BASIC DIODE ELECTRONICS INTRODUCTION TO DIODES The p-n Junction The p-n junction is a homojunction between a p-type and an n-type semiconductor. It acts as a diode, which can serve in electronics as a rectifier, logic gate, voltage regulator (Zener diode), switching or tuner (varactor diode); and in optoelectronics as a light-emitting diode (LED), laser diode, photodetector, or solar cell. In a relatively simplified view of semiconductor materials, we can envision a semiconductor as having two types of charge carriers-holes and free electrons which travel in opposite directions when the semiconductor is subject to an external electric field, giving rise to a net flow of current in the direction of the electric field. Figure 1 illustrates the concept. A p-n junction consists of a p-type and n-type section of the same semiconductor materials in metallurgical contact. The p-type region has an abundance of holes (majority carriers) and a few mobile electrons (minority carriers); the n-type region has an abundance of mobile electrons and a few holes (Fig. 2). Both charge carriers are in continuous random thermal motion in all directions.

Fig. 2. Energy levels and carrier concentrations for a p-type and n-type semiconductor before contact.

1

When a section of p-type material and a section of n-type material are brought in contact to form a pn junction, a number of interesting properties arise. The pn junction forms the basis of the semiconductor diode.

Electrons and holes diffuse from areas of high concentration toward areas of low concentration. Thus, electrons diffuse from the n-region to the p-region., leaving behind positively charged ionized donor atoms. In the p-region the electrons recombine with the abundant holes. Similarly, holes diffuse from the p-region into the n-region, leaving behind negatively charged ionized acceptor atoms. In the n-region the holes recombine with the abundant mobile electrons. This diffusion process does not continue indefinitely, however, because it causes a disruption of the charge balance in the two regions.

As a result, a narrow region on both sides of the junction becomes nearly depleted of the mobile charge carriers. This region is called the depletion layer. It contains only the fixed charges (positive ions on the n-side and negative ions on the p-side). The thickness of the depletion layer in each region is inversely proportional to the concentration of dopants in the region. The net effect is that, the depletion region sees a separation of charge, giving rise to an electric field pointing from the n side to the p side.

The fixed charges create an electric field in the depletion layer that points from the n-side towards the p-side of the junction. The charge separation therefore causes a contact potential (also known as built-in potential) to exist at the junction. This built-in field obstructs the diffusion of further mobile carriers through the junction region.

An equilibrium condition is established that results in a net contact potential difference Vo between the two sides of the depletion layer, with the n-side exhibiting a higher potential than the p-side. This contact potential is typically on the order of a few tenths of a volt and depends on the material (about 0.5 to 0.7 V for silicon).

The built-in potential provides a lower potential energy for an electron on the n-side relative to the p-side. As a result, the energy bands bend as shown in Fig. 3. In thermal equilibrium there is only a single Fermi function for the entire structure so that the Fermi levels in the p- and the n-regions must align.

No net current flows across the junction. The currents associated with the diffusion and built-in field (drift current) cancel for both the electrons and holes.

Fig. 3. A p-n junction in the Thermal equilibrium at T > 0º K. The depletion-layer, energy-band diagram, and concentrations (on a logarithmic scale) of the mobile electrons n(x) and holes p(x) are shown as a functions

of the position x. The built-in potential difference V0 corresponds to the energy eV0

where e is the electron charge.

2

The Biased p-n Junction An externally applied potential will alter the potential difference between the p- and n-regions. This in turn will modify the flow of majority carriers, so that the junction can be used as a “gate”. If the junction is forward biased by applying a positive voltage V to the p-region (Fig. 4), its potential is increased with respect to the n-region, so that an electric field is produced in a direction opposite to that of the built-in field. The presence of the external bias voltage causes a departure from equilibrium and a misalignment of the Fermi levels in the p- and n-regions, as well as in the depletion layer. The presence of the two Fermi levels in the depletion layer, Efc and Efv represents a state of quasi-equilibrium.

Fig. 4. Energy band diagram and carrier concentrations for a forward-biased p-n junction.

In effect, then, if one were to connect the two terminals of the p-n junction to form a closed circuit, two currents would be present. First, a small current, called reverse saturation current, is, exists because of the presence of the contact potential and the associated electric field. In addition, it also happens that holes and free electrons with sufficient thermal energy can cross the junction. This current across the junction flows opposite to the reverse saturation current and is called diffusion current. Of course, if a hole from the p side enters, it is quite likely that it will quickly recombine with one of the n-type carriers on the n side. (Fig. 4) The net effect of the forward bias is to reduce the height of the potential-energy hill by an amount eV. The majority carrier current turns out to increase by an exponential factor exp(eV/kT). So that the net current becomes i = isexp(eV/kT) – is, where is is nearly a constant. The excess majority carrier holes and electrons that enter the n and p regions, respectively, become minority carriers and recombine with the local majority carriers. Their concentration therefore decreases with the distance from the junction as shown in Fig. 4. This process is known as minority carrier injection. If the junction is reversed biased by applying a negative voltage V to the p-region, the height of the potential energy hill is augmented by eV. This impedes the flow of majority carriers. The corresponding current is multiplied by the exponential factor exp(eV/kT) where V is negative; i.e. it is reduced. The net result for the current is i = is exp(eV/kT) - is, so that a small current of magnitude ≈ is flows in the reverse direction when IVI » kT/c. A p-n junction therefore acts as a diode with a current-voltage (i-V) characteristic as illustrated.

⎥⎦

⎤⎢⎣

⎡−⎟

⎠⎞

⎜⎝⎛= 1exp

kTeVii s Ideal Diode Characteristic

3

The ideal diode characteristic equation is known as the Shockley equation, or simply the diode equation. Figure 5 depicts the real diode i-V characteristic for a fairly typical silicon diode for positive diode voltages. Since the reverse saturation current, is is typically very small (10-9 to 10-15 A), the expression

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛=

kTeVii s exp

is a good approximation if the diode voltage V is greater than a few tenths of a volt.

Fig. 5. i -V characteristic of a real p-n junction diode. Note that the conduction takes off for voltages larger than the contact potential.

Fig. 6 summarizes the behaviour of the semiconductor diode in terms of its i-V characteristic. Note that a third region appears in the diode i-V curve that has not been discussed yet. The reverse-breakdown region to the far left (typically > 50V) of the curve represents the behavior of the diode when a sufficiently high reverse bias is applied. Under such a large reverse bias, the diode conducts current again, this time in the reverse direction. To explain the mechanism of reverse conduction, one needs to visualize the phenomenon of avalanche breakdown. When a very large negative bias is applied to the p-n junction, sufficient energy is imparted to charge carriers that reverse current can flow, well beyond the normal reverse, saturation current. In addition, because of the large electric field, electrons are energized to such levels that if they collide with other charge carriers at a lower energy level, some of their energy is transferred to the carriers with low energy,

4

and these can now contribute to the reverse conduction process, as well. This process is called impact ionization. Now, these new carriers may also have enough energy to energize other low-energy electrons by impact ionization, so that once a sufficiently high reverse bias is provided, this process of conduction takes place very much like an avalanche: a single electron can ionize several others.

Fig. 6. The reverse breakdown region The phenomenon of Zener breakdown is related to avalanche breakdown. It is usually achieved by means of heavily doped regions in the neighbourhood of the metal-semiconductor junction (the ohmic contact) .The high density of charge carriers provides the means for a substantial reverse breakdown current to be sustained at a much lower specific voltage than normal diode, at a nearly constant reverse bias known as the Zener voltage, Vz. This phenomenon is very useful in applications where one would like to hold some load voltage constant for example, in voltage regulators. The response time of a p-n junction to a dynamic (ac) applied voltage is determined by solving the set of differential equations governing the processes of electrons and hole diffusion, drift (under the influence of the built-in and external electric fields), and recombination. These effects are important for determining the speed at which the diode can be operated. They may be conveniently modeled by two capacitances, a junction capacitance and diffusion capacitance, in parallel with an ideal diode. The junction capacitance for the time necessary to change the fixed positive and negative charges stored in the depletion layer when the applied voltage changes. The thickness l of the depletion layer turns out to be proportional to √(Vo-V); it therefore increases under the reverse-bias conditions (negative V) and decreases under the forward-bias conditions (positive V). The junction capacitance C=ЄA/l (where A is the area of the junction) is therefore inversely proportional to √(Vo-V). The junction capacitance of a reverse-biased diode is smaller (and the RC response time is therefore shorter) than that of a forward-biased diode. The dependence of C on V is used to make voltage-variable capacitors (varactors).

5

Experiment l(a) : i-v characteristics of a semiconductor diode Procedure Connect the diode according to the circuit diagram as shown in Fig.8.

Fig 8 Vary the voltage V on the power supply between 0-30V. The current i may be deduced using

RVi R= , converting the recorded voltage VR in the Y-input into current scale. The voltage across

the diode is recorded as the X-input as V. Use suitable input scales and calibrate the scale on the recording sheet. Carry out the recording for both forward and reversed bias characteristics. Calculation Do a curve fitting using the diode equation. Note down the contact potential. Experiment l(b): i-v characteristics of a Zener dioide(5.6V). Procedure Same as in Experiment l(a) . Calculation Just note down the Zener breakdown voltage. Curve fitting is not required.

6

EXPERIMENT 2 RECTIFYING AND FILTER CIRCUITS Objective To study the operating principles and performance of various rectifying circuits and filters. Introduction Many electronic circuits require d.c. power. But our power lines are all a.c. So a way to change one to the other is necessary. Because of their property of unidirectional conduction, pn-junction diodes can be used to convert a.c. to d.c. To most common circuits are the full-wave and half-wave rectifiers. Their circuit are as follows.

7

and the waveform appearing across RL are FILTERING The rectified signal may be thought of as a combination of a fixed (d.c.) voltage and a superimposed alternating voltage. The purpose of the filtering stage is to remove the remaining alternating portion of the signal. The principle of operation of all filters may be thought of as either or both of two concepts. The first is that, by offering a low impedance path to ground for the a.c. portion of the signal while maintaining a higher resistance path for the d.c. portion, the a.c. is removed. This is. accomplished by a series resistor and capacitor to ground. Alternately, the second concept is that the blocking action of an inductor stops the a.c. portion while the d.c. portion passes without much attenuation.

Note: For filtering, large capacitance (hundreds to tens of hundreds microfarad) is needed. These are generally electrolytic capacitors, which consist of a repeating sandwich of aluminum sheets and a conducting paste, rolled into a cylinder for miminmun size. The aluminum sheets are polarized to form thin layers of aluminum oxide, a dielectric insulating material. The thinner the the dielectric the higher the capacitance will be. The voltage polarity of the electrolytic capacitance must be observed. The polarity is indicated on the capacitor itself. If the incorrect polarity is used the oxide will be reduced and conduction (shorting) will occur between the plates. Since this type of capacitor is sealed to keep the conducting paste from drying out, explosion may result from wrong polarity.

The most common filter is the RC type, shown in Figure 5(a). The resistor represents the load on the power supply. The value of the capacitance is chosen so that the impendance Xc « RL. For example, if RL = 10 KΩ, then a typical value of Xc is 100.0. Since the frequency of the full wave rectified signal is 100 Hz (not 50 Hz!), the capacitance needed is

uf16)100)(100)(2(

1Xf2

1Cc

=π

=π

=

8

Figure 5(b) shows the output of the circuit. In the first cycle, the capacitor is charged to the peak value, and then begins to decay at a slower rate governed by the RC time constant. In the next cycle the charge on the capacitor is restored to its peak value, before it has decreased appreciably. If most of the alternating portion of the signal is to be removed, the RC time constant must be very long compared to the period of the signal. In this example, RC = 0.16 second, while the period is 1/100 Hz = 0.01 second; correct operation is thus ensured. With the exception of the first cycle, the output waveform is a good approximation to a triangular wave. Thus, it is possible to calculate the Fourier series for it. Analysis of the Fourier series for this waveform is useful in evaluating the magnitudes of d.c. voltage, the fundamental a.c. wave, and its higher harmonics. This is of particular interest in filtering, where the value of the d.c. portion and the amount of a.c. signal remaining after filtering are important design considerations. Fourier analysis of a half wave rectified waveform is V = 0.318 Vo + 0.5 Vo sinωt - 0.212 Vo cos2 t – ω KK

and for a full wave rectified waveform it is V = 0.636 Vo - 0.424 Vo cos2ωt - 0.085 Vo cos4 t + ω KK When Vo is the peak value of the half waveform, Vo = 1.4Vrms. Note: To observe the accurate waveform of the rectified signal on an oscilloscope, use the input marked DC. The input marked AC has an input coupling capacitor. It will alter the profile of the signal due to the charging and discharging.

It serves to block the DC component when desires.

C

R

AC

DC

Y-Amplifier

9

Note that there is a phase shift between the peak of the ripple voltage and the peak of the sinusoidal wave. The ripple factor, r, is defined as the ratio of the rms ripple voltage(Vac) to the background dc voltage(Vdc)

dc

ac V V

r = ____ where Vac = RMS of 1 Vripple (peak to peak) __

2

The complete mathematical development of expressions for this quantity is given in many texts. Our interest is only in the results of this process. For the RC filter introduced earlier, these results are

r = CRf3

1

L

and

CRf11

VV

L

pdc

+

=

10

4

4

where Vp is the rms voltage of the transformer output

These equations can be simplified by inserting the frequency of the a.c. signal. For all full wave rectified situations, the ripple frequency is twice the line frequency, or 100 Hz. Then and

and

CR11

VpV

CR1r

L

dc

L

+=

=

(2)

If the load current is zero (RL→∞ ), it will be seen from equation (2) that the ripple factor is zero arid Vdc →Vp. If the current increases (which is equivalent to a decrease in the load resistance), the ripple factor will increase and Vdc will decrease. This suggests that the filter lacks regulation in situations where the load current varies considerably. Let us rewrite equations 2, using the fact that Idc ≅ Vdc/RL:

C400I

VV dcpdc −=

From this equation, two facts are clear: the output voltage decreases linearly with increasing current through the load, and the magnitude of the slope of that variation decreases with increasing capacitance. Thus, better regulation can be achieved by using the largest practical capacitance. RC -Section π A significant improvement in filtering is obtained when an additional resistor and capacitor are connected to the RC filter of Figure 6. This configuration is called a π -section filter, and is shown schematically in Figure 6. Note that, as the name "section" implies, any number of these units may be strung together to improve filtering and regulation.

11

680

400

When RL >> R, as is usually the case, the ripple factor for this filter is approximately

L

2C1CRR

XX3

r π= (RL >> R) 4

If, as before, we take the ripple frequency to be 100 Hz, and remember that Xci = l/ωCi = 1/2π fCi, then

r = C1C2RRL

1.16x10-6

showing that the voltage regulation is a function of C1 improving with large values. It is usually said that the major effect of C1 is on the regulation and of C2 is ripple.

Experiment

(i) Construct the half-wave rectifying circuit in Fig.1, using a 6 Vac supply. Connect one channel of the scope across the secondary of the transformer and the second channel across the load. Record the waveforms observed.

(ii) In most practical applications full-wave rectification is the norm. Rewire the diode in the full wave rectifier circuit shown in Fig. 2. Connect one channel of the scope across the secondary of the transformer and the second channel across the load. Record the waveforms observed

(iii) Construct the full-wave rectifier with the filtering as shown in Fig.5. Record the resulting waveform for two values of C = (47µf and 470µf), and RL = 1KΩ (> 1watt). Calculate the ripple factors.

