Characterization of high-speed electronic devices using ultrafast lasers
Creators: name: Zeng, An
Publisher: name: University of British Columbia
Year: 2009

description: This thesis describes experimental studies on high-speed modulation-doped field-effect transistors (MODFETs), using electro-optic sampling (EOS). The author has built an EOS system with subpicosecond temporal resolution and TeraHertz bandwidth. Due to the design of the EOS system, signal propagation delay time through an electronic device can be accurately determined in addition to the input and output waveforms. We performed an experimental study of the measurement errors and invasiveness caused by an external LiTaO₃ probe used in the EOS system. We found that contact (air gap free) external electro-optic sampling, which has been widely used in EOS measurement, could lead to more serious measurement errors than previously reported due to probe-tip induced dispersion and reflection. These unwanted effects can be minimized in the non-contact sampling geometry. By comparing sensