PHYSICAL ELECTRONICS(ECE3540) CHAPTER 9 METAL SEMICONDUCTOR AND SEMICONDUCTOR HETERO-JUNCTIONS Tennessee Technological University Monday, November 11, 013 1
Introduction Chapter 4: we considered the semiconductor in equilibrium and determined electron and hole concentrations in the conduction and valence bands, respectively. The net flow of the electrons and holes in a semiconductor generates current. The process by which these charged particles move is called transport. Chapter 5: we considered the two basic transport mechanisms in a semiconductor crystal: drift: the movement of charge due to electric fields, and diffusion: the flow of charge due to density gradients. Tennessee Technological University Monday, November 11, 013
Introduction Chapter 6: we discussed the behavior of nonequilibrium electron and hole concentrations as functions of time and space. We developed the ambi-polar transport equation which describes the behavior of the excess electrons and holes. Chapter 7: We considered the situation in which a p-type and an n-type semiconductor are brought into contact with one another to form a PN junction. Tennessee Technological University Monday, November 11, 013 3
Introduction Chapter 8: We considered the PN junction with a forward-bias applied voltage and determined the current-voltage characteristics. When holes flow from the p region across the space charge region into the n region, they become excess minority carrier holes and are subject to excess minority carrier diffusion, drift, and recombination. When electrons from the n region flow across the space charge region into the p region, they become excess minority carrier electrons and are subject to these same processes. Tennessee Technological University Monday, November 11, 013 4
Introduction When a sufficiently large reverse-bias voltage is applied across a PN junction, breakdown can occur, producing a large reverse-bias current in the junction, which can cause heating effects and catastrophic failure of the diode. Zener diodes are designed to operate in the breakdown region. Breakdown puts limits on the amount of voltage that can be applied across a PN junction. Tennessee Technological University Monday, November 11, 013 5
Introduction Chapter 9: we will consider the metalsemiconductor junction and the semiconductor hetero-junction, in which the material on each side of thejunctionisnotthesame.thesejunctionscan also produce diodes. An Ohmic contact is a low-resistance junction providing current conduction in both directions. We will examine the conditions that yield metalsemiconductor Ohmic contacts. Tennessee Technological University Monday, November 11, 013 6
Metal-Semiconductor Junction There are two kinds of metal-semiconductor contacts: Rectifying Schottky diodes: metal on lightly doped Silicon. Low-resistance Ohmic contacts: metal on heavily doped Silicon. Tennessee Technological University Monday, November 11, 013 7
The Schottky Barrier Diode Rectifying contacts are mostly made of n-type semiconductors; for this reason we will concentrate on this type of diode. In the ideal energy-band diagram for a particular metal and n- type semiconductor, the vacuum level is used as a reference. The parameter M is the metal work function (in volts), s is the semiconductor work function, and is known as the electron affinity. Before contact, the Fermi level in the semiconductor was above that in the metal. In order for the Fermi level to become a constant through the system in thermal equilibrium, electrons from the semiconductor flow into the lower energy states in the metal. Tennessee Technological University Monday, November 11, 013 8
The Schottky Barrier Diode The parameter B0 is the ideal barrier height of the semiconductor contact, the potential barrier seen by electrons in the metal trying to move into the semiconductor. The barrier is known as the Schottky barrier and is given as: 0 ( B M ) On the semiconductor side, is the built-in potential barrier. This barrier, similar to the case of the PN Junction, is the barrier seen by electrons in the conduction band trying to move into the metal V bi is given as: Vbi ( 0 n) B Tennessee Technological University Monday, November 11, 013 9
The Schottky Barrier Diode Bn Increases with Increasing Metal Work Function Vacuum level, E 0 q M Si = 4.05 ev M : Work Function of metal q Bn E c Si : Electron Affinity of Si E f Theoretically, Bn = M Si E v x = 0 x = x n Fig. 9.1: Ideal energy-band diagram of a metal-semiconductor junction Tennessee Technological University Monday, November 11, 013 10
