Basic Electronics

1. Basic Electronics Prof. Rajput Sandeep Assist. Prof., EC Dept. HCET ,Siddhpur

2. Lecture : 1 Junction Diode Characteristics

3. Open Circuited p-n Junction • In Equilibrium (no bias) Total current balances due to the sum of the individual components no net current! Electron Diffusion Current Electron Drift Current Hole Drift Current Hole Diffusion Current

4. Open Circuited p-n Junction EC EC EF Ei Ei EF EV EV • In Equilibrium (no bias) • Total current balances due to the sum of the individual components n vs. E n-Type Material p-Type Material - qVBI + + + + + + + + + + + + + + + + + + + p vs. E no net current!

5. PN Junction I-V Characteristics I • Forward Bias (VA > 0) Electron Diffusion Current IN Electron Drift Current Current flow is proportional to e(Va/Vref) due to the exponential decay of carriers into the majority carrier bands Lowering of potential hill by VA VA Hole Drift Current Hole Diffusion Current IP Current flow is dominated by majority carriers flowing across the junction and becoming minority carriers

6. PNJunction I-V Characteristics • Reverse Bias (VA< 0) Increase of potential hill by VA Electron Drift Current Electron Diffusion Current negligible due to large energy barrier Current flow is constant due to thermally generated carriers swept out by E fields in the depletion region Hole Diffusion Current negligible due to large energy barrier Current flow is dominated by minority carriers flowing across the junction and becoming majority carriers Hole Drift Current

7. PNJunction I-V Characteristics • Where does the Reverse Bias Current come from? • Generation near the depletion region edges “replenishes” the current source.

8. PNJunction I-V Characteristics • Putting it all together -I0 for Ideal diode Vref = kT/q

9. Current-Voltage Characteristics of a Typical Silicon PNJunction • Diode Equation

10. Lecture : 2 Current Components in a P-N Diode Prof. Rajput Sandeep Assist. Prof., EC Dept. HCET ,Siddhpur

11. Quisineutral Region Quisineutral Region x”=0 x’=0 Current Components in a P-N Diode Total on current is constant throughout the device. Thus, we can characterize the current flow components as… J -xp xn

12. Current Components in a P-N Diode PN-junction diode structure used in the discussion of currents. The sketch shows the dimensions and the bias convention. The cross-sectional area A is assumed to be uniform.

13. Current Components in a P-N Diode Hole current (solid line) and recombining electron current (dashed line) in the quasi-neutral n-region of the long-base diode of Figure 5.5. The sum of the two currents J (dot-dash line) is constant.

14. Current Components in a P-N Diode Hole density in the quasi-neutral n-region of an ideal short base diode under forward bias of Vavolts.

15. Current Components in a P-N Diode The ratio of generation region width Xito space charge region width Xdas a function of reverse voltage for several donor concentrations in a one-sided step junction.

16. Current Components in a P-N Diode The current components in the quasi-neutral regions of a long-base diode under moderate forward bias: J(1) injected minority-carrier current, J(2) majority-carrier current recombining with J(1), J(3) majority-carrier current injected across the junction. J(4) space-charge-region recombination current.

17. J n - r e g i o n p - r e g i o n S C L J = J + J e l e c h o l e T o t a l c u r r e n t M a j o r i t y c a r r i e r d i f f u s i o n a n d d r i f t c u r r e n t J h o l e J M i n o r i t y c a r r i e r d i f f u s i o n e l e c c u r r e n t x W – W n p Current Components in a P-N Diode The total current anywhere in the device is constant. Just outside the depletion region it is due to the diffusion of minority carriers.

18. Lecture : 3 Breakdown Diodes and Temperature Effect on Diode Prof. Rajput Sandeep Assist. Prof., EC Dept. HCET ,Siddhpur

19. Breakdown Diodes • When the reverse voltage applied across diode becomes greater than the breakdown voltage, then the diode breaks down and very high current starts flowing in the circuit. There are generally two types of breakdowns in a diode: • Zener breakdown • Avalanche breakdown • And based on the above classifications of breakdown of diode, we have the two special types of diode as • Zener breakdown • Avalanche breakdown • The difference between the Zener Diode and avalanche Diode is the doping level. The doping level of Zener diode is more than avalanche diode or we can say diodes which have higher doping level undergo Zener breakdown when reverse bias voltage is increased while diodes with lesser doping level undergo Avalanche breakdown. 

