4 Analysis of Interband Tunneling in MOS Devices

The drain current in MOSFETs biased in the off-state can originate due to:

• The two-dimensional drain-source interaction effects
  • The conventional subthreshold diffusion current, due to the gradient of nonvanishing inversion-layer charge under the threshold voltage. In short-channel devices, this current is strongly influenced by the channel length due to the drain-induced-barrier-lowering (DIBL) effect.
  • The bulk conduction due to the lowering of the barrier at the source side (saddle point) by the depletion region of the drain junction (the punch-through effect). This is a typical two-dimensional effect caused by the vicinity of the junctions.
  • Both effects, 1) and 2) can be enhanced by the impact ionization, leading to the breakdown at higher drain biases.
  • The positive feedback effect, known as the parasitic bipolar-transistor action, produces an increased bulk conduction, which eventually leads to breakdown.
  The current caused by these effects flows mainly between the drain and source, whereas a substantial bulk current can be generated in the impact-ionization processes as well. This leakage current component is dependent on the channel length.
• The drain-bulk gated-diode effects
  • The thermal-generation leakage due to the interface and bulk traps [156][133]. These effects are complicated in the heavily doped junctions [344][308].
  • The avalanche multiplication. The position of the ``hot-point'' strongly depends on the gate voltage, doping level and the oxide thickness [158][123][44]. The breakdown occurs in the shallow deeply-depleted region between the heavily-doped junction and the oxide when the channel region is strongly accumulated, at the junction corner near the intersection of the junction and the interface for an intermediate gate bias and in the bulk when the channel is inverted.
  • The interband tunneling or the band-to-band tunneling (BBT) in the deeply-depleted region which is induced between the oxide and the heavily doped junction by an accumulating gate field and a lateral field (the drain-bulk bias) [211]. For the interband tunneling to occur two conditions must be fulfilled:
    - the band-bending is larger than the tunneling band gap and
    - the tunneling path is short to enable a significant tunneling generation rate.
  • The tunneling over the interface states and the bulk traps (e.g. those produced as the implantation damage). It has been shown that the anomalous leakage characteristics of heavily-doped gated diodes can be satisfactorily described by accounting for the field-enhanced capture cross-sections in the emission due to the Poole-Frenkel effect and for trap-assisted tunneling [173], [181], [221] and [491][231]. This includes the dependences on the doping level, terminal bias and silicon-lattice temperature.
  The gated-diode current flows between the drain and bulk, but also between the source and bulk for a nonvanishing bulk-source bias. This leakage current component is essentially independent of the channel length. Since it is directly influenced by the gate bias, it is called the gate-induced drain leakage (GIDL).
• The effects at the bottom region of the drain-bulk diode
  • The inversion saturation current (diffusion of minority carriers through the junction). This current is very small.
  • The band-to-band tunneling is impossible to occur for the reason of a low substrate or well dopant concentrations.
  • The tunneling through the bulk traps.
  • The avalanche multiplication of the carriers induced by other processes.

• 4.1 Interband Tunneling in ULSI MOS Devices
• 4.2 Numerical Analysis of Interband Tunneling in MOSFETs
  • Two-Dimensional Numerical Approach
  • The Potential and Field Distributions in the Critical Area