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MOSFET Threshold Voltage Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp Introduction to Electronic Devices (Course Number 300331) Fall 2006 Field Effect Transistors (FETs) Information: Dr. Dietmar Knipp http://www.faculty.iu- bremen.de/dknipp/ Assistant Professor of Electrical Engineering Source: Apple Ref.: Apple Ref.: IBM Critical 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 101 dimension (m) Ref.: Palo Alto Research Center Field-Effect Transistors (FETs) 1 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp Introduction to Electronic Devices 6 Field-Effect-Transistors (FETs) 6.1 Introduction 6.2 Basic Device Structures 6.3 The MOS structure 6.3.1 Accumulation 6.3.2 Depletion 6.3.3 Weak Inversion 6.3.4 Strong Inversion 6.3.5 Carrier Distribution for an ideal MOS structure 6.3.6 Potential Distribution for an ideal MOS structure 6.3.7 The ideal MOS structure 6.4 The silicon oxide MOS structure 6.4.1 The Work Function Difference 6.4.2 Interface Traps and Oxide Charges 6.4.3 The MOSFET Threshold voltage Field-Effect Transistors (FETs) 2 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.5 Metal Oxide Semiconductor Field Effect Transistor (MOSFET) 6.5.1 Basic characteristics of MOS Field Effect Transistor 6.5.2 MOSFETs modeling 6.5.2.1 Gradual Channel Approximation 6.5.2.2 I/V characteristic in the linear region 6.5.2.3 I/V characteristic in the saturation region 6.5.2.4 The output characteristic 6.5.2.5 The transfer characteristic 6.5.2.6 Conductance and Transconductance 6.5.2.7 The Sub threshold Region 6.5.3 Types of MOSFETs 6.5.4 Threshold voltage control 6.5.5 Scaling of MOSFETs 5.5.5.1 Threshold Voltage Roll-off in the linear region 5.5.5.2 Drain Induced Barrier Lowering in the saturation region 5.5.5.3 Bulk punch through 5.5.5.4 Scaling Rules 6.6 Silicon on Insulator (SOI) 6.7 Thin Film Transistors (TFTs) 6.7.1 Amorphous silicon TFTs 6.7.2 Poly silicon TFTs 6.7.3 Organic TFTs References Field-Effect Transistors (FETs) 3 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6 Field Effect Transistors (FETs) The concept of the Field Effect Transistor (FET) was proposed by Lilienfeld (1930). The idea got practical after the pioneering work of Shockley in the early 1950. Today the Field-Effect Transistor is the most important electronic device used for microprocessors and semiconductor memories. Throughout the last 50 years several field-effect transistor concepts have been developed and implemented. The most important group of field effect transistors is the class of metal insulator semiconductor field effect transistor (MISFET). In this case the gate is insulated from the channel of the transistor by an insulator. Out of this class the metal-oxide-semiconductor field-effect transistor (MOSFET) structure is by far the most important structure. Another class of transistors, which belongs to the group of field effect transistors is the MESFET. The Metal-semiconductor field effect transistor (MESFET) structure is different from the MISFET structure. Instead of using a MOS structure to modulate the channel conductivity a metal semiconductor (Schottky barrier / Schottky diode) is used control the conductivity of the channel. In the following the operating principle of the Metal-Oxide-Semiconductor Field- Effect Transistor will be discussed. Field-Effect Transistors (FETs) 4 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.1 Introduction The metal-oxide-semiconductor field-effect transistor (MOSFET) is composed of a MOS structure (MOS diode / MOS capacitor) and two pn-junctions placed immediately adjacent to the MOS structure. A MOSFET is a charge controlled device. Charges have to be accumulated on the gate to control the device properties. As a consequence of voltage applied to the gate a channel is formed at the interface between dielectric an substrate. Source Gate Drain VGS I V G ID DS IS n+ n+ Schematic cross section Induced n-channel Dielectric of an enhanced-type p-type substrate NMOS transistor. Bulk Field-Effect Transistors (FETs) 5 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.1 Introduction The induced channel of a MOSFET can be a n-channel (high concentration of electrons) or a p-channel (high concentration of holes). Therefore, a MOSFET is an unipolar device. Either electrons or holes contribute to the current flow. In terms of the real implementation the statement is not completely right, because pn-junctions are placed adjacent to the MOS structure. The output current of the transistor is defined to be the current between the drain and source contact. The drain current is controlled by the gate bias. The FET is in the off-state, when only a few electrons (holes) move from the