EC T34 ELECTRONIC DEVICES AND CIRCUITS

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1 RAJIV GANDHI COLLEGE OF ENGINEERING AND TECHNOLOGY PONDY-CUDDALORE MAIN ROAD, KIRUMAMPAKKAM-PUDUCHERRY DEPARTMENT OF ECE EC T34 ELECTRONIC DEVICES AND CIRCUITS II YEAR Mr.L.ARUNJEEVA., AP/ECE 1

2 PN JUNCTION DIODE UNIT I SEMICONDUCTOR DIODES In a piece of semiconductor material if one half is doped by p- type impurity and the the other half is doped by n-type impurity, a PN junction is formed. The plane dividing the two halves is called PN junction. The N-type material has high concentration of free electrons while p- type material has high concentration of holes. At the junction, there is a tendency for the free electrons to diffuse over to P-side and holes to N-side. This process is called diffusion. As the free electrons move across the junction from N-type to P-type, the donor ions becomes positively charged. Hence a positive charge is built on the N-side of the junction. The free electrons that cross the junction uncover the negative acceptor ions by filling in the holes. Therefore a net negative charge is established on the p-side of the junction. This net negative charge on the p side prevents further diffusion of electrons onto the p-side. Similarly the net positive charge on the N-side repels the hole crossing from P-side to N-side. Thus a barrier is set-up near the junction which prevents further movement of charge carriers i.e. electrons and holes. As a consequence of the induced electric field across the depletion layer, an electrostatic potential difference is established between P & N regions, called potential Barrier, junction barrier, diffusion potential or contact potential.(v o ) Note: V o = 0.3 V for Ge = 0.72 V for Si UNDER FORWARD BIAS CONDITION When positive terminal of the battery is connected to the P-type and negative terminal to the N-type of the PN junction diode, the bias applied is known as Forward Bias. OPERATION As shown in the figure, the applied potential with external battery acts in opposition to the internal potential barrier and disturbs the equilibrium. 2

Under the forward bias condition, the applied positive potential repels the holes in the p-type region so that the holes move towards the junction and the applied negative potential repels the

Eventually, when the applied potential is more than the internal barrier potential the depletion region and the internal potential barrier disappear.

3 Under the forward bias condition, the applied positive potential repels the holes in the p-type region so that the holes move towards the junction and the applied negative potential repels the electrons in the N-type region and the electrons move towards the junction. Eventually, when the applied potential is more than the internal barrier potential the depletion region and the internal potential barrier disappear. VI CHARACTERISTICS OF A DIODE UNDER FORWARD BIAS As the forward voltage (V f ) is increased, for V f <V o, the forward current I f is almost zero because the potential barrier prevents the holes from P-region and electrons from N-region to flow across the depletion region in the opposite direction. For V f >V o, the potential barrier completely disappears and hence, the holes cross the junction from P-type to N-type and the electrons cross the junction in the opposite direction, resulting in relatively large current flow. UNDER REVERSE BIAS CONDITION When the negative terminal of the battery is connected to the P-type and positive terminal of the battery is connected to N-type, the bias applied is known as reverse bias. OPERATION 3

Under applied reverse bias, holes from P-side move towards the negative terminal of the battery and electrons from N-side are attracted towards the positive terminal of the battery.

4 Under applied reverse bias, holes from P-side move towards the negative terminal of the battery and electrons from N-side are attracted towards the positive terminal of the battery. Hence, the width of the depletion region increases. Hence, the resultant potential barrier is increased which prevents the flow of majority carriers in both directions; the depletion width (W) is proportional to V o under reverse bias. But a very small current (µa) flows under reverse bias as shown in the characteristics curve. Electrons forming covalent bonds of the semiconductor atoms in P and N- type regions may absorb sufficient energy from heat, causing breaking of some covalent bonds. Hence electron-hole pairs are produced in both regions. Thus holes in p- regions are attracted towards the negative terminal and electrons in the n region are attracted towards the positive terminal of the battery. Consequently, the minority carriers, electrons in P-region and holes in N-region, wander over to the junction and flow towards their majority carrier side giving rise to small reverse current, called Reverse Saturation Current, Io. THEORY OF DIODE CURRENT EQUATION Ideal Diodes The diode equation gives an expression for the current through a diode as a function of voltage. The Ideal Diode Law, expressed as: where: I = the net current flowing through the diode; I 0 = "dark saturation current", the diode leakage current density in the absence of light; V = applied voltage across the terminals of the diode; q = absolute value of electron charge; k = Boltzmann's constant; and T = absolute temperature (K). 4

The "dark saturation current" (I 0 ) is an extremely important parameter which differentiates one diode from another. I 0 is a measure of the recombination in a device.

