# Semiconductor Physics fall 2012 problems

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1 Semiconductor Physics fall 2012 problems 1. An n-type sample of silicon has a uniform density N D = atoms cm -3 of arsenic, and a p-type silicon sample has N A = atoms cm -3 of boron. For each semiconductor material determine the following: a) Consider the n-type material. Find the temperature at which half the impurity atoms are ionized. Assume that all mobile electrons come from the dopant impurities. Here, find the temperature in two ways 1) using the Fermi distribution (eq. 16 on page 17), and 2) using the distributions functions on page 22 of Sze. b) The temperature at which the intrinsic concentration n i exceeds the impurity density by a factor of 10 (for each materials). c) The equilibrium minority-carrier concentrations in each materials at 300 K. Assume full ionization of impurities. d) The Fermi level referred to the valence band edge E V in each material at 300 K. The Fermi level if both types of impurities are present in a single sample (compensation doping). Hint: Use equation 42 on page 25 but without assuming that N D >>N A, but still assuming that N D -N A >>n i. 2. From fig. 13 on page 26 of Sze, determine what donor atoms are used to dope the Si sample. 3. Consider the Fermi level for a boron-doped Si sample with an impurity concentration of cm -3 at 300 K. What fraction of the impurity atoms are ionized? Could we just say that all of them are ionized? Hint: select a value for the concentration of ionized acceptors, which also gives you the concentration of holes. Use this to calculate a Fermi level. Use this to evaluate the percentage of ionized acceptors from the Fermi distribution, which you can use to evaluate the Fermi level again. If necessary, repeat until consistent. You don t need to go on iterating forever Consider a compensated (contains both donor and acceptor impurity atoms in the same region) n- type Si sample at T = 300 K, with a conductivity of = 16 ( cm) -1 and an acceptor doping concentration of cm -3. Determine the donor concentration. Assume complete ionization and an electron mobility of n = 1500 cm 2 /Vs. n i = cm a) Find the resistivity at 300 K for a Si sample doped with cm -3 of phosphorous atoms, cm -3 of arsenic atoms and cm -3 of boron atoms. Assume that the impurities are completely ionized and the mobilities are n = 1500 cm 2 /Vs, p = 500 cm 2 /Vs, independent of impurity concentrations. n i = cm -3 in Si at 300 K. (If you need to, see hint in problem 1) b) Find the Fermi level in the sample. HAND-IN

2 6. Calculate the electron and hole concentration under steady-state illumination in n-type silicon with excess generation rate G L = cm -3 s -1, N D = cm -3 and n = p =10 s. 7. Assume that an n-type semiconductor is uniformly illuminated, producing a uniform excess generation rate G. Show that in steady state the change in semiconductor conductivity is given by = q( n + p ) p G. HAND-IN 8. In a Haynes-Shockley experiment, the maximum (excess) amplitudes of the minority carriers at t 1 = 25 s and t 2 = 100 s differ by a factor of 10. Find the minority carrier lifetime. 9. The band structure of GaAs is shown to the left. There is a central conduction band minima at the -point and also secondary minima at the L-points, with effective masses as indicated in the figure. At high electric fields in n-type GaAs, electrons may be excited from the lower to the higher valley. How is the sample conductivity changed? 10. In an experiment the voltage across a uniform, 2 m long region of 1 cm, n-type silicon is doubled, but the current only increases by 50%. Explain.