(iv) Construct the RC π-section as shown in Fig 6. Record the wave form for various value of RC and calculate the ripple factors, for two values of C where C1 = C2(47f and 470f),

R = 10Ω (> 1watt) and RL = 1k.

12

12

Note: Use the oscilloscope to measure the RMS value of the ripple voltage. ( Do not use the multimeter for ripple voltage )

True RMS Multimeters Certain multimeters will compute the RMS voltage using an IC chip. That is, the meter actually performs squaring, averaging and taking square root of the signal. This is important if the signal is not sinusoidal. Multimeters that do not claimed to be TRUE RMS simply assume a factor of 1.41 for RMS, which is correct only for sinusoidal signal.

13

EXPERIMENT 3 (Optional 4 credit hours) REGULATION WITH ZENER DIODE Considerations involved in regulation were presented in the preceding section. Regulation may be significantly improved by adding a regulator stage to the filtered output. One device often used as a voltage regulator is the Zener diode. Our previous discussion of the junction diode omitted an important feature of the diode's operation, the avalanche or reverse breakdown process. Suppose that the reverse bias on the diode is greatly increased. When the voltage is sufficient, valence electrons will be freed from their positions around the nuclei that bind them. Since these electrons possess excess energy, their collisions with other atoms will knock loose additional electrons; these, in turn, will knock loose more electrons, and the reverse current becomes an "avalanche." This effect was first noted and utilized by Clarence Zener, for whom the phenomenon is named. Note that, for reverse voltages greater than the breakdown voltage, it takes only a very small change in voltage to cause a large change in current; essentially, the diode goes from OFF to ON as the voltage becomes more negative. The breakdown or Zener voltage of a diode can be controlled in the manufacturing process, and diodes are available in various values from 1.2 V to several hundred volts. It is apparent from Figure 1 that, as long as the Zener diode is reverse biased, any current flow greater than a few microamperes must be accompanied by a voltage at least as large as the Zener voltage. This is the key to the use of the Zener diode as a voltage regulator. Figure 2 shows a Zener diode placed in parallel with a variable load resistance RL; the fixed resistance R is included for reasons that will become clear below. The unregulated power supply must be designed so that its filtered output voltage, V, will at an times exceed the desired regulated voltage; the breakdown voltage, Vz of the diode is chosen to be the same as the desired regulated voltage. A further design consideration is the maximum allowable current through the diode, Iz, which is given in the manufacturer's specifications as power rating in Watts. Normally, the diode will be chosen so that the expected current through the diode is about 20 per cent of Iz. Having these known factors in hand, we can write, for the circuit of Figure 2, the equation

V = IR + Vz where

I= I1 + I2 = 0.2Iz + I2 Solving these equations for R, we obtain

2z

zI21.0

VVR+

−= (Note: Iz = 5mA)

If we now take I2 to be the maximum current in the load resistance, we

14

15

obtain from the value of R needed in the circuit. In operation, the Zener diode will maintain the voltage Vz between its terminals (and thus across the load) at all times; the excess of V over Vz will appear across R. The regulation under these conditions is excellent, regardless of the current demands of the load. One can place two or more Zener diodes in series to obtain almost any regulated voltage; the values of the individual Vz's are simply added. Note, however, that the maximum allowable current through such an arrangement is that of the diode with the lowest Iz. Other arrangements, such as the one in Figure 3, can be used to obtain regulated voltages of any specified value. Close examination of the curve in Figure 1 in the Zener region shows that there is a slight variation in voltage as the current through the diode increases. This variation, however, is so small that one percent regulation is reasonably easy to achieve. Unless the current is essentially constant, though, better regulation than this must be accomplished by electronic regulation. Zener Diode Experiments (i) Construct the circuit as shown in Fig 2. using a variable DC power supply. Record the output voltage for a range of input voltages. (ii) Construct the circuit as shown in Fig 3. using a variable DC power supply. Record the output voltage for a range of input voltages.

Figure. 3. Circuit using a combination of zener diodes to achieve a voltage not available with a single zener diode.

16