Valerio Di Lecce

Graphene-Base Heterojunction Transistor: An Attractive Device for Terahertz Operation

For the first time, a simulation study is reported of a device formed by stacking an n+-Si layer (emitter), a monolayer graphene sheet (base), and a second n-Si layer (collector), operating as a graphene-base heterojunction transistor. The device differs from the recently proposed hot-electron graphene-base transistor (GBT), where graphene is sandwiched between the two dielectric layers, in the current flow being regulated mainly by thermionic emission over the potential-energy barrier, rather than by tunneling through the emitter-contact Schottky barrier. The simulations are based on a 1-D quantum transport model with the effective mass approximation and nonparabolic corrections. In addition to being much easier to fabricate compared with the GBT, the device is shown to be able to provide 104 ON/OFF current ratio, current densities well in excess of 0.1 A/μm2 and cutoff frequencies well above 1 THz, together with an intrinsic dc small-signal voltage gain larger than 10. Even though the simulation model is somewhat idealized, since ballistic transport is assumed and Si-graphene interfaces are ideal, our results show that this device is a serious competitor for high-frequency RF applications.

Correlation between DC and rf degradation due to deep levels in AlGaN/GaN HEMTs

We investigate the role of the 0.5 eV traps in determining GaN HEMT degradation by means of DC and rf testing, and 2D numerical simulation. We demonstrate that generation of deep levels, having an activation energy of 0.5 eV, is responsible for the degradation observed during rf aging; we show that the occurrence of trap-induced degradation depends on rf driving conditions. We also show that degradation can be explained by the generation of a damaged region within the AlGaN layer at the gate-drain edge, and that the DC and pulsed device degradation effects have a different dependence on the width and depth of the damaged region.

Graphene Base Transistors: A Simulation Study of DC and Small-Signal Operation

A simulation study aimed at investigating the main features in dc and small-signal operating conditions of the hot-electron graphene base transistor (GBT) for analog terahertz operation is presented. Intrinsic silicon is used as reference material. The numerical model is based on a self-consistent Schrödinger-Poisson solution, using a 1-D transport approximation and accounting for multiple-valley and nonparabolicity band effects. Some limitations in the extension of the saturation region and in the output conductance related to the finite quantum capacitance of graphene and to space charge effects are discussed. A small-signal model is developed that catches the essential physics behind the voltage gain and the cutoff frequency, which shows that the graphene quantum capacitance severely limits the former but not the latter. According to simulations carried out within the ballistic transport approximation, a 20-nm-long GBT can achieve at the same time a voltage gain larger than 10 and a cutoff frequency largely above 1 THz within a reasonably wide bias range.

Exploiting Negative Differential Resistance in Monolayer Graphene FETs for High Voltage Gains

Through self-consistent quantum transport simulations, we evaluate the RF performance of monolayer graphene field-effect transistors in the bias region of negative output differential resistance. We show that, compared with the region of quasi-saturation, a voltage gain larger than 10 can be obtained, at the cost of a decrease in the maximum oscillation frequency of about a factor of 1.5-3 and the need for a careful circuit stabilization.

DC and small-signal numerical simulation of graphene base transistor for terahertz operation

The working principles of the hot-electron graphene base transistor (GBT) for analog terahertz operation have been investigated by means of a self-consistent Schrodinger-Polsson solver code. Its regions of operation are defined and discussed. With the help of a small-signal model, it is shown that the cutoff frequency does not depend on the quantum capacitance of the graphene layer, which on the contrary severely affects the intrinsic voltage gain, and that terahertz operation is possible.

Impact of crystallographic orientation and impurity scattering in Graphene-Base Heterojunction Transistors for Terahertz Operation

The influence of the crystal orientation on the performance of silicon-based Graphene-Base Heterojunction Transistors (GBHTs) for terahertz operation is investigated by means of an in-house developed simulator based on quantum transport coupled with Poisson equation. The effect of impurity scattering is included, finding that terahertz operation is possible even considering the reduction of the mobility due to dopants.

Semianalytical quantum model for graphene field-effect transistors

We develop a semianalytical model for monolayer graphene field-effect transistors in the ballistic limit. Two types of devices are considered: in the first device, the source and drain regions are doped by charge transfer with Schottky contacts, while, in the second device, the source and drain regions are doped electrostatically by a back gate. The model captures two important effects that influence the operation of both devices: (i) the finite density of states in the source and drain regions, which limits the number of states available for transport and can be responsible for negative output differential resistance effects, and (ii) quantum tunneling across the potential steps at the source-channel and drain-channel interfaces. By comparison with a self-consistent non-equilibrium Greens function solver, we show that our model provides very accurate results for both types of devices, in the bias region of quasi-saturation as well as in that of negative differential resistance.

Graphene-base heterojunction transistors for post-CMOS high-speed applications: Hopes and challenges

We compare through numerical simulations a Si GBHT and a SiGe HBT: the fT limit of GBHTs is predicted to be more than twice as high as for HBTs assuming a transparent graphene/Si interface; if a more realistic interface model extrapolated from existing experiments is used, the fT limit drops by about two orders of magnitude.

Simulations of Graphene Base Transistors With Improved Graphene Interface Model

A simulation study of the graphene base heterojunction transistor (GBHT) is presented based on a novel realistic graphene-Si interface model, calibrated on the experimental graphene-Si Schottky diodes, whose current-voltage-temperature characteristics are well reproduced. The GBHT simulations predict fT in the tens-of-gigahertz range and confirm the need for an improved quality of the graphene interface for the terahertz operation to be reached.

Comparison of Cu-gate and Ni/Au-gate GaN HEMTs large signal characteristics

In this paper a complete comparison between Copper (Cu) gate and Nickel-Gold (Ni/Au) gate passivated AlGaN/GaN High Electron Mobility Transistors (HEMTs) is presented. DC and Radio Frequency (RF) performance was compared in order to evaluate the behaviour of the two Schottky contacts in the standard HEMT structure. From the obtained data a critical drain current collapse was observed in the Cu-gate devices, with detrimental effects on the RF performance, while the Ni/Au-gate performed nicely both during pulsed I–V and RF measurements. An investigation on the drain current transients and on ID – VGS characteristics, obtained by pulsed signals showed that an acceptor trap at the Cu/AlGaN interface, with activation energy of about 0.43 eV, could be responsible for the Cu-gate HEMT poorer performance. The results suggest that a detailed investigation on surface treatments, gate metal quality and deposition methods is needed in order to fabricate Cu-gate GaN HEMTs.