Review of Semiconductor Devices

Si vs. GaAs: crystal lattice Both GaAs and Si have FCC lattice with two basis at (0,0,0) and (¼, ¼, ¼) First brillouin zone for fcc lattice

Band Structure For Si, there are six equivalent minima at 0.8X, along the (100) direction (in the reciprocal lattice). For GaAs, secondary minimum at the L point about 0.3 eV above the  minima

Ingot Growth and Epitaxy Substrate growth Si : Czochralski GaAs, GaP : Liquid encapsulated Czochralski with arsenic or phosphorus over-pressure Semi-insulating properties: Si : difficult to get semi-insulating wafers ( ~ 104 -cm) GaAs: deep acceptors compensated wafers have  ~ 108 -cm. Recently semi-insulating wafers realized without compensation Epitaxial growth (mainly for compound semiconductors) MBE CVD Czochralski technique

Processing Doping Diffusion, ion implantation, and oxidation Etching Si : As (54 meV), P (45 meV), B (45 meV) GaAs: Si (5.8 meV), Be (28 meV), Mg (28 meV) Diffusion, ion implantation, and oxidation Standard for Si Difficult for compound semiconductors Etching Si: very anisotropic etch GaAs: fairly isotropic

Mobility and velocity-field plots For Si: 2107 cm2V-1s-1 107 cm2V-1s-1 107 Carrier drift velocity (cm/s) ~ 2, for electrons ~ 1, for holes 106 For GaAs: 105 103 104 105 Electric field (V/cm) vs = 1x107 cm/s for GaAs Room temperature low field mobility of GaAs is 8500 and of Si is 1500 cm2V-1s-1 (for undoped samples).

Transferred Electron Effects Oscillations due to TE effect I GaAs sample Vdc Pulse width Time Electrons get transferred from the (0,0,0) minima at  point to a higher minima at L point along the 111 direction having mobility of 100 cm2V-1s-1

MOSFET G S D n+ n+ p 0,Si > 0,SiO2 Ec EF Ev qVg qVg Vg < 0 Accumulation Vg > 0 Depletion/Inversion Vg = 0 Flat band

Gate Contacts and Current Thermionic emission (Reverse bias sat. current) Direct (MOS) Fowler-Nordheim (MOS) qB qB qVg qVg qVg A* = effective Richardson’s constant Vg > 0, d > 4.5 nm Vg > 0, d < 4.5 nm

MOSFETs vs. MESFETs Both MOSFETs and MESFETs are homo-structure devices MOSFET is enhancement mode, while MESFET is depletion mode device MOSFET uses MOS junction to control channel current while MESFET uses Schottky junction MOSFET gate leakage can be very low while MESFET gate leakages can be considerable MESFETs are usually much faster than MOSFETs Difficult to realize complimentary devices for GaAs because of very low mobility of p-type devices and lack of MOS type junctions Inversion difficult to achieve in GaAs base devices due to absence of good insulator and high surface trap density GaAs based devices are radiation hard

HEMTs vs. HBTs AlGaAs/GaAs HEMT n+ AlGaAs/p-GaAs/n- GaAs HBT Emitter Vn Vp Emitter (n+ AlGaAs) Base (p-GaAs) Gate metal AlGaAs donor layer GaAs buffer Collector (n-GaAs) AlGaAs spacer Due to separation of the electrons from the parent donors the mobility is higher Due to wider emitter bandgap, the base current is smaller and gain is higher

Integration Issues Difficult to make a Si device based MMIC due to parasitic capacitances due to lack of semi-insulating substrate (lower bandgap) MMICs are usually made of depletion mode compound semiconductor devices (higher bandgap, semi-insulating substrate) Difficult to have very high integration with GaAs due to unavailibility of complementary devices and higher leakages Non-planar devices such as HBTs are quite difficult to integrate at a very large scale

Detectors, Emitters, Memory Devices Photo-detectors such as avalanche and p-i-n photodiodes can be made of both Si and GaAs depending on the applications Solar cells are made primarily of Si due to lower cost and smaller bandgap LEDs and Lasers are made primarily from III-arsenide and III-phosphide compounds (also recently III-nitrides) Floating gate devices are usually made of Si and Si based insulators. Typical examples are floating gate device (metal-insulator-oxide-semiconductor) are flash memories

Common Semiconductors  Properties  Si (----)  GaAs (AlGaAs/ InGaAs)  InP (InAlAs/  4H- SiC  GaN (AlGaN/ GaN)  Bandgap (eV)   1.11 1.42 1.35 3.26 3.42  e (cm2/Vs) 1500 8500 (10000) 5400 700 900 (2000) Vsat ( 107 cm/s) 1 (2.1) (2.3) 2 1.5 (2.7) 2DEG density (cm-2) NA < 41012 1-2 1013 EB (106 V/cm) 0.3 0.4 0.5 3.3 Dielectric constant 11.8 12.8 12.5 10 9

Wide Bandgap Applications GaN is projected to be a $3 billion industry by 2007

Figures of merit for high frequency/high power devices Semiconductor Electron mobility (cm2/Vsec) Relative permittivity e Bandgap Eg (eV) BFOM Ratio JFM Ratio Tmax (°C) Si 1300 11.4 1.1 1.0 300 GaAs 5000 13.1 1.4 9.6 3.5 SiC 260 9.7 2.9 3.1 60 600 GaN 1500 9.5 3.4 24.6 80 700 BFOM: Baliga’s figure-of-merit JFM: Johnson’s figure-of-merit 2 ce m v E Figure of Merit o s B CFOM = ( ) 2 ce m v E o s B silicon c is thermal conductivity EB is breakdown field m is low field mobility vs is saturation velocity eo is dielectric constant

Nitride Advantages for Electronic Devices Properties Advantages High mobility High saturation velocity High sheet carrier concentration High breakdown field High microwave power, Power electronic devices Wide bandgap ( ) Growth on SiC substrate High temperature operation Chemical inertness Good ohmic contacts No micropipes Holds promise for reliable device fabrication Insulated Gate transistors possible SiO2/AlGaN and SiO2/GaN good quality interfaces

Power Densities for GaN and SiC Microwave Devices Output power density (W/mm) Highest reported value of power density of 12 W/mm at DRC, 2003

Size Reduction with Same Output Power