Random Telegraph Noise (RTN) – an often encountered problem
use QPC device as charge noise sensor control RTN with Vtop
power spectral density
two-level RTN: Lorentzian, 1/f2 above corner freq.
ensemble of fluctuators with homogeneous distribution of eff : 1/f
Equivalent Gate Voltage Noise VEG
noise is reduced exponentially with less negative VG
RTN time scales?
device with single fluctuator
independent of temperature (to 4.2K), i.e. tunneling, not thermally activated process
typical for INchanges in OUT less pronounced, with both increasing and decreasing trends
Origin of RTN?
simulations:- 1D not enough (full screening)- 2D self-consistent “nextnano”
comparison: VTOP = 0 and VTOP = -1.7 V
steeper slope for VTOP = 0trap energy lifter for VTOP = -1.7 V“smaller” barrier
leakage eliminated for ETRAP > m
partial depletion using VTOP reduces oreliminates tunneling from Schottky gate
deep trapping state
comparison: VTOP = 0 and VTOP = -1.7 Videntical carrier depletion width
more radial fields for VTOP = 0tunneling in more sideways directions(access traps at surface)
OUT depends on E-field at trap locationwide range of behaviors as seen
bias cooling vs. surface gate
two techniques consistent
bias cooling:- apply (positive) bias to Schottky gates during cool-down- carriers (negative) are frozen in deep traps (DX-centers) at low T- when bias removed, non-equilibrium trap occupation maintained
presence of additional negative charges lowers VG
distributed doping delta doping
Role of DX centers?
x = 0.1 : no DX centers available (above conduction band edge)i.e. here, DX cannot be the intermediate charge trap for noise
x= 0.2, 0.3 : DX levels well below C band
noise for bias-cooled distributed doping
noise for bias-cooled delta doping
noise for bias-cooled delta doping
observations:• x = 0.1 most, x = 0.3 least noise(low band gap x=0.1, more tunneling)
• lower noise with less neg. VGabove VG ~ -0.4 V constant noise
• very negative VG, ~ 2(few active traps)
• VG > -0.4 V : ~< 1(other ensemble charge noise sample)( < 1 : nonuniform distribution corner freq.
• DX centers excluded
conclusions
• charge noise in GaAs/AlGaAs devices dominated by electron trapping leakingfrom Schottky gates
• noise reduced if tunneling is eliminated, baseline charge noise remains (of different origin)
• bias cooling or top gates to reduce tunneling / noise(top gate more versatile, in-situ, works in some cases where bias fails)
• introducing additional high bandgap AlAs layer under cap, or thin insulator under gates for noise reduction