20. Breakdown Diodes • Zener diode : As we have already mentioned doping level of Zener diode is very high and hence width of depletion region is less. As we know • E = VB / d •                                 VB is the barrier voltage •                                 E is the electric field •                                 d is the depletion width • As doping is high, hence width (d) is less and as barrier voltage varies with doping as stated by the formula: • From the formula we can get that the voltage varies proportional to log of doping and hence the barrier voltage is almost constant. • So from the above discuss we find that Electric field in the depletion region would be large as VB is almost constant and d has decreased. Due to this large electric field, electrons from the outer shell of the atom in the depletion region are expelled out and hence carriers are generated within the depletion region. The high electric field in the depletion region pulls out large number of electrons from the large number of atoms. This leads to large current flow and this type of breakdown is called Zener breakdown. 

21. Breakdown Diodes • Avalanche diode : The diode which have lesser doping undergo avalanche breakdown when high reverse voltage is applied. The lesser doping means the depletion width is large and so electric field within depletion region is not so high. • Hence the electric field would not be able to pull out electrons from the outer shell of atoms and breakdown doesn’t occur in depletion region. But as the depletion region is large and hence when the minority charge carriers move through the depletion region, they get accelerated by the electric field and that even for larger time (as distance through which acceleration is provided is large). • Hence minority charge carriers acquire high velocity and so high kinetic energy. When these charge carriers strike with atoms in the n-type and p-type regions, the high kinetic energy gets converted to thermal energy and hence due to this energy electrons from the outermost shell are pulled out and large current starts flowing. This type of breakdown is called avalanche breakdown. •  But due to the high thermal energy, the temperature rises and diode gets burned. Due to this reason the simple diodes (where avalanche breakdown occurs) is not used in the applications and instead Zener diode is used in the application circuits of breakdown diodes such as regulating power supply.

22. Breakdown Diodes • Differences between Zener breakdown and Avalanche breakdown: • Zener breakdown                                                  Avalanche breakdown • 1. The Zener breakdown occurs in HIGH         1.  The avalanche breakdown occurs in doping diodes.                                                    LOW doping diodes. • 2. The breakdown occurs within the                 2.  The breakdown occurs outside the depletion region.                                                  depletion region. • 3. The breakdown voltage is lesser than             3. The breakdown voltage is more than zener that of avalanche breakdown.                       breakdown voltage. • As Zener breakdown voltage is less than that of avalanche breakdown voltage, hence Zener breakdown is said to occur before the avalanche breakdown. • Hence we can say if we increase the doping of a diode, the chances of zener breakdown increases and hence breakdown voltage decreases.

23. Temperature Effect on Diode • The following graph shows the effect of temperature on the characteristics of diode • A-B curve: This curve shows the characteristics of diode for different temperatures in the forward biase. As we can see from the figure given above, that curve moves towards left as we increase the temperature. We know with increase in temperature, conductivity of semiconductors increase.The intrinsic concentration (ni) of the semiconductors is dependent on temperature as given by: • Eg is the energy gap • K is a voltage man constant • A is a constant independent of temperature

24. Temperature Effect on Diode • When temperature is high, the electrons of the outermost shell take the thermal energy and become free. So conductivity increases with temperature. Hence with increase in temperature, the A-B curve would shift towards left i.e. curve would rise sharply and the breakdown voltage would also decrease with increase in temperature. • A-C curve: This curve shows the characteristics of diode in the reverse biased region till the breakdown voltage for different temperatures. We know ni concentration would increase with increase in temperature and hence minority charges would increase with increase in temperature. The minority charge carriers are also known as thermally generated carriers and the reverse current depends on minority carriers only. Hence as the number of minority charge carriers increase, the reverse current would also increase with temperature as shown in the figure given on the previous page.    • The reverse saturation current gets double with every 10 C increase in temperature. • C-D curve: This curve shows the characteristics of a diode in reverse biased region from the breakdown voltage point onwards. As with increase in temperature, loosely bonded electrons are already free and to free the other electrons, it would take more voltage than earlier.

