Optimization of Source/Drain Schottky Barrier Height for be SOI Mosfet
Authors: Yojo Leonardo Shimizu , Rangel Ricardo Cardoso , Sasaki Katia Regina Akemi , Martino Joao Antonio 
Published: February 2020

New devices architectures have been proposed to enable the evolution of semiconductors technology. One possibility is the reconfigurable transistors [1], i.e., transistor that can act as an n-type or as a p-type, which could enable circuits performing more than one function [2]. A new reconfigurable planar device was proposed/fabricated at Laboratorio de Sistemas Integraveis (LSI) from University of Sao Paulo (USP) using SOI substrate [3]. The named BE (Back Enhanced) SOI MOSFET (Patent BR 102015020974-6, 2015) is a kind of undopped junctionless SOI transistor and it shows the benefit of a simpler fabrication process compared to non-planar similar devices. In order to enable the good operation of the BE SOI for both types of transistors, the source/drain Schottky contact electrode perform an essential role. This work investigates the influence of the source/drain electrode metal workfunction on the drain current levels of the n-type and p-type BE SOI MOSFET in order to optimize its behavior.

Figure 1 shows a schematic profile of the device. The work principle of the BE SOI MOSFET is based on different back bias (VGB) in each case. For a positive enough VGB, an electron channel is formed at the back interface that connects the source, channel and drain. In this case it is working like the n-type BE SOI MOSFET. This current path can be interrupted by the front gate (VGF). The electrostatic coupling due to VGB and VGF can deplete the channel, thus ceasing the drain current. Analogously, for a negative enough VGB, a hole channel is formed at the back interface and the p-type BE SOI MOSFET is obtained. The drain current characteristics of this transistor were explored in previous work [4].

Schottky source and drain contacts are required in this device. When the current flows, the charges tunnel through the Schottky barriers as indicated in figure 2. Consequently, the Schottky Barrier Height (SBH) influences at the current level. Higher SBH reduces the amount of tunneling charges. Figures 3 and 4 shows the simulated drain current in the BE SOI MOSFET as a function of VGF for metal workfunctions (ΦM) varying between 4.1V and 5.1V. The simulations performed in numerical Synopsys Sentaurus TCAD [5] considered only the dependence of the SBH on the metal workfunction and were calibrated using experimental data. The |VGB| value used is 25V and the drain to source voltage |VDS| is 100mV.

The SBH for electrons directly increases with ΦM, therefore the simulated drain current in the n-type BE SOI (figure 3) decreases as ΦM rises. For lower values of ΦM (near conduction band level of the silicon), it was observed higher currents. In these situations the SBH is small, thus most of the electron current is by thermionic emission over the barrier and the contact exhibit an ohmic behavior. An analogous phenomenon is observed in the p-type device. The SBH for holes directly decreases with ΦM, hence the simulated drain current in the p-type BE SOI (figure 4) increases as ΦM reduces. In the same manner, the hole current for higher values of ΦM (near valence band level) is mostly by thermionic emission.

In order to evaluate similar current levels for the n-type and p-type transistors, an optimal ΦM value must be selected. Figure 5 shows the comparison of the simulations of both types of transistors for an overdrive voltage VGT=VGF-VT=1V, where VT is the threshold voltage. It is possible to observe that the ratio of the maximum current levels is compatible with the charge mobility difference between electrons and holes. The inset graph shows the details of the region where the current of the n-type and p-type transistors intersect (at ΦM=4.57V). This workfunction value corresponds the BE SOI MOSFET with similar current levels for both types.

The parameter that most affects the current level in the n-type and p-type BE SOI MOSFET is the metal workfunction of the drain and source Schottky contacts. Other optimizations can be performed to aim higher currents such the use of UTBB SOI substrate or alternative materials in order to enhance the performance of this device. This work evaluated the difference of the current levels in both types of transistors. Simulations results showed that ΦM=4.57V enables similar SBH, and therefore, similar electron and hole current levels. The simplicity of fabrication and the reconfigurability aspect makes the BE SOI MOSFET an interesting candidate for future applications.

Figure 1