinite. It is helpful to think of an ideal diode as a bi-stable switch which is closed in the forward direction and open in the reverse direction. Hence, it has two stable states : ON or OFF.
The Real Diode
A real diode neither conducts in the forward direction with zero resistance nor it offers infinite resistance in the reverse direction.
In this case, we have to take two factors into account. One is that forward current does not start flowing until the voltage applied to the diode exceeds its threshold or knee voltage V (0.3 V for Ge and 0.7 V for Si). Hence, a real diode is shown as equivalent to an ideal diode in series with a small oppositely-connected battery of e.m.f. V as shown in Fig. 25 (a). The second factor to be considered is the forward dynamic or ac resistance (rac) offered by the circuit. So far, we considered the resistance to be zero implying that forward characteristic is a straight vertical line [25 (a)). If we take into account, the forward characteristic becomes as shown in Fig. 25 (b), Here, the reciprocal of the slope of this characteristic represents r.
(i) Large Signal Operation Large signal sources are those whose voltage is much greater than the diode knee voltage VK (nearly equal to barrier potential VB). Under such conditions, forward current would be large, so that would be negligible,
(ii) Small Signal Operation In this case, the signal voltage is much smaller than Vk (0.3 V for Ge and 0,7V for Si). Since If would be small, r, would be very large as compared to rB.
An actual or real diode does not have infinite resistance in the reverse direction because it wilI always have some reverse saturation current prior to breakdown. For example, if with a, is 10 uA, then . Silicon diodes have reverse resistance of many thousands of megaohms. Hence, an actual diode in the reverse direction can be thought of as equivalent to a mg” resistor. This would be true only in the case of signals of low frequencies. For high-frequency signals we will have to take into account the capacitive effects .
Diode Circuits with DC and AC Voltage Sources
We will often come across diode and transistor circuits which will contain both dc and ac voltage sources. Such circuits can be easily analysed by using Superposition Theorem. We will first draw the de equivalent circuit while neglecting ac-sources and find the required current and voltage values. Next, we will draw the ac equivalent circuit while neglecting the dc sources and again find the voltage and current values. Finally, we will superimpose the two sets of values to get the final result. While drawing the equivalent circuits, following points must be remembered :
- Direct current cannot flow through’ a capacitor. Hence, all capacitors look like an open switch to a dc source.
2. Usually, capacitors used in most circuits are large enough to look like a short to an ac source particularly one of very high frequency. Similarly, de batteries would also act as short circuits unless they have very high internal resistances.
DC Equivalent Circuit of P-N Junction Diode
It is seen from Fig. 26 that the circuit to the left of point A is ‘open’ to the de source of 20 V because of capacitor C. Hence, the equivalent dc circuit is as shown in Fig. 29 (a). As seen, diode is forward-biased by the battery. Hence, only 0.7 V de appear across the diode. The dc current I = (20 – 0.7)/20 K = 1 mA.
AC Equivalent Circuit of P-N Junction Diode
Here, the capacitor C and the 20-V battery would be treated as shorts thereby giving us the ac equivalent circuit of Fig. 29 (b). Since, it is a silicon diode
As shown in Fig. 29 (b), so far as the signal source is concerned, 20 K resistor and ac resistance of the diode are connected in parallel at point A. Now, 20 K ||. Hence, 1 K and 51 Q are put in series across the signal source of peak value 10 mV. The peak value of the ac voltage drop over 51 2 resistance is
The total drop across the diode is the sum of the ac and dc drops. The combined voltage wave- form is shown in Fig. 52.29 (c). It consists of a de voltage of 0.7 V over which rides an ac voltage of peak value ± 0.48 mV.