The Intrinsic Silicon Thermally generated electrons and holes
- Slides: 26
The Intrinsic Silicon • Thermally generated electrons and holes • Carrier concentration pi =ni ni=1. 45 X 1010 cm-3 @ room temp Generally: ni= 3. 1 X 1016 T 3/2 e-1. 21/2 KT cm-3 T= temperature in Ko (Degrees Kelvin) K= Boltzmann Constant = 8. 63 X 10 -5 e. V/Ko
The Extrinsic Silicon Number of carriers is increased by introducing foreign atoms called impurities The process of introducing impurities is called doping Two Types of dopants: p type and n type p-type dopants: Boron (B), Gallium (G), Aluminum (Al) n-type dopants: Arsenics (Ar), Phosphorous (P), Antimony (Sb)
Doping Concentration P-type: concentration = p = NA+pth NA= concentration of p type do pant (atoms/cm 3) pth= concentration of thermally generated holes (holes/cm 3) p NA (NA>> pth ) n-type: concentration = ND+nth ND= concentration of n type do pant (atoms/cm 3) nth= concentration of thermally generated electrons (electrons/cm 3) n ND (ND>> nth )
Degrees of Doping Degree of concentration • N- - or P - - : ND or NA<1014 cm-3 • N- or P - : 1014 cm-3 <ND or NA<1016 cm-3 (lightly doped) • N or P : • N+ or P + : 1018 cm-3<ND or NA<1020 cm-3 (heavily doped) • N++ or P ++ : 1016 cm-3<ND or NA<1018 cm-3 (moderately doped ND or NA>1020 cm-3
Review of the pn Junction p type Depletion Region n type Potential across pn junction: D = (KT/q) ln( NA. ND/ni 2 ) Depletion) region length: Xd = K [ [ (1/NA) + (1/ND)] K constant a function of ( ε , q ) si D ]0. 5 Junction Capacitance: P- P+ N+ N- Cj = Cjo / (1+V/ D)0. 5 It is a function of the applied voltage and doping concentration D Xd
Two terminal MOS Structure Vg Insulator Sio 2 P+ Substrate v. B Depth of Depletion region: Xd = { 2 εSi. | s - F │ }0. 5 The Charge Density: Q = - { 2 q NA εSi. | s - F │ }0. 5 Gate Metal
The Physical Structure Al Si. O 2 P+ N+ n Cross-section of pn-junction in an IC process n P+ Al Al
3 D Perspective
Nmos Transistor D Polycrystalline silicon G Gate oxide Gate S NMOS Enhancement D (Bulk) G B S NMOS with Bulk Contact
The Physical Structure (NMOS) Gate oxide Polysilicon Gate Al Si. O 2 Field Oxide Al Si. O 2 S n+ D n+ channel Field Oxide L P Substrate contact (G) (S) n+ L n+ (D) W Poly The process and sequence is designed by the fabrication house You design the MASKS Metal
Regimes of Operation G S 1. Accumulation VGS is negative Majority carries attracted to the surface D N+ N+ -Ve charge p-type G S 2. Depletion VGS increased by a small amount Majority carriers depleted Space charge (depletion) region formed D p-type Inverted surface to N-type 3. Inversion VGS increased further Minority carriers attracted to surface Inverted surface provides conduction S p-type G D
The Threshold Voltage §The voltage applied between the gate and the source which causes the beginning of the channel surface strong inversion. §Threshold voltage Vt is a function of : n n n Vfb = flatband voltage; depends on difference in work function between gate and substrate and on fixed surface charge. s = surface potential. Gate oxide thickness. Charge in the channel area. Additional ion implantation. n Typical values: 0. 2 V to 1. 0 V for NMOS and -0. 2 to -1. 0 V for PMOS
