MSD2024_25_Aula02_junctions (4) PDF

1 MICROELETRÓNICA: SISTEMAS E DISPOSITIVOS Junções p-n e Schottky  Outline p-n junction – Overview – Formation – Carrier diffusion – Ideal junction – Built-in potential – Bias – Breakdown...

1 MICROELETRÓNICA: SISTEMAS E DISPOSITIVOS Junções p-n e Schottky  Outline p-n junction – Overview – Formation – Carrier diffusion – Ideal junction – Built-in potential – Bias – Breakdown – Capacitance Schottky junction – Metal-semiconductor junction – Bias – Current characteristics – Depletion capacitance – Comparison with p-n junction – Applications and limitations Special p-n junctions – Zener diode – Variable capacitance diode (varactor) – Tunneling diode (Esaki diode) Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 2  p-n Junction Overview – The most important device is a junction between a p-type region and an n-type region – The p-n junction is the basic element of all bipolar devices. – Its main electrical property is that it rectifies (allow current to flow easily in one direction only). – The p-n junction is often just called a DIODE. Applications – photodiode, light sensitive diode, – LED- light emitting diode, – varactor diode-variable capacitance diode – Basic element of several types of transistors Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 3  p-n Junction Applications (expanded) – p-n Junction Diode Junction diode – Rectifiers – Switching Diode – Zener Diode – Varactor Diode Tunnel Diode Photo-Diode – Solar Cell – Photo Detector Light Emitting diode & Laser Diode – BJT (Bipolar Junction Transistor) – HBT (Heterojunction Bipolar Transistor) – FET (Field Effect Transistor) JFET MOSFET – memory MESFET - HEM Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky  p-n Junction Formation – The p-n junction can be formed by pushing a piece of p-type of a semiconductor (ex: silicon) into close contact with a piece of n-type. – But forming a p-n junction is not so simply. Because; There will only be very few points of contact and any current flow would be restricted to these few points instead of the whole surface area of the junction. Silicon that has been exposed to the air always has a thin oxide coating on its surface called the “native oxide”. This oxide is a very good insulator and will prevent current flow. Bonding arrangement is interrupted at the surface; dangling bonds. – To overcome these surface states problems p-n junction can be formed in the bulk of the semiconductor, away from the surface as much as possible. Processing – Dopant diffusion – Ionic implantation – Epitaxy – Low damage deposition (CVD, PVD, etc): normally only for heterojunctions Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 5  p-n Junction Formation – Typical process starts with p-type substrate – Create n-well to house diode – p and n+ diffusion regions are the cathode and anode – N-well must be reverse biased from substrate – Parasitic resistance due to well resistance Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 6  p-n Junction There is a discontinuity in the Fermi level across the junction. Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 7  p-n Junction Carrier diffusion – Diffusion occurs when there exists a concentration gradient – In the figure below, imagine that we fill the left chamber with a gas at temperate T. If we suddenly remove the divider, what happens? The gas will fill the entire volume of the new chamber. How does this occur? – The net motion of gas molecules to the right chamber was due to the concentration gradient – If each particle moves on average left or right then eventually half will be in the right chamber – If the molecules were charged (or electrons), then there would be a net current flow. The diffusion current flows from high concentration to low concentration Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 8  p-n Junction At time = 0, slam the two pieces together Gradient is the driving force for diffusion Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 9  p-n Junction Carrier diffusion – The donors and acceptors impurities are incorporated into lattice (ex: B or P in silicon) and are “fixed” (unless temperature increase, so they can diffuse). However, the electrons and holes that “come from them” are free to move. – Electrons on the right side of the junction will diffuse to the left and holes on the left side of the junction will move to the right. – Holes will combine with the electrons on right side of the metallurgical junction leaving behind negatively charged acceptor centers. Similarly, electrons diffusing to the left leave behind positively charged donor centers. – The increasing amount of fixed charge wants to electrostatically attract the carriers that are trying to diffuse away and equilibrium is reached. – These fixed charges produce an electric field which slows down the diffusion process. – This fixed charge region is known as depletion region or space charge region since it is depleted of free carriers. Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 10  p-n Junction Ideal junction at equilibrium – When both drift and diffusion are present, the total current is given by the sum: dn J = J drift + J diff = q n nE + qDn dx – In resistors, the carrier density is approximately uniform and the second term is nearly zero – Consider the PN junction in thermal equilibrium, the sum of the currents has to be zero dno electrons − Dn dno = dx = − kT 1 dn0 J n = 0 = qn0  n E0 + qDn E 0 n0  n q n0 dx dx dpo holes dn Dp qn0  n E0 = −qDn o E0 = dx = − kT 1 dp0 dx n0  p q p0 dx Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 11  Depletion region p-n Junction Ef must be the same everywhere in a solid under thermal equilibrium, even at a junction of different materials Charge density Electric field Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 12  p-n Junction Built-in potential – The potential barrier height across a p-n junction is known as the built in potential and also as the junction potential. – The potential energy that this potential barrier correspond is qVbi Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 13  Space charge neutrality: p-n Junction n+Na=p+Nd Built-in potential Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 14  p-n Junction Built-in potential – One side very heavily doped so that Fermi level is at band edge. e.g. p+-n junction (heavy B implant into lightly doped substrate) (E i − E F )Left  E i − EV = EG / 2 N  (E F − E i )Right = kT ln D   ni  EG kT  Nd   Vbi = + ln  2q q  ni  Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 15  p-n Junction Depletion region width 1  2K S  0 NA  2  N A + ND  W = Vbi    (  q ND N A + ND )   NA   2K  (N + ND )  1 2 W =  S 0 A Vbi   q ND N A   N + ND   N + ND   N  N + N D  W = xn  A  W = xn  A  = x p  A  A   N A   N A   D  N N A  or or  NA   ND  xn = W   xp = W    A N + N D   A N + N D  Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 16  p-n Junction Depletion region width "P+ - N" => N a >> Nd => xp N a xp >> xn Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 17  p-n Junction Without external bias – Diffusion small since few carriers have enough energy to penetrate barrier – Drift current is small since minority carriers are few and far between: Only minority carriers generated within a diffusion length can contribute current – Important Point: Minority drift current independent of barrier! – Diffusion current strong (exponential) function of barrier Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 18  p-n Junction Without external bias – Diffusion small since few carriers have enough energy to penetrate barrier – Drift current is small since minority carriers are few and far between: Only minority carriers generated within a diffusion length can contribute current – Important Point: Minority drift current independent of barrier! – Diffusion current strong (exponential) function of barrier Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 19  p-n Junction Reverse bias – Applied voltage disturbs equilibrium, EF no longer constant – Reverse bias adds to the effect of built-in voltage and increases barrier to diffusion – Diffusion current is reduced exponentially – Drift current does not change – Net result: Small reverse current  2K S  0 (N A + ND ) 1  2 W =  (V bi + Vrev )  q ND N A  Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 20  p-n Junction Forward bias – Forward bias causes an exponential increase in the number of carriers with sufficient energy to penetrate barrier – Diffusion current increases exponentially – Drift current does not change (both n- and p- components are decreased) – Net result: Large forward current – Drift current is very similar to that of the equilibrium case. This current is due to the minority carriers on each side of the junction and the movement of minority carriers is due to the built-in field across the depletion region.  