3 11. The energy band diagram for a semiconductor is shown in the figure. This may for instance represent the variation of potential along the radius of a n- type semiconductor nanowire with the Fermi level pinned close to the valence band edge at the nanowire surface by surface states. Sketch the general form of a) n and p versus x, b) the electrostatic potential V as a function of x, c) the electric field as a function of x, d) J n and J p versus x. 12. Consider an ideal silicon pn abrupt junction with N A = cm -3 and N D = cm -3. Assume that all impurities are completely ionized. a) One way of finding an expression for built-in voltage/potential is to consider the p- and n-part of the junction separately. V bi can then be found as the difference between the Fermi-level E Fn on the n- side and the Fermi level E Fp on the p-side, as qv bi = E Fn - E Fp. Convince yourself that this gives an equation corresponding to equation 6 on page 81 in Sze. b) Calculate V bi ( bi ) at T = 300, 350, 400, 450 and 500 K and plot V bi vs T. c) Try to give a physical explanation for the trend observed in b). Hint: Maybe you can base your reasoning on the procedure in part a). d) Find the depletion layer width and the maximum field at zero bias for T = 300 K. 13.a) Find and sketch the charge density, the built-in electric field and the potential for a silicon pin junction with the doping profile shown in the figure above. (The symbol i represents a very lightly doped or nearly intrinsic region.) b) Calculate V bi and the length of each depletion region and thermal equilibrium. c) Compare the maximum field in the pin junction to the maximum field in a pn junction that contains no lightly doped intermediate region, but has the same dopant concentrations as in part (a) in the other regions. d) Make a sketch indicating how the built-in electrical field in the junction changes when a reverse bias is applied. HAND-IN

4 14. The depletion layer capacitance C D (denoted C j in the plot above) of a pn junction diode with area 10-5 cm 2 is measured. A plot of (1/C D 2 ) vs. the applied voltage V a is shown in the figure below. a) If the diode is considered as a one-sided step junction, where the p-side is heavily doped, find the doping level on the n-side. b) Sketch (qualitatively) the doping density on the n-side of the p + n junction. Calculate the location of the point (distance from the actual junction) on the n-side at which the dopant density changes, and include this point in your sketch. Indicate how the dopant density changes at this point does it increase or decrease compared to the value you found in a)? HAND-IN 15. In a p + n silicon diode with N D = cm -3 and N A = cm -3 the lifetime of the holes is 1 s and their mobility 450 cm 2 /Vs. The area of the diode is 0.1 mm 2. ( Ideal implies that effects within the space-charge region are negligible and that minority carriers flow only by diffusion mechanisms in the charge neutral regions.) a) Determine the current in the diode when reverse biased as above. b) Estimate the stored minority carrier charge when the diode is forward biased in the circuit above. c) Turning the switch will reverse bias the diode. Determine the change in fix charge in the depletion region on the n side when the diode is switched from forward to reverse bias. HAND-IN

5 16. Consider an ideal, long-base, silicon abrupt pn-junction diode with uniform cross section and constant doping on either side of the junction. The diode is made from 1 -cm p-type and 0,2 - cm n-type materials in which the minority-carrier lifetimes are n = 10-6 s and p = 10-8 s, respectively. ( Ideal implies that effects within the space-charge region are negligible and that minority carriers flow only by diffusion mechanisms in the charge neutral regions.) I used N A = cm -3 and N D = cm -3 (from fig 18 on page 32, and I approximated the N A a little) and I used D n = 30 cm 2 /s and D p = 10 cm 2 /s when I needed it. a) What is the value of the built-in voltage (built-in potential)? b) Calculate the density of the minority carriers at the edge of the space-charge region when the applied voltage is 0,589 V (which is 23 kt/q). c) Sketch the majority- and minority-carrier currents as functions of distance from the junction on both sides of the junction, under the applied bias voltage of part b. d) Calculate the location(s) of the plane or planes at which the minority-carrier and majority-carrier currents are equal in magnitude for the applied bias of part b. 17. For forward voltages between 0 and 0.75 V, plot (on a logarithmic scale) the diffusion current in a Si p + n-junction. In the same plot, include also the recombination current (eq 74 on page 98). You may assume a donor concentrations of N D = cm -3, p = 500 cm 2 /Vs and D p = 10 cm 2 /s and a life-time p = = 1 s. For the purpose of calculating the built-in potential you can use for instance N A = cm -3. Finally, plot the sum of the two current contributions, also in the same plot. Try to identify regions with ideality factor 1 or 2 (see eq. 78 on page 98 for definition of ideality factor) 18. A Si npn transistor has impurity concentrations of , and cm -3 in the emitter, base and collector, respectively. The p-type base width is 1.0 m, and the device cross-sectional area is 0.2 mm 2. When the emitter-base junction is forward biased to 0.5 V and the base-collector junction is reverse biased to 5 V, calculate a) the neutral base width and b) the minority carrier concentration in the base at the edge of the emitter-base junction depletion region.

