High-Gain Graphene Transistors with a Thin AlOx Top-Gate Oxide

Abstract

The high-frequency performance of transistors is usually assessed by speed and gain figures of merit, such as the maximum oscillation frequency f _max, cutoff frequency f _T, ratio f _max/f _T, forward transmission coefficient S ₂₁, and open-circuit voltage gain A _v. All these figures of merit must be as large as possible for transistors to be useful in practical electronics applications. Here we demonstrate high-performance graphene field-effect transistors (GFETs) with a thin AlOx gate dielectric which outperform previous state-of-the-art GFETs: we obtained f _max/f _T > 3, A _v > 30 dB, and S ₂₁ = 12.5 dB (at 10 MHz and depending on the transistor geometry) from S-parameter measurements. A dc characterization of GFETs in ambient conditions reveals good current saturation and relatively large transconductance ~600 S/m. The realized GFETs offer the prospect of using graphene in a much wider range of electronic applications which require substantial gain.

Figure 1

High-frequency GFET. (a) Optical image of one of the fabricated GFETs with the gate length L = 1.1 μm. Graphene stripe cannot be seen as it is completely covered by the contacts. (b) Schematic of the central part of the GFET. Source (S) and drain (D) contacts (Au; yellow) are separated by an underlap (length u) from the gate (G) contact (Al; red core), which is covered by an insulating layer (AlOx; gray shell). Width of graphene stripes $W_{\mathrm{s}}$ was 5, 20 and 50 μm. All GFETs had the same contact width (~50 μm) regardless of the stripe width. The channel width $W=2W_{\mathrm{s}}$. (c) Scanning electron microscopy image of the central part of one the fabricated GFETs with the gate length L = 1 μm. The underlap is $u=100$ nm.

Figure 2

Output characteristics and small-signal conductances of the fabricated GFETs in ambient air. (a) Drain current $I_{\mathrm{D}}$ as a function of $V_{\mathrm{DS}}$ for different $V_{\mathrm{GS}}$ in a GFET with $W=10$  μm and $L=1$  μm. The onset of saturation is at $V_{\mathrm{DS}}=V_{\mathrm{GS}}-V_0$ and it moves to larger $\left|V_{\mathrm{D}S}\right|$ at larger $\left|V_{\mathrm{G}S}\right|$. (b) The transconductance $g_{\mathrm{m}}$ and output conductance $g_{\mathrm{d}}$ of the fabricated GFETs at the operating point at which they exhibit the largest voltage gain $A_{\mathrm{v}}$.

Figure 3

The highest gain in each of the fabricated GFETs at 10 MHz. (a) The open circuit voltage gain $A_{\mathrm{v}}$ as a function of the gate length L. (b) The forward gain $S_{21}$ as a function of $W/L$. The highest value of 12.5 dB was obtained for $W=100$  μm and $L=1.1$  μm. A $W/L$ fit is suggested by the black line because $S_{21}$ scales with $g_{\mathrm{m}}$ and therefore with $W/L$.

Figure 4

The largest values of the high-frequency transistor response parameters of each fabricated GFET as a function of gate length $L$. (a) The cuttof frequency $f_{\mathrm{T}}$ and (b) maximum oscillation frequency $f_{\max }$. A $L^{-1}$ fit is suggested by the black line in both plots.

Figure 5

The maximum oscillation frequency $f_{\max }$ as a function of the cutoff frequency $f_{\mathrm{T}}$ of each fabricated GFET for different gate lengths $L$ and channel widths $W$. The ratio $f_{\max }/f_{\mathrm{T}}$ varies between 1 and 3.