25. Lecture : 4 Junction Diode Switching Times Prof. Rajput Sandeep Assist. Prof., EC Dept. HCET ,Siddhpur.

26. Junction Diode Switching Times • The switching time of a diode is defined as the time which a diode takes to change its state from forward biased state to reverse biased state or in other words the forward current through diode doesn’t reduce to reverse saturation current immediately as the reverse voltage is applied. In fact it takes time for the current to reduce from forward current to reverse saturation current. This time is also called reverse recovery time.  • To discuss more about the switching time, we first need to discuss charge distribution of diode in normal state, forward biased state and reverse biased state assuming doping of p-type is more than n-type. • Apply the relation given below • n * p = ni2 at constant temperature    (Mass action law)

27. Junction Diode Switching Times • Now we apply the above relation to p-type : P i.e. the concentration of majority carriers (holes) is larger as doping of p-side is high and we have the value of ni2 as constant at fixed temperature. Hence from the above relation we find that number of minority carriers (electrons) is less in p-type material while as doping of n-side is normal, hence number of majority carriers (i.e. electrons) in n-side is not large with the value of ni2as constant and hence number of minority carriers is larger as compared to that in p-side. • Npo is defined as the concentration of minority carriers in N-type material i.e. holes and Pno is defined as the concentration of minority carriers in P-type material i.e. electrons when diode is in un-biased.

28. Junction Diode Switching Times • Charge distribution of diode in Forward Biased state : When diode is forward biased, the majority carriers of both sides cross the junction and after reaching the other side, the charge carriers start combining. So holes from p-side start moving towards n-side and electrons from n-side start moving to p-side. When holes enter the n-side they become the minority carriers and just at the junction there would be high concentration of holes in n-side as the recombining has just started. Also all the holes can not recombine at the junction. • Hence when we move away from the junction in the n-side, the concentration of holes is decreasing as more and more holes are recombining. This is also shown in the figure below. Similarly in the p-side, concentration of the electrons is high near the junction and it starts decreasing as we move away from the junction in the p-side.

29. Junction Diode Switching Times • Charge distribution of diode in Reverse Biased state : When we reverse biased any diode, the minority carriers from both sides cross the junction and then recombine after reaching the other side. Hence the holes from n-side move towards p-side and after reaching p-type material become majority carriers. These holes combine with minority carriers of p-side i.e. electrons. • So the minority carriers at junction i.e. holes in the n-side which are near junction would immediately cross the junction on reverse biased and other holes move slowly. Similar to the above, electrons of p-side move to n-side.

30. Junction Diode Switching Times • Diode Reverse Recovery Time : Consider the following circuit of diode to analyze the switching time of diode. • So to change state from forward to reverse biase, the whole minority charge distribution needs to be inverted as we can see from the figures above.

31. Junction Diode Switching Times • Diode Reverse Recovery Time : Now let’s see what happens during the period in which state changes. Firstly we are in forward biased state when voltage applied is +V. • So there are many minority carriers near the junction and then there is an exponential decrease in the concentration of minority carriers and there is a continuous flow of majority carriers across the junction. We assume the current as I in the forward biased. We depict this in the following graph of current across the junction with time.

32. Junction Diode Switching Times • Now we change the applied voltage to –V at time t=t1 i.e. diode is now reverse biased. As minority carrier concentration in both sides was large near junction in the forward biased, when we have instantly changed the state to reverse biased, those minority arriers start moving in the opposite direction. • And due to large concentration of such minority carriers, the amount of current flowing remains the same.

33. Junction Diode Switching Times • But the high reverse current continues for small time because the concentration of the stored minority carriers start decreasing and the current also starts decreasing exponentially as shown below: • The time gap t2 - t1 in which the reverse current is high (i.e. equal to I) is known as storage time and the time gap from t2 to t3 i.e. the time reverse current becomes equal to reverse saturation current is known as transient time. The total time from t1 to t3 is known as reverse recovery time.

34. Junction Diode Switching Times • Effect Of Doping On Reverse Recovery Time : As we have already known that reverse recovery time is the time it takes to invert the minority charge distribution of diode from forward biased to minority charge distribution in reverse biased. • Hence when we increase the doping of material, the concentration for minority charge carriers decrease. • Hence as the peaks of charge distribution have fallen, it takes lesser time to invert the charge distribution. • Hence we can say that with increase in doping, the reverse recovery time decrease and with decrease in doping level the reverse recovery time increases.

35. Lecture : 5 Transistor Characteristics Prof. Rajput Sandeep Assist. Prof., EC Dept. HCET ,Siddhpur

36. Semiconductors: ability to change from conductor to insulator Can either allow current or prohibit current to flow Useful as a switch, but also as an amplifier Essential part of many technological advances What is a Transistor?

