Other FET’s and Optoelectronic Devices Chapter Other FET’s and Optoelectronic Devices Sung June Kim kimsj@snu.ac.kr http://nanobio.snu.ac.kr

CONTENTS Other things you should know about MOSFET’s JFET’s and MESFET’s Photodiodes and Laser Diodes

Transistor Action Load Line Analysis E = iD R + VD

Amplification & switching Fig 6-2 A three-terminal nonlinear device that can be controlled by the voltage at the third terminal vG: (a) biasing circuit; (b) I-V characteristic and load line. If VG = 0.5Vm the d-c value of ID and VD are as shown by the dashed lines

N-MOSFET’s have higher Currents and gm than p-MOSFET’s. Note: These are short channel devices.

VT adjustment I2 (ion implantation) Adjustment of VT in a p-channel transistor by boron implantation : (a) boron ions are implanted through the thin gate oxide but are absorbed within the thick oxide regions; (b) variation of implanted boron concentration in the gate region – here the peak of the boron distribution lies just below the Si surface. Qd control

VT adjustment I2

Equivalent circuit Equivalent circuit of a MOSFET, showing the passive capacitive and resistive components. The gate capacitance Ci is the sum of the distributed capacitances from the gate to the source-end of the channel(CGS) and the drain-end(CGD). In addition, we have an overlap capacitance (where the gate electrode overlaps the source/drain junctions) from the gate-to-source(COS) and gate-to-drain(COD). COD is also known as the miler overlap capacitance. We also have p-n junction depletion capacitances associated with the source (CJS) and drain(CJD). The parasitic resistances include the source/drain series resistances (Rs and RD), and the resistances in the substrate between the bulk contact and the source and drain(RBS and RBD). The drain current can be modeled as a (gate) voltage-controlled constant-current source.

MOSFET scaling base = constant electric field Table 6-1 scaling factor surface dimensions (L,Z) 1/k vertical dimensions (d,xi) impurity concentrations k current, voltages current density Capacitance (per unit area) transconductance 1 circuit delay time power dissipation 1/k2 power density power delay product 1/k3

JFET Fig 6-3 Simplified cross-sectional view of a junction FET: (a) transistor geometry; (b) detail of the channel and voltage variation along the channel with VG=0 and small ID

JFET Pinch-off

JFET I-V by inspection

MESFET

HEMT (a) Simplified view of modulation doping, showing only the conduction band. Electrons in the donor doped AlGaAs fall into the GaAs potential well and become trapped. As a result, the undoped GaAs becomes n-type, without the scattering by ionized donors which is typical of bulk n-type material. (b) use of a single AlGaAs/GaAs heterojunction to trap electrons in the undoped GaAs. The thin sheet of charge due to free electron at the interface forms a two dimensional gas(2-DEG), which can be exploited in HEMT devices.

Photodiodes are Detectors of photons.

LED(Light Emitting Diode)

Laser Diodes : Stimulated Emission of Photons.

A double heterojunction laser structure: (a) multiple layers used to confine injected carriers and provide waveguiding for the light; (b) a stripe geometry designed to restrict the current injection to a narrow stripe along the lasing direction. One of many methods for obtaining the stripe geometry, this example is obtained by proton bombardment of the shaded regions in (b), which converts the GaAs and AlGaAs to semi-insulating form.

Separate confinement of carriers and waveguiding: (a) use of separate changes in AlGaAs alloy composition to confine carriers in the region (d) of smallest band gap, and to obtain waveguiding (w) at the larger step in refractive index; (b) grading the alloy composition, and therefore the refractive index, for better waveguiding and carrier confinement.