Browsing by Author "Jimenez, David"

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Does carrier velocity saturation help to enhance f(max) in graphene field-effect transistors?
(2020-09-01) Feijoo, Pedro C.; Pasadas, Francisco; Bonmann, Marlene; Asad, Muhammad; Yang, Xinxin; Generalov, Andrey; Vorobiev, Andrei; Banszerus, Luca; Stampfer, Christoph; Otto, Martin; Neumaier, Daniel; Stake, Jan; Jimenez, David
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (f(max)). This paper investigates whether velocity saturation can help to get better current saturation and if that correlates with enhancedf(max). We have fabricated 500 nm GFETs with high extrinsicf(max)(37 GHz), and later simulated with a drift-diffusion model augmented with the relevant factors that influence carrier velocity, namely: short-channel electrostatics, saturation velocity effect, graphene/dielectric interface traps, and self-heating effects. Crucially, the model provides microscopic details of channel parameters such as carrier concentration, drift and saturation velocities, allowing us to correlate the observed macroscopic behavior with the local magnitudes. When biasing the GFET so all carriers in the channel are of the same sign resulting in highly concentrated unipolar channel, we find that the larger the drain bias is, both closer the carrier velocity to its saturation value and the higher thef(max)are. However, the highestf(max)can be achieved at biases where there exists a depletion of carriers near source or drain. In such a situation, the highestf(max)is not found in the velocity saturation regime, but where carrier velocity is far below its saturated value and the contribution of the diffusion mechanism to the current is comparable to the drift mechanism. The position and magnitude of the highestf(max)depend on the carrier concentration and total velocity, which are interdependent and are also affected by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the highestf(max)from similar to 60 to similar to 40 GHz.
Photoresponse of Graphene-Gated Graphene-GaSe Heterojunction Devices
(2018-08) Kim, Wonjae; Arpiainen, Sanna; Xue, Hui; Soikkeli, Miika; Qi, Mei; Sun, Zhipei; Lipsanen, Harri; Chaves, Ferney A.; Jimenez, David; Prunnila, Mika
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Because of their extraordinary physical properties, low-dimensional materials including graphene and gallium selenide (GaSe) are promising for future electronic and optoelectronic applications, particularly in transparent-flexible photodetectors. Currently, the photodetectors working at the near-infrared spectral range are highly indispensable in optical communications. However, the current photodetector architectures are typically complex, and it is normally difficult to control the architecture parameters. Here, we report graphene-GaSe heterojunction-based field-effect transistors with broadband photodetection from 730-1550 nm. Chemical-vapor-deposited graphene was employed as transparent gate and contact electrodes with tunable resistance, which enables effective photocurrent generation in the heterojunctions. The photoresponsivity was shown from 10 to 0.05 mA/W in the near-infrared region under the gate control. To understand behavior of the transistor, we analyzed the results via simulation performed using a model for the gate-tunable graphene-semiconductor heterojunction where possible Fermi level pinning effect is considered.