37. A Brief History • Guglielmo Marconi invents radio in 1895 • Problem: For long distance travel, signal must be amplified • Lee De Forest improves on Fleming’s original vacuum tube to amplify signals • Made use of third electrode • Too bulky for most applications

38. Bell Labs (1947): Bardeen, Brattain, and Shockley Originally made of germanium Current transistors made of doped silicon The Transistor is Born

39. How Transistors Work • Doping: adding small amounts of other elements to create additional protons or electrons. • P-Type: dopants lack a fourth valence electron (Boron, Aluminum). • N-Type: dopants have an additional (5th) valence electron (Phosphorus, Arsenic). • Importance: Current only flows from P to N.

40. Physical Structure of Transistor

41. Diode: simple P-N junction. Forward Bias: allows current to flow from P to N. Reverse Bias: no current allowed to flow from N to P. Breakdown Voltage: sufficient N to P voltage of a Zener Diode will allow for current to flow in this direction. Diodes and Bias

42. The Bipolar Junction Transistor • Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping. The Two Types of BJT Transistors PNP NPN p n p E C n p n E C B C Cross Section B Cross Section B B C E E • Collector doping is usually ~ 109 • Base doping is slightly higher ~ 1010 – 1011 • Emitter doping is much higher ~ 1017 Schematic Symbol Schematic Symbol

43. Junction Transistor IE IC IE IC - VCE + + VEC - C E C E - - + + VBE VBC VEB VCB IB IB - - + + B B NPN : IE = IB + IC VCE = -VBC + VBE PNP : IE = IB + IC VEC = VEB - VCB

44. Lecture : 6 Transistor Current Components Prof. Rajput Sandeep Assist. Prof., EC Dept. HCET ,Siddhpur

45. Transistor Current Components • In the figure we show the various components which flow across the forward-based emitter junction and the reverse-biased collector junction. The emitter current IE consists of hole current IpE (holes crossing from the emitter into base) and electron current InE (electron crossing from base into the emitter). • The ratio of hole to electron currents, IpE/InE, crossing the emitter junction is proportional to the ratio of the conductivity of the p material to that of the n material. In the commercial transistor the doping of the emitter is made much larger than the doping of the base. This future ensures (in a p-n-p transistor) that the emitter current consists almost entirely of the holes. Such a situation is desired since the current which results from electrons crossing the emitter junction from base to emitter does not contribute carriers which can reach the collector.

46. Transistor Current Components • Not all the holes crossing the emitter junction JE reach the collector junction Jc because some of them combine with the electrons in the n – type base. If Ipc is the hole current at Jc, there must be a bulk recombination current IpE - I­pC leaving the base, as indicated in figure. (actually, electrons enter the base region through the base lead to supply those charges which have been lost by recombination with the holes injected into the base across JE). • If the emitter were open-circuited so that IE = 0, then IpC would be zero. Under these circumstances, the base and collector would act as a reverse-biased diode, and the collector current Icwould equal the reverse saturation current ICO. If IE ≠ 0, then, from figure, we note that, • Ic = Ico – IpC • For a p-n-p transistor, Ico consists of holes moving across Jc from left to right (base to collector) and electrons crossing Jc in the opposite direction. Since the assumed reference direction for I­co in figure is from right to left, then for a p-n-p transistor, Ico is negative. For an n-p-n transistor, Ico is positive.

47. Current flow for an NPN BJT in the active region n I co - Inc + VCB - p- - Electrons + Holes + Ipe Ine n+ VBE - Bulk-recombination Current • Most of the current is due to electrons moving from the emitter through base to the collector. Base current consists of holes crossing from the base into the emitter and of holes that recombine with electrons in the base.

48. Current flow for an NPN BJT in the active region For CB Transistor IE= Ine+ Ipe Ic= Inc- Ico And Ic= - αIE + Ico CB Current Gain, α ═ (Ic- Ico) (IE- 0) For CE Transistor, IC = βIb + (1+β) Ico where β═α , 1- α is CE Gain. ICO Inc Bulk-recombination current Ipe Ine

49. Lecture : 7 Transistor Configurations Prof. Rajput Sandeep Assist. Prof., EC Dept. HCET ,Siddhpur

50. Various Regions (Modes) of Operation of BJT Active: • Most important mode of operation • Central to amplifier operation • The region where current curves are practically flat • Barrier potential of the junctions cancel each other out causing a virtual short (behaves as on state Switch) Saturation: Cutoff: • Current reduced to zero • Ideal transistor behaves like an open switch • There is also a mode of operation called inverse active mode, but it is rarely used.