2K  (N + ND ) 1 (Vbi − Vfwd ) 2 W = S 0 A  q ND N A  Fermi level is not constant  Current Flow Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 21  p-n Junction Ideal diode – This equation is valid for both forward (VF) and reverse biases (VR); just change the sign of V. – Change V with –V for reverse bias. When qV>a few kT exponential term goes to zero as Reverse saturation current I = − I 0 = − I sat (increases exponentially with temperature but it is independent of applied reverse voltage) Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 22  p-n Junction Ideal diode – Diode IV relation is an exponential function – This exponential is due to the Boltzmann distribution of carriers versus energy – For reverse bias the current saturation is due to the drift current (minority carriers) Current Forward Bias VB I0 Voltage VB ; Breakdown voltage I0 ; Reverse saturation current Reverse Bias Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 23  p-n Junction Reverse breakdown – An applied reverse bias (voltage) will result in a small current to flow through the device. – At a particular high voltage value, which is called as breakdown voltage VB, large currents start to flow. If there is no current limiting resistor which is connected in series to the diode, the diode will be destroyed. There are two physical effects which cause this breakdown Zener breakdown is observed in highly doped p-n junctions and occurs for voltages of about 5 V or less. The conduction and valance bands on opposite side of the junction become so close during the reverse-bias that the electrons on the p-side can tunnel directly from VB into the CB on the n-side. EB Zener≈106 V/cm Possible for doping >1018 cm-3 Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 24  p-n Junction Reverse breakdown – An applied reverse bias (voltage) will result in a small current to flow through the device. – At a particular high voltage value, which is called as breakdown voltage VB, large currents start to flow. If there is no current limiting resistor which is connected in series to the diode, the diode will be destroyed. There are two physical effects which cause this breakdown Avalanche breakdown is observed in less doped p-n junctions. This mechanism occurs when electrons and holes move through the DR and acquire sufficient energy from the electric field to break a bond i.e. create electron-hole pairs by colliding with atomic electrons within the depletion region. The newly created electrons and holes move in opposite directions due to the electric field and thereby add to the existing reverse bias current. This is the most important Collision probability grows breakdown mechanism in p-n junction. with kinetic energy of carriers (i.e., higher electric fields) Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 25  p-n Junction Depletion capacitance – When a reverse bias is applied to p-n junction diode, the depletion region width, W, increases. This cause an increase in the number of the uncovered space charge in depletion region. – Whereas when a forward bias is applied depletion region width of the p-n junction diode decreases so the amount of the uncovered space charge decreases as well. – So the p-n junction diode behaves as a device in which the amount of charge in depletion region depends on the voltage across the device. So, it looks like a capacitor with a variable capacitance. Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 26  p-n Junction Depletion capacitance xp Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 27  p-n Junction Depletion capacitance Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 28  p-n Junction Exercise – A P+N junction has Na=1020 cm-3 and Nd =1017cm-3. What is: a) its built in potential, b) Wdep , c) xN , and d) xP ? kT N d N a 10 20 1017 cm −6 a) bi = ln 2 = 0.026V ln 20 −6 1V q ni 10 cm 1/ 2 2 sbi  2 12  8.85 10 1  −14 b) Wdep  =  −19  = 0.12 μm  1.6 10 10 17 qN d  c) xN  Wdep = 0.12 μm d) xP = xN N d N a = 1.2  10−4 μm = 1.2 Å  0 Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 29  p-n Junction Exercise – If the slope of the line in 1/C2 plot is 2x1023 F-2 V-1, the intercept is 0.84V, and A is 1 mm2, find the lighter and heavier doping concentrations Nl and Nh – Solution N l = 2 /( slope  q s A2 ) = 2 /( 2  10 23  1.6  10 −19  12  8.85 10 −14  10 −8 cm 2 ) = 6 1015 cm −3 2 q 0.84 kT N h N l ni kTbi 10 20 0.026 bi = ln  N = e = e = 1.8  1018 cm −3 6 1015 2 h q ni Nl Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 30  Schottky junction Metal-semiconductor junction – Two kinds of metal-semiconductor contacts: Low-resistance ohmic contacts: metal on heavily doped silicon Rectifying Schottky diodes: metal on lightly doped silicon – Many of the properties of pn junctions can be realized by forming an appropriate metal-semiconductor rectifying contact (Schottky contact) Simple to fabricate Switching speed is much higher than that of p-n junction diodes – Metal-Semiconductor junctions are also used as ohmic-contact to carry current into and out of the semiconductor device Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 31  Schottky junction Metal-semiconductor junction q(s-) = EC – EF = kT ln(NC/ND) for n-type = EG – kT ln(Nv/NA) for p-type Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 32  Schottky junction Metal-semiconductor junction Built-in potential Barrier height qBn = qms + kT ln(NC/ND) Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 33  Schottky junction Metal-semiconductor junction – Vacuum level, E0 - corresponds to energy of free electrons. – The difference between vacuum level and Fermi-level is called workfunction,  of materials. – Workfunction, m is an invariant property of metal. It is the minimum energy required to free up electrons from metal. (3.66 eV for Mg, 5.15eV for Ni etc.) – The semiconductor workfunction, s, depends on the doping.  s =  + ( EC − EF ) FB – where  = (E0 – EC)|SURFACE is a fundamental property of the semiconductor. (Example:  = 4.0 eV, 4.03 eV and 4.07 eV for Ge, Si and GaAs respectively) Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 34  Schottky junction Metal-semiconductor junction – Schottky barrier heights for electrons and holes Metal Mg Ti Cr W Mo Pd Au Pt  Bn (V) 0.4 0.5 0.61 0.67 0.68 0.77 0.8 0.9  Bp (V) 0.61 0.5 0.42 0.3 Work Function 3.7 4.3 4.5 4.6 4.6 5.1 5.1 5.7 y m (V) Bn + Bp  Eg Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 35  Schottky junction Metal-semiconductor junction – Measured barrier height Bn for metal-Si and metal-GaAs contacts n-type p-type M > S rectifying ohmic M < S ohmic rectifying Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 36  Schottky junction Metal-semiconductor junction – Silicides A silicon and metal compound. It is conductive, similar to a metal. Silicide-Si interfaces are more stable than metal-silicon interfaces. After metal is deposited on Si, an annealing step is applied to form a silicide-Si contact. The term metal-silicon contact includes and almost always means silicide-Si contacts. Silicide ErSi1.7 HfSi MoSi2 ZrSi2 TiSi2 CoSi2 WSi2 NiSi2 Pd2Si PtSi BnBn (V) 0.28 0.45 0.55 0.55 0.61 0.65 0.67 0.67 0.75 0.87 BpBp (V) 0.55 0.49 0.45 0.45 0.43 0.43 0.35 0.23 Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 37  Schottky junction Metal-semiconductor junction – MS (n-type) contact with m < s No barrier for electron flow from S to M. So, even a small V > 0 results in large current. As drawn, small barrier exists for electron flow from M to S, but vanishes when V < 0 is applied to the metal. Large current flows when V < 0. The MS(n-type) contact when m < s behaves like an ohmic contact. – MS (n-type) contact with m > s Soon after the contact formation, electrons will begin to flow from S to M near junction. Creates surface depletion layer, and hence a built-in electric field (similar to p+-n junction). Under equilibrium, net flow of carriers will be zero, and Fermi-level will be constant. A barrier B forms for electron flow from M to S. B = m – ... ideal MS (n-type) contact. B is called “barrier height”. Electrons in semiconductor will encounter an energy barrier equal to m – s while flowing from S to M. Still, ohmic behavior can be achieved if highly-doped S is used (depletion region very thin, carriers can tunnel instead of surpassing potential barrier) Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 38  Schottky junction Bias Equilibrium Forward bias: -Built-in potential reduced, electrons in the S move to M Forward bias -Barrier height unchanged Reverse bias: Reverse bias -Built-in potential raised, electrons in the S hardly move to M -Barrier height unchanged Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 39  Schottky junction Bias Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky 40  Schottky junction Current characteristics Electrons emitted from S to M are called “hot” (energy higher by qΦB than in M) I 0 = AKT 2 e − qB / kT Collide in the M to give up free energy (equilibrium in > R0. Vex = 1…..5 V Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky  Special p-n junctions Tunneling diode – The maximum |NDR| can be found as – The peak voltage Vp – Iexcess is an additional tunneling current related to parasitic tunneling via impurities. – This current usually determines the minimum (valley) current, Iv Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky  Special p-n junctions Tunneling diode – Static resistance vs. Differential resistance Static resistance , R=V/I Differential resistance, Rd=R=V/I For linear (“Ohmic”) components, R = Rd. For many semiconductor devices, R ≠ Rd Microeletrónica: Sistemas e Dispositivos Junções p-n e Schottky  Special p-n junctions Tunneling diode – Power = Voltage x Current = I2 R – For R