About this applet

This Java applet illustrates the energy levels in various materials such as: metals, dielectrics and semiconductors.

How to use this applet

By clicking on the button on the right hand of the applet one can switch between semiconductor types (N or P). The Fermi level (Ef) as well as the affinity function (χsi), work function (Φ si) and doping parameter (φ f) are displayed. On the right hand one can choose the metal material to display its work function (Φm). In the center is the dielectric.

Discussion

Some of the energy characteristics of metals are: overlap of conduction with valence band (ie. no energy gap -Eg- ) and the Fermi level is always in the conduction band. Their work function varie from one material to the other and can exceed 5eV. Dielectrics in the other hand are materials whose distance between conduction band and valence band (Eg) is large leading to a low probability of finding electrons in the conduction band. The semiconducturs, however, have narrow energy gap with respect to dieclectric and their Fermi level is set by the doping specy and concentration and temperature.

The difference between the metal work function and that of semiconductor q*(Φm - Φ si) dictates the characteristics of the device these two materials form:

  • two semiconductors with opposit dopings to form diodes
  • metal - semiconductor to form ohmic contact
  • metal - semiconductor to form Schottky diode
  • Metal- Oxide - Semiconductor (MOS) if a dielectric is placed between the two materials

Junction Diodes: By flipping N-Si or P-Si buttons in this applet one can change the Fermi level in silicon as well as its position with respect to the intrinsic level by changing the doping concentration. When the material is doped N (N-type silicon) the Fermi level is above the intrinsic level. When the material in doped P (P-type silicon) the Fermi level is below the intrinsic level. Contacting the two materials leads to a junction diode whose barrier is the sum of the doping parameters (φ f) in N-type and P-type; repectively. This barrier exceeds 600mV at room temperature.

Schottky diodes: This device consist of metal whose work function exceeds that of the semiconductor such as nickel, palladium and platinum. When the metal is contacting an N-type semiconductor, for example, a junction is formed with a barrier (Schottky barrier) high enough to prevent current flow. Platinum, for example, is one of the material that lead to a Schottky diode with a useful barrier comparable to that in the pn-junctions. In manufacturing these metals are in fact a silicide such as PtSi2 with a slight shift in work function than that in pure platinum.

Ohmic contact: In the other hand, in metals such as aluminum, magnesium, molybden, etc, the work function is low and is comparable to that of a N-type silicon with moderate to high doping. Therefore the barrier is low leading to a less resistive contact called ohmic contact. Aluminum, however is a P-type dopant that can easily compensate and invert the N-silicon.

MOS: MOS devices consist of a gate (conductive material), an insulator (dielectric) and a semiconductor material (substrate). Two substrate contacts on each side of the gate form the source and drain nodes. The work function difference between that of silicon and gate dictates the threshold voltage for the channel formation. The channel is defined as surface inversion of the substrate from majority carriers to minority carriers. In normal operation, minority carriers flow from the source to the drain. The device is called enhancement MOS device and constitutes the fundamental component of the logic technologies. Below, is a general information on past, current and future trends in MOS devices:

  • Gate work function choice: As a general rule, metal work-functions that are close to silicon conduction band are good for nMOS devices, whereas metal work functions that are close to the silicon valence band are good for pMOS devices. In a P-type silicon (nMOS), if a gate work function is low such as in aluminum, tantalum, molybden or "degenerated N-Polysilicon (N-Poly) ", surface depletion of the P-Type silicon occurs easily without additional voltage. Surface inversion to minority carriers occurs with a modest theshold voltage. However, for large work function closer to the silicon valence band the threshold voltage is high and in general inacceptable (see MOS capacitor applet).
  • Early MOS devices: Aluminum (Φm = 4.25 eV) was used for the gate because of adequate work function leading to useful threshold voltage in n-Type MOS devices.
  • Advanced MOS Processing: Progress in semiconductor processing has radically eliminated aluminum as a gate for the last two decades by introducing polysilicon material. The work function for complementary MOS devices (CMOS) are undependently set to typically 4.1 and 5.2 eV by doping the polysilicon gates n-type and p-type, respectively. The doping level in polysilicon gate is high.
  • Challenges in sub 100nm technologies: Polysilicon as a gate is reaching some limitation due to polysilicon depletion, high gate resistance, etc. Some techniques such as polysilicon gate silicidation reduce gate resistance. Silicidation consists of a reaction between silicon with metals to form partial (surface of polysilicon gate only) or fully silicided gate such as: PtSi2, TiSi2, CoSi2, NiSi2 for even lower resistance and depletion effect elimination.
  • Metal Gate: Refractory metals for gate in MOS devices are under considerations today. For complementary MOS (CMOS), two work functions are necessary and should be close to the Silicon conduction band (~4.1eV) in nMOS, and close to the silicon valence band (~5.2eV) in pMOS. Metal work function engineering using metal alloy or chemically altered properties of the metal for useful threshold voltages is necessary.