5 The "dark saturation current" (I 0 ) is an extremely important parameter which differentiates one diode from another. I 0 is a measure of the recombination in a device. A diode with a larger recombination will have a larger I 0. Note that: I 0 increases as T increases; and I 0 decreases as material quality increases. For actual diodes, the expression becomes: where: n = ideality factor, a number between 1 and 2 which typically increases as the current decreases. TEMPERATURE EFFECTS Temperature plays an important role in determining the characteristic of diodes. As temperature increases, the turn-on voltage, v ON, decreases. Alternatively, a decrease in temperature results in an increase in v ON. This is illustrated in figure below where V ON varies linearly with temperature which is evidenced by the evenly spaced curves for increasing temperature in 25 C increments. The temperature relationship is described by equation V ON (T New ) V ON (T room ) = k T (T New T room ) where, Dependence of id on temperature versus vd for real diode (kt = -2.0 mv / C) 5

6 T room = room temperature, or 25 C. T New = new temperature of diode in C. V ON (T room ) = diode voltage at room temperature. V ON (T New ) = diode voltage at new temperature. k T = temperature coefficient in V/ C. Although k T varies with changing operating parameters, standard engineering practice permits approximation as a constant. Values of k T for the various types of diodes at room temperature are given as follows: k T = -2.5 mv/ C for germanium diodes k T = -2.0 mv/ C for silicon diodes The reverse saturation current, I O also depends on temperature. At room temperature, it increases approximately 16% per C for silicon and 10% per C for germanium diodes. In other words, I O approximately doubles for every 5 C increase in temperature for silicon, and for every 7 C for germanium. The expression for the reverse saturation current as a function of temperature can be approximated as where K i = 0.15/ C ( for silicon) and T1 and T2 are two arbitrary temperatures. DIODE RESISTANCE DC or STATIC RESISTANCE The resistance of the diode at the operating point can be found by using. The DC resistance levels at the knee and below will be greater than the resistance levels obtained for the vertical rise section of the characteristics. Resistance in reverse bias will be quite high. AC or DYNAMIC RESISTANCE A straight line drawn tangent to the curve through the q-point will define a particular change in voltage and current that can be used to determine the ac or dynamic resistance for this region of the diode characteristics. 6

7 Note: keep the change in voltage and current as small as possible and equidistant to either side of Q-point. AVERAGE AC RESISTANCE The average ac resistance is the resistance determined by a straight line drawn between the two intersections established by the maxi mum and minimum values of input voltage. ( ) DIODE EQUIVALENT CIRCUITS An equivalent circuit is a combination of elements properly chosen to best represent the actual terminal characteristics of a device, system etc. (i) Piecewise-Linear Equivalent Circuit One technique for obtaining an equivalent circuit for a diode is to approximate the characteristics of the device by straight-line segments, as shown below. The resulting equivalent circuit is naturally called the piecewise-linear equivalent circuit. But the straight-line segments do not result in an exact duplication of the actual characteristics, especially in the knee region. For the sloping section of the equivalence, the average ac resistance is included next to the actual device. It defines the resistance level of the device when it is in the on state. The ideal diode 7

is included to establish that there is only one direction of conduction through the device, and a reverse-bias condition will result in the open-circuit state for the device.

8 is included to establish that there is only one direction of conduction through the device, and a reverse-bias condition will result in the open-circuit state for the device. Since a silicon semiconductor diode does not reach the conduction state until V D reaches 0.7 V with a forward bias, a battery V T opposing the conduction direction must appear in the equivalent circuit as shown below. V T represents the horizontal offset in the curve. Defining the piece wise-linear equivalent circuit using straight-line segments to approximate the characteristic curve. Components of the piecewise-linear equivalent circuit (ii) Simplified Equivalent Circuit For most applications, the resistance rav is sufficiently small to be ignored in comparison to the other elements of the network. The removal of r av from the equivalent circuit is the same as implying that the characteristics of the diode appear as shown in Figure below. The reduced equivalent circuit appears in the same figure. It states that a forward-biased silicon diode in an electronic system under dc 8

9 conditions has a drop of 0.7 V across it in the conduction state at any level of diode current. (iii) Simplified equivalent circuit for the silicon semiconductor diode Ideal Equivalent Circuit 0.7-V level can be ignored when compared to the applied voltage level. In this case the equivalent circuit will be reduced to that of an ideal diode as shown in Figure below with its characteristics. Ideal diode and its characteristics SPACE CHARGE (or) TRANSISTION CAPACITANCE C T OF DIODE Reverse bias causes majority carriers to move away from the junction, thereby creating more ions. Hence the thickness of depletion region increases. This region behaves as the dielectric material used for making capacitors. The p-type and n-type conducting on each side of dielectric act as the plate. The incremental capacitance C T is defined by Since Therefore, where, dq is the increase in charge caused by a change dv in voltage. C T is not constant, it depends upon applied voltage and therefore it is defined as dq / dv. 9