6 19. The figure above depicts a pnp bipolar transistor and all the currents which we consider in it. Make: a) an equivalent figure, still for a pnp-transistor, where you instead of currents draw electron and hole flows. b) an equivalent figure for a npn transistor, where you draw currents. Make nice big figures, so that everything is clearly visible HAND-IN 20. The functionality of a bipolar transistor depends on the collector current being related to the emitter current. Consider equations 2-4 on page in Sze for the distribution of minority carriers (electrons) in the base of an npn bipolar transistor in the active mode, that is when the emitter-base junction is forward biased and the base-collector junction is reverse biased. The equations can be reorganized to read: ( ) ( ) where x = 0 and x = W are the edges of the neutral base (as in fig 3b on page 246 in Sze) (The equation now look quite like what we find on page 56 in the handout, only there we consider minority carriers (holes) in the n-type base of a pnp-transistor) For a Si npn-transistor, plot n p (x)/n p (0) n p (x)/n p0 as a function of x/w for the ratios W/L n = 10, 5, 2, 1 and 0.1. All five graphs should be in the same plot (this is the reason why the x-axis should be normalized to x/w). Your plot should show that the distribution of electrons in the p-type base approaches a straight line for W/L n being small. You will have to assume reasonable values for V EB, V CB, the base doping and the minority carrier diffusion length L n in the base. HAND-IN

7 21. Consider a Si npn transistor with N B = cm -3 and N E = cm -3. The area of the device is cm 2. The neutral base width is cm. For the transistor we further have: L E = cm L B = cm L C = cm D E = 5.18 cm 2 /s D B = 22 cm 2 /s D C = 15.6 cm 2 /s a) Calculate I ne and I pe (it is OK to use the expressions for current that Sze finds for short bases, that is, when W/L n < 1) and the emitter efficiency for a forward bias of V EB = 0.6 V on the emitterbase junction. b) What is the emitter efficiency if the emitter is doped to the same level as the base, that is, N E = N B = cm -3? HAND-IN 22. Consider the lower image on p 59 of the notes. It shows band diagrams for a pnp-transistor in the four modes of operations. Draw the equivalent images for a npn transistor. 23.The transistor in the circuit shown in the figure has a common-emitter current gain =100. The resistors in the circuit are R C = 1 k and R B = 20 k. Determine v out for v in = 1 V and 2 V. Assume V f = 0.6 V for the forward biased EB junction.

8 24. a) Find the barrier height and the donor concentration of the W- GaAs Schottky barrier shown in Fig 30 on page 176 in Sze (use bottom figure on page 70 in the handout). We ignore the image-force lowering It appears that the image in the hand-out and the image in the book differ in how the data are displayed (notation on the y-axis) as well as in what the data actually are. We ll get back about it soon which-ever one you use the strategy for obtaining what is asked for is the same. Both figures refer to the same original publication, which we haven t been able to obtain. However, using the data in the book yields a doping concentration higher than N C and thus a Fermi level in the conduction band. This complicates the analysis beyond what we had in mind. We therefore suggest that you solve the problem using the figure in the handout. b) Compare the barrier height with that obtained from the saturation current density shown in Fig 25 page 171, assuming that A**= 4 A/(cm-K) 2. (Equation 111 is a more elaborate version of what we have on page 72 in the handouts we again ignored. A** is the reduced Richardson factor which involves effects of tunneling, but which differs less than an order of magnitude from A**, so you can use A*=A** here.) HAND-IN 25. Compare a Schottky diode fabricated on n-gaas and a p + n diode (also GaAs). The reverse saturation currents are I s = A and I 0 = A for the Schottky diode and the pn-junction, respectively. What voltage is required for each device in order to have 0.5 ma pass through it? 26. a) In figure 6 on page 143 we notice that c 2 = 0.27 is relevant for Si. This number thus tells us something about the nature of the population of surface states in Si. Assuming an interface layer with thickness 5 Å and permittivity i = 0 and a semiconductor doping of N D = cm -3, calculate the density of interface states D it. (check your answer on page 142). b) If the Fermi level does not coincide with the neutral level, there will be a net charge at the interface. By how much can the Fermi level deviate from the neutral level in order for the interface charge Q ss to not become greater than the semiconductor charge Q sc (from ionized dopants)? Here you will have to obtain a value for V bi ( bi ) at some point. Either you can just assume some reasonable value, or use fig. 5 on page 140 in the book. bi = E g-q n-q 0, if we make the approximation that E F coincides with q 0 (not completely true, but not far from the truth either, it will do as an estimate). q 0 is calculated at the bottom of p. 142.

