PHYSICAL ELECTRONICS(ECE3540) CHAPTER 9 METAL SEMICONDUCTOR AND - PowerPoint PPT Presentation

PHYSICAL ELECTRONICS(ECE3540) CHAPTER 9 – METAL SEMICONDUCTOR AND CHAPTER 9 – METAL SEMICONDUCTOR AND SEMICONDUCTOR HETERO-JUNCTIONS SEMICONDUCTOR HETERO-JUNCTIONS 1 Tennessee Technological University Monday, November 11, 2013 Brook Abegaz

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. 2 Tennessee Technological University Monday, November 11, 2013

Introduction  Chapter 6 : we discussed the behavior of non- equilibrium 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. 3 Tennessee Technological University Monday, November 11, 2013

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. 4 Tennessee Technological University Monday, November 11, 2013

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. 5 Tennessee Technological University Monday, November 11, 2013

Introduction  Chapter 9 : we will consider the metal- semiconductor junction and the semiconductor hetero-junction, in which the material on each side of the junction is not the same. These junctions can also produce diodes.  An Ohmic contact is a low-resistance junction providing current conduction in both directions. We will examine the conditions that yield metal- semiconductor Ohmic contacts. 6 Tennessee Technological University Monday, November 11, 2013

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. 7 Tennessee Technological University Monday, November 11, 2013

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. 8 Tennessee Technological University Monday, November 11, 2013

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:     ( ) V 0 bi B n 9 Tennessee Technological University Monday, November 11, 2013

The Schottky Barrier Diode  Bn Increases with Increasing Metal Work Function Vacuum level, E 0  M  Si = 4.05 eV : Work Function q  M of metal  Si : Electron Affinity of Si q  Bn E c 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 10 Tennessee Technological University Monday, November 11, 2013

The Schottky Barrier Diode Depletion Metal Neutral region layer q  Bn E c • Schottky barrier height,  B , E f is a function of the metal N-Si material. E v •  B is the most important E c parameter. The sum of q  Bn P-Si and q  Bp is equal to E g . E f q  Bp E v Fig. 9.2: Energy Band Diagram of Schottky Contact 11 Tennessee Technological University Monday, November 11, 2013

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 0.42 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 12 Tennessee Technological University Monday, November 11, 2013

The Schottky Barrier Diode • A high density of energy Vacuum level, E 0 states in the band gap at  Si = 4.05 eV the metal-semiconductor q  M interface pins E f to a narrow range and  Bn is q  Bn E c typically 0.4 to 0.9 V +  E f • Question: What is the typical range of  Bp ? E v Fig. 9.3: Fermi Level Pinning 13 Tennessee Technological University Monday, November 11, 2013

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 2 ZrSi 2 TiSi 2 CoSi 2 WSi 2 NiSi 2 Pd 2 Si PtSi f Bn (V) 0.28 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 0.23 Table. 9.1: Schottky Contacts of Metal Silicide on Si 14 Tennessee Technological University Monday, November 11, 2013

The Schottky Barrier Diode     ( ) qV q E E q  bi q  Bn bi Bn c f E c N    E f c ln q kT Bn N d   2 ( ) V V   E v s bi R W x dep n qN d q  Bn  q(  bi + V) eN x   s d n | | C A E max  W qV dep s E c Question: E f How should we plot the CV data to extract  bi ? E v Fig. 9.4: Using C-V Data to Determine  B 15 Tennessee Technological University Monday, November 11, 2013

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.          ( ) V ( ) 0 bi B n 0 B M   eN d x 2 ( ) V V    | | n E s bi R W x  max dep n qN s d 16 Tennessee Technological University Monday, November 11, 2013

Solution  B0 is the ideal Schottky barrier height.         ( ) 4 . 55 4 . 01 0 . 54 V 0 B M The space charge width at a zero bias is:     19 2 . 8 10 kT N x         c ln 0 . 0259 ln 0 . 206 V     n 16    10  e N d        ( ) 0 . 54 0 . 206 0 . 33 V V 0 bi B n    14 2 ( ) 2 ( 11 . 7 )( 8 . 85 * 10 )( 0 . 33 ) V V     - 4 s bi R 0.207 * 10 cm W x  dep n 19 16 ( 1 . 6 * 10 )( 10 ) qN d   19 16 4 ( 1 . 6 * 10 )( 10 )( 0 . 207 * 10 ) eN x    4 d n | | 3 . 2 * 10 / E V cm   max 14 ( 11 . 7 )( 8 . 85 * 10 ) s 17 Tennessee Technological University Monday, November 11, 2013

The Schottky Barrier Diode Using CV Data to Determine  B   1 2 ( ) V  bi  2 2 C qN A d s 2 1/C q  bi q  Bn E c E f V E  bi v Fig. 9.5: Using C-V Data to Determine C 18 Tennessee Technological University Monday, November 11, 2013