10 When p-n junction is forward biased, then a capacitance is defined called diffusion capacitance C D (rate of change of injected charge with voltage) to take into account the time delay in moving the charges across the junction by the diffusion process. If the amount of charge to be moved across the junction is increased, the time delay is greater, it follows that diffusion capacitance varies directly with the magnitude of forward current. Alloy junction The junction in which there is an abrupt change from acceptor ions on one side to donor ions on the other side is called alloy or fusion junction. Since net charge = 0, = Acceptor concentration Donor concentration. If << ; Neglected and assume that that the entire barrier potential V B appears across the uncovered acceptor ions. By poisson s equation, = ε premitivity of the semiconductor ε = where, relative dielectric constant permitivity of free space Integrating the above equation, At x = Wp W, V= V B, and if V B = V 0 V V 0 V = Thus the thickness of depletion layer increases with applied voltage ie. W = V B If A = Area of junction, then Q = Therefore = 10

11 Hence C T = DIFFUSION CAPACITANCE If the bias is in the forward direction the potential barrier at the junction is lowered and holes from the p- side enter the n side. similarly the electrons from the n side move into the p- side. This is the process of minority carrier injection, where the excess hole density falls off exponentially with the distance. The diffusion or storage capacitance (C D ) is defined as the rate of change of injected charge with applied voltage. Expression for C D : Assume : one side of the diode i.e p-type is heavily doped compared with n-type and current due to electrons crossing the junction from n type to p type is zero. Therefore, total diode current crossing the junction is the hole current moving from p side to n side. Ie. I = I pn (o) The excess minority charge Q is given by, Q = ( ) Wher A diode cross section E charge of an electron Q = [ ( ) ] And C D = = Ae L P ( ) The hole current Ipn(x), at x= 0 is I = ( ) Differentiating Pn(o) with respect to voltage is and simplifying, C D = PN DIODE SWITCHING TIMES When a diode is driven from the reversed condition to the forwardstste or in the opposite direction, the diode response is accompanied by a transient and an interval of time before the diode recovers to its steady state. Forward Recovery time t fr 11

12 It is the time difference between the 10% point of the diode voltage and the time when this voltage reaches and remains within 10% of this final value. Diode Reverse Recovery time When an external voltage forward biases PN junctions, the number of minority carriers is very large. They are supplied from the other side of the junction. As the minority carriers approach the junction they rapidly swept across and the density of minority carriers diminishes to zero at the junction. If the external voltage is suddenly reversed in a diode in FB, the diode current will not immediately fall to its steady state reverse voltage value. The current cannot attain its steady state value until the minority carrier distributionat the moment of voltage reversal had the form(a) reduces to form (b). Storage and transition times Consider the voltage in (b) is applied to the diode resistor circuit in figure (a).upto time t 1, the voltage vi = v f, forward biases the diode. Then the current is i = At t 1, the vi = -v r, but the current does not drop to zero, but instead reverses and remains at the value i = until t = t 2. At t = t 2 as in, the injected minority carrier density at x= 0 has reached its equilibrium state.if the diode ohmic resistance is R d, at t 1 the diode voltage falls of slightly, but does not reverse. At t = t 2, the diode voltage begins to reverse and the magnitude of the diode current begins to decrease. Storage time t s The interval t 1 to t 2, for the stored minority charge to become zero, is called the storage time t s. Transition Time t 1 The time which elapses between t 2 and the time when the diode has nominally recovered is called the transistion time t t.this recovery interval will be completed when the minority carriers at some distance from the junction have diffused to the junction and crossed it and C T has charged to voltage V R. Reverse recovery time (t rr ) The reverse recovery time (or) turn off time is the interval from the current reversal at t = t 1 until the diode has recovered to a specified extent in terms of diode current or resistance ie. t rr = t s + t t 12

ZENER DIODES When the reverse voltage reaches breakdown voltage in normal PN junction diode, the current through the junction and power dissipated at the junction will be high.

13 ZENER DIODES When the reverse voltage reaches breakdown voltage in normal PN junction diode, the current through the junction and power dissipated at the junction will be high. Such an operation is destructive and diode gets damaged. Diodes which are designed with adequate power dissipation capabilities to operate in the breakdown region may be employed as voltage reference or constant voltage devices. Such diodes are known as avalanche, breakdown or zener diodes. Under forward biased condition, the zener works in similar with the PN diode. Under reverse bias, breakdown of the junction occurs and voltage remains constant and current varies largely. Two mechanisms of diode breakdown for increasing reverse voltage are: Avalanche Breakdown The thermally generated electrons and holes acquire sufficient energy from the applied reverse potential to produce new carriers by removing valence electrons. These new carriers, in turn produce additional carriers again through the process of disrupting bonds. This cumulative process is referred to as avalanche multiplication. It results in the flow of large reverse currents. The diode is said to be in avalanche breakdown region. Zener Breakdown When the P and N regions are heavily doped, direct rupture of covalent bonds takes place because of strong electric fields at the junction of PN diode. The new electron hole pairs so created increase the reverse current in a reverse biased PN diode. The decrease in current takes place at a constant value of reverse bias ie. below 6V for heavily doped diodes. 13

14 14

EDC Lecture Notes UNIT-1

EDC Lecture Notes UNIT-1 P-N Junction Diode EDC Lecture Notes Diode: A pure silicon crystal or germanium crystal is known as an intrinsic semiconductor. There are not enough free electrons and holes in an intrinsic semi-conductor