The Schottky Barrier Diode Metal Depletion layer Neutral region q Bn N-Si E c E f Schottky barrier height, B, is a function of the metal material. E v P-Si E c E f B is the most important parameter. The sum of q Bn and q Bp is equal to E g. q Bp E v Fig. 9.: Energy Band Diagram of Schottky Contact Tennessee Technological University Monday, November 11, 013 11
The Schottky Barrier Diode 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 0.4 0.3 Work Function 3.7 4.3 4.5 4.6 4.6 5.1 5.1 5.7 m (V) Bn + Bp E g Bn increases with increasing metal work function Tennessee Technological University Monday, November 11, 013 1
The Schottky Barrier Diode q M q Bn Si = 4.05 ev + Vacuum level, E 0 Ec E f A high density of energy states in the band gap at the metal-semiconductor interface pins E f to a narrow range and Bn is typically 0.4 to 0.9 V Question: What is the typical range of Bp? E v Fig. 9.3: Fermi Level Pinning Tennessee Technological University Monday, November 11, 013 13
The Schottky Barrier Diode Schottky Contacts of Metal Silicide on Si Silicide: 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 ErSi 1.7 HfSi MoSi ZrSi TiSi CoSi WSi NiSi Pd Si PtSi f Bn (V) 0.8 0.45 0.55 0.55 0.61 0.65 0.67 0.67 0.75 0.87 f Bp (V) 0.55 0.49 0.45 0.45 0.43 0.43 0.35 0.3 Table. 9.1: Schottky Contacts of Metal Silicide on Si Tennessee Technological University Monday, November 11, 013 14
The Schottky Barrier Diode q q Bn bi q Bn q( bi + V) qv E c E f E v E c E f E v qv W bi dep q q x n Bn Bn s C W dep ( E A c kt ln E E f N N d d c ) s( Vbi V qn max R ) end x Question: How should we plot the CV data to extract bi? s n Fig. 9.4: Using C-V Data to Determine B Tennessee Technological University Monday, November 11, 013 15
Exercise 1. Consider a contact between Tungsten and an n-type Silicon doped to N d = 10 16 cm -3 at T = 300K. Calculate the theoretical barrier height, built-in potential barrier and maximum electric field in the metal-semiconductor diode for a zero applied bias. Use the metal work function for Tungsten as M = 4.55V and electron affinity for Silicon = 4.01V. 0 ( B M ) Vbi ( 0 n) B W dep x n s( Vbi V qn d R ) Emax en x d n s Tennessee Technological University Monday, November 11, 013 16
Solution B0 is the ideal Schottky barrier height. B0 ( M ) 4.554.01 0. 54V The space charge width at a zero bias is: 19 kt N c.8x10 n ln 0.059ln 0. 06V 16 e N d 10 V bi ( B0 n) 0.540.06 0. 33 V W dep x n 14 s ( V bi V R ) (11.7 )( 8.85 * 10 )( 0.33 ) - 4 0.07 * 10 cm 19 16 qn (1.6 * 10 )(10 ) d E max end x s n 19 16 (1.6*10 )(10 )(0.07*10 14 (11.7)(8.85*10 ) 4 ) 3.*10 4 V / cm Tennessee Technological University Monday, November 11, 013 17
The Schottky Barrier Diode Using CV Data to Determine B 1/C 1 C ( bi V ) qn A d s q Bn q bi E c E f V E bi v Fig. 9.5: Using C-V Data to Determine C Tennessee Technological University Monday, November 11, 013 18
The Schottky Barrier Diode v thx - q( B V) E c V Metal N-type Silicon E fm q B qv E fn E v n v J th N SM c e q( V )/ kt 3kT J 1 st B / m e n qnv thx qv / kt mn kt h v thx 4qmnk 3 h, where J st 3/ kt T e e 100e q( V )/ kt / m n q / kt B B q / kt B e qv / kt A/cm Tennessee Technological University Monday, November 11, 013 19 x Richardson's Constant A * 4qmnk 3 h
The Schottky Barrier Diode Schottky Diodes V = 0 Forward biased I Reverse biased Reverse bias V Forward bias Tennessee Technological University Monday, November 11, 013 0
The Schottky Barrier Diode Schottky Diodes I 0 A I * I * A KT 4qmnk 3 h I SM e q / kt B M S 100 A/(cm I st e qv / kt K I st ) I st 1) Tennessee Technological University Monday, November 11, 013 1 ( e qv / kt Richardson's Constant A * 4qmn k 3 h
The Schottky Barrier Diode Applications of Schottky Diodes I I Schottky diode I I st ( e qv / kt 1) I st AKT e q / kt B B PN junction diode I st of a Schottky diode is 10 3 to 10 8 times larger than a PN junction diode, depending on B. Alarger I 0 means a smaller forward drop V. A Schottky diode is the preferred rectifier in low voltage, high current applications. V Tennessee Technological University Monday, November 11, 013
Exercise. Consider a Tungsten-Silicon diode with a barrier height of BN = 0.67V and J st =6*10-5 A/cm. Calculate the effective Richardson constant. J st * A T e q Bn / kt Richardson's Constant A * 4qmnk 3 h Tennessee Technological University Monday, November 11, 013 3