Threshold Adjust D G , Φs =2ΦF s is the surface potential ~ -0. 6 V for NMOS is the body factor ~ 0. 6 to 1. 2 V 1/2 Fermi potential ΦF is is –ve in n. MOS, +ve in p. MOS The body effect coefficient γ is +ve in n. MOS, -ve in p. MOS The substrate bias voltage VSB is +ve in n. MOS, -ve in p. MOS B S g Threshold voltage is a function of source to substrate voltage VSB. g Body factor is the coefficient for the VSB dependence factor. VSB
Threshold Adjust Ion Implantation (dopant) S gn. MOS transistors implanted with ntype dopant results in a decrease in threshold voltage g. An effective mean to adjust the threshold is to change the doping concentration through an ion implantation dose. gn. MOS transistors implanted with ptype dopant results in an increase in the threshold voltage. D channel p Substrate VTO’=VTO + (q. DI /Cox) DI = dose of dopant in the channel area(atoms/cm 2) Cox = gate oxide capacitance per unit area
Threshold Adjust… Continued Example of Numerical Values for our process
Threshold Adjust D n Depletion NMOS transistor n n G Heavy ion implantation of n dopant in the channel area results in negative threshold voltage Transistor conducts with zero gate to source voltage. It is called Depletion mode transistor S NMOS Depletion Al n+ Field threshold adjust n n n Required to minimize interaction between transistors. Heavy implantation called p-guard/nguard VTF = 12 to 22 V P+ n+ n+ P+
Current-Voltage Relations S VGS VDS G n+ ID D n+ p-substrate MOS transistor and its bias conditions
Gate Voltage and the Channel gate VGS > VT VDS< (VGS-VT) channel source drain Id gate VGS > VT VDS = (VGS –VT ) VGS > VT VDS > (VGS-VT ) source drain Id gate source drain Id
Qualitative Operation of NMOS Transistor 1. Cut-Off Region VGS < VT • No Inversion or Weak Inversion • IDS = leakage current or sub-threshold current 2. Linear Region VGS > VT and VDS < VGS-VT • Channel surface is inverted • Output current depends on VGS and VDS • The relationship between IDS and VDS is almost linear
NMOS Operation-Linear Process Tranconductance u. A/V 2 for 0. 35 u, K’ (Kp)=196 u. A/ V 2 Gate oxide capacitance per unit area eox = 3. 9 x eo = 3. 45 x 10 -11 F/m tox Oxide thickness for 0. 35 u and tox=100 Ao Quick calculation of Cox: Cox= 0. 345 / tox (Ao) pf/um 2 u = mobility of electrons 550 cm 2/V-sec for 0. 35 u process
NMOS Operation-Linear Effect of W/L W Ids Effect of temperature temp u Ids K’ Ids Impact of oxide thickness tox W
Transistor in Saturation Electrons leaving channel are injected in depletion region and accelerated towards drain VGS g g Voltage across channel tends to remain constant D S n+ The current IDS saturates with very weak dependence on VDS g = channel length modulation parameter typical values 0. 01 V-1 to 0. 1 VDS > VGS - VT G - VGS - VT + n+
V GS = 5 V 2 Saturation IDS (m A) Linear V GS = 4 V 1 V GS = 3 V 0. 020 ÖIDS V DS = V GS -VT Square D ependence I-V Relation 0. 010 Subthreshold Current V GS = 2 V V GS = 1 V 0. 0 1. 0 2. 0 3. 0 4. 0 5. 0 0. 0 V DS (V) (a) I DS as a function of VDS 2. 0 VT 1. 0 V GS (V) (b) Ö IDS as a function of VGS (for V NMOS Enhancement Transistor DS = 5 V) . 3. 0
Variations in Width and Length 1. Width Oxide encroachment Weff = Wdrawn- 2 WD polysilicon Weff Wdrawn WD polysilicon 2. Length Lateral diffusion LD = 0. 7 Xj Leff = Ldrawn- 2 LD Ldrawn LD Leff LD WD
The PMOS Transistor Polysilicon Gate oxide Al Si. O 2 S p+ Field Oxide channel D p+ Field Oxide L N Substrate D D G G S PMOS Enhancement B S PMOS with Bulk Contact
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