source to the drain. In the on-state electrons or holes are injected via the source and flow to the drain. Therefore, the operating principle of a MOSFET is quite different from the behavior of a bipolar junction transistor (BJT). A BJT is a current/voltage controlled device, whereas the MOSFET is a charge controlled device. A MOSFET is a unipolar device, whereas BJTs or diodes are bipolar devices. Field-Effect Transistors (FETs) 6 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.2 Basic Device Structures The MOSFET is a charge controlled device, so that the charge on the gate should be maximized. The charge on the gate can be calculated by: ε ⋅ε Q = C V = 0 dielectric ⋅V G G G d G The charge on the gate can be maximized by increasing the dielectric constant of the gate dielectric and decreasing the thickness of the gate dielectric. For example: The gate capacitance can be increased by changing the dielectric material from silicon oxide SiO2 to Aluminum oxide Al2O3. Aluminum oxide exhibits a higher dielectric constant. The gate electrode and the channel form a plate capacitor or a MOS structure, so that the electrons (holes) are located in a thin region close to the dielectric interface. Field-Effect Transistors (FETs) 7 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.3 The MOS structure The MOS structure is the „heart“ of a MOSFET. In the following the operating principle of the MOS structure will be discussed before addressing the fundamentals of the MOSFET. In general a MOS structure can be used to store charges. The concept of storing charge in a MOS structure is used for example to realize Charge Coupled Devices (CCD) and MOSFETs. Cross section of a metal oxide semiconductor (MOS) structure. Ref.: M.S. Sze, Semiconductor Devices Field-Effect Transistors (FETs) 8 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.3 The MOS structure The energy band diagram of a MOS structure using a p-type semiconductor is shown in the figure under thermal equilibrium (V=0). Before discussing the energy diagram the following functions and energies are introduced: The work function qΦ is defined as the energy required to remove an electron from the Fermi level EF to a position outside the material (vacuum level). The work function can be defined for a semiconductor, a metal or an insulator. qϕB The electron affinity qχ is the energy required to remove an electron from the bottom of the conduction band to the vacuum level. Energy band diagram of an ideal MOS structure for V=0 (flat band condition). Ref.: M.S. Sze, Semiconductor Devices Field-Effect Transistors (FETs) 9 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.3 The MOS structure E g qφms ≡ qφm − qφs = qφm − qχ + + qϕB = 0 2 At zero applied bias (V=0) the energy difference between the metal work function qφm and the semiconductor work function qφs is zero. As a the consequence the work function difference qφms is zero. In this case the energy band diagram is flat. This case is called flat-band condition! Before we discuss the influence of different gate voltages on the band energy diagram and the carrier distribution of a MOS structure, we will introduce the surface electrostatic potential (or simply surface potential). The gate voltage leads to a bending of the band diagram and we will describe the bending of the band by using the bulk and the surface potential. Field-Effect Transistors (FETs) 10 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.3 The MOS structure The surface potential ϕS is defined as zero in the bulk of the semiconductor. At the semiconductor surface the electric potential is equal to the surface potential. If now a voltage is applied to MOS structure charges are displaced due to coulomb interaction. In the following it is assumed that the MOS structure acts as a plate capacitor. The formation of charges in the semiconductor leads to the formation of opposite charges on the metal electrode. There is no carrier transport through the gate dielectric. Therefore, the MOS structure is in thermal equilibrium even though a voltage is applied to the structure. qϕ qϕB qϕS Energy band diagram at the surface of a p-type semiconductor Ref.: M.S. Sze, Semiconductor Devices Field-Effect Transistors (FETs) 11 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 6.3 The MOS structure In the following it is assumed that the semiconductor material is p-type doped. 5.3.1 Accumulation If a negative voltages (V<0) is Accumulation applied to the MOS structure excess carriers (holes) will be accumulated at the semiconductor / dielectric interface. In this case the energy band at the interface between the semiconductor and the dielectric is bended upwards.
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