Solution 1. Using the relation for the reverse saturation current density: J st * A T e q Bn / kt A * J T st qbn / kt e 114 K A cm Tennessee Technological University Monday, November 11, 013 4
The Schottky Barrier Diode 110V/0V AC utility power PN Junction rectifier Hi-voltage 100kHz Hi-voltage Transformer Lo-voltage Schottky rectifier 50A DC AC AC 1V MOSFET DC inverter feedback to modulate the pulse width to keep V out = 1V Fig. 9.6: Switching Power Supply Tennessee Technological University Monday, November 11, 013 5
Applications of Schottky Barrier Diode Synchronous Rectifier: For an even lower forward drop, replace the diode with a wide-w MOSFET which is not bound by the tradeoff between diode V and leakage current. There is no minority carrier injection at the Schottky junction. Therefore, Schottky diodes can operate at higher frequencies than PN junction diodes. Tennessee Technological University Monday, November 11, 013 6
Comparison of Schottky Barrier Diode and the PN Junction Diode The ideal current-voltage relationship of the Schottky barrier diode are of the same form as the PN Junction Diode, there is only a magnitude difference in the reverse-saturation current densities and the switching characteristics. The current in a PN Junction is determined by the diffusion of minority carriers while the current in a Schottky barrier diode is determined by thermionic emission of majority carriers over a potential barrier. The effective turn-on voltage of the Schottky diode is less than the PN Junction diode. The Schottky diode is a high-frequency device than the PN Junction diode, therefore can be used in fast-switching application in pico-second time. Tennessee Technological University Monday, November 11, 013 7
Metal-Semiconductor Ohmic Contacts Ohmic Contacts: metal-to-semiconductor contacts providing conduction in both directions, and having the current throught the ohmic contact as a linear function of applied voltage. Ohmic contacts can be classified as ideal (non-rectifying barrier) or tunnel barrier. A) Ideal (Non-rectifying) Barrier Check the Energy band Diagram (the case where m < s ) Tennessee Technological University Monday, November 11, 013 8
Metal-Semiconductor Ohmic Contacts Now, if a positive voltage is applied into the metal, there is no barrier to electrons flowing from the semiconductor into the metal. If a positive voltage is applied to the semiconductor, the effective barrier height for electrons flowing from the metal into the semiconductor will be Bn = n which is fairly small for a moderately doped semiconductor therefore electrons can easily flow from the metal into the semiconductor. Tennessee Technological University Monday, November 11, 013 9
Metal-Semiconductor Ohmic Contacts B) Tunneling Barrier In a metal to semiconductor contact, the space charge width is inversely proportional to the square root of the semiconductor doping. As the doping concentration in the semiconductor increases, the probability of tunneling through the barrier increases. Tennessee Technological University Monday, November 11, 013 30
Semiconductor Heterojunctions Semiconductor Heterojunctions are formed between two semiconductor materials with different bandgap energies. Such a junction is useful because it can create a potential well at the interface where electrons are confined to in the direction perpendicular to the interface but are free to move in the other direction. In order to have a useful heterojunction, the latice constants of the two materials must be well-matched. Examples of Heterojunctions: GaAs - AlGaAs Tennessee Technological University Monday, November 11, 013 31
Exercise 3. Consider Silicon at T = 300K doped at N d =7*10 18 cm -3. Assume a rectifying Schottky barrier with Bn =0.67V. Consider the density of energy states for Silicon N csi =.8*10 19 cm -3. Calculate the space charge width for the Schottky rectifying diode. x n sv qnd bi 1 Tennessee Technological University Monday, November 11, 013 3
Solution 1. Using the relation for the reverse saturation current density: J st * A T e q Bn / kt A * J T st qbn / kt e 114 K A cm Tennessee Technological University Monday, November 11, 013 33
Picture Credits Semiconductor Physics and Devices, Donald Neaman, 4th Edition, McGraw Hill Publications. Modern Semiconductor Devices for Integrated Circuits, Prof. Chenming Calvin Hu, UC Berkeley (Free e-book Download) http://www.eecs.berkeley.edu/~hu/book-chapters-and-lecture-slides-download.html Tennessee Technological University Monday, November 11, 013 34