# Performance Comparison of Graphene Nanoribbon FETs With Schottky Contacts and Doped Reservoirs

## Summary (2 min read)

### Introduction

- Graphene is a zero-gap Manuscript received March 24, 2008.
- GNRFETs that are demonstrated experimentally to date are realized by connecting the channel to metals with Schottky contacts [8], [14], therefore obtaining a Schottky-barrier FET .
- Different types of nonidealities have been investigated.
- Vacancies and edge roughness can greatly affect device electrical performance more than ionized impurities actually do.

### II. APPROACH

- Device characteristics of GNRFETs are calculated by solving the Schrödinger equation using the NEGF formalism [25] selfconsistently with the 3-D Poisson equation [18]–[21].
- Lattice vacancies or edge roughness are considered as atomistic defects of the channel GNR, where the existence of carriers is essentially prohibited.
- An ionized impurity is treated as an external fixed charge, which can play an important role for the electrostatic potential of the device.
- In other words, in the self-consistent iterative loop between the transport equation and the Poisson equation, the input charge into the Poisson equation always includes a fixed external charge as well as the output charge from the Schrödinger equation.

### III. RESULTS

- The authors first present results for an SBFET and a MOSFET under ideal conditions.
- Fig. 2(a) and (b) shows the transfer characteristics for each device.
- The MOSFET has 50% larger Ion (i.e., current for VG = VDD and VD = VDD) and larger transconductance gm than the SBFET.
- In Fig. 4(a), the cutoff frequency fT as a function of the applied gate voltage is shown and computed by using the quasi-static approximation [27] as fT = gm 2πCG ∣∣∣∣ VD=VDD (1) where gm is the transconductance and CG is the gate capacitance computed as the derivative of the charge in the channel with respect to the gate voltage.
- Fig. 4(b) shows the intrinsic delay as a function of on–off ratio:.

### B. Atomistic Vacancy

- Fig. 5(a) and (b) shows the transfer characteristics for SBFET and MOSFET, both in the linear and the logarithmic scale, for different positions of a defect.
- As shown in Fig. 5, the defect near the source has the largest effect in both devices.
- For an SBFET with a defect near the source, thicker SB is induced [Fig. 6(a)] due to the electron accumulation, and quantum transmission is reduced [Fig. 6(b)] at the ON state, which result in a smaller Ion.
- Instead, the reduced number of propagating states due to the lattice vacancy reduces the transmission probability [Fig. 6(d)], which results in a smaller on current.
- Transport is, indeed, mostly determined by the top-of-the-barrier potential, which, as shown in Fig. 6(c), is only partially influenced by the presence of the defect in correspondence of the drain (and in the middle of the channel).

### IV. CONCLUSION

- GNR SBFETs and MOSFETs are compared by solving the Schrödinger equation self-consistently with the 3-D Poisson equation.
- His research interests include the physics, modeling, and simulation of nanodevices.
- Dr. Guo is a member of the technical program committees of the International Electron Devices Meeting and the Device Research Conference.

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