9 27. Here, we want to find the threshold voltage V T for inversion in a p-type Si MOS in terms of material constants, such as dielectric constants, and in terms of parameters that we control, such as doping level. The voltage V T will be divided between the oxide and the semiconductor, so that. The band bending (in inversion) is primarily given by the depleted region in the semiconductor. Here, we have a space charge density for (see figure). From that we find the potential, where is the potential on the semiconductor surface. a) Deep in the p-type semiconductor. Starting from show that. b) In strong inversion the electron concentration at the surface.starting from, where is the intrinsic Fermi level at the surface, show that when. The potential at the surface thus has to be for inversion. c) Now consider the voltage drop V o over the oxide. From electrostatics, we found that. At the onset of strong inversion, can be approximated by where Q sc is the charge from the depleted acceptors. In inversion,. Show that the depletion region width W can be written in terms of the surface potential as. (see beginning of problem) and that, consequently, in inversion, d) Now we put our results together. Show that the threshold voltage

10 28. For the MOS structure above, the work function in the metal is smaller than that of the semiconductor by 1V (but we have no charges in or on the oxide). The potential difference V FB between metal and semiconductor is composed of a potential drop over the oxide and a potential drop over the depleted region of the semiconductor. Consider the semiconductor to be grounded. The oxide thickness d =10 nm, the oxide permittivity is and N A = cm -3. a) What is the value of the surface potential? b) What is the electric field in the oxide? HAND-IN

11 29.a) A n-channel MOSFET has been made in p-type silicon with N A = cm -3. Determine the threshold voltage under the assumption that the oxide (relative dielectric constant 3.9) is free of charge and that difference in work functions may be neglected. The gate oxide is 100 nm thick. b) Sketch the drain current vs the gate voltage for the MOSFET of part a) if drain and gate are connected and source and substrate are grounded. Can the threshold voltage be determined from the characteristic? 30 a) A MOSFET has a threshold voltage of V T = 0.5 V, a subthreshold swing of 100 mv/decade, and a drain current of 0.1 ma at V T. What is the subthreshold leakage current at V G = 0V? (Hint: think about what the subthreshold swing really means.) b) In the MOSFET lab, you measured drain current as a function of gate and drain voltage. The figure shows the drain current at a fixed V D > V Dsat) for varying V G for one of the devices in the lab. Was it the NMOS or the PMOS and what is the subthreshold swing? HAND-IN 31. Consider an n-channel Si MOSFET with 20 nm thick gate oxide (relative dielectric constant 3.9) and uniform p-type substrate doping of cm -3. The dimensions are Z = L =1 m and the electron mobility in the inversion layer is 800 cm 2 /Vs. The semiconductor work function is 3.8 V and the gate work function is 4 V. a) Sketch the band diagram at thermal equilibrium. Sketch the band diagram at threshold condition s = 2 B. What is the threshold voltage? b) Calculate the drain current for V G = +2.2 V and V D = 0.4 V. c) What length can the channel be reduced to before severe short-channel effects take place? (See handout on short-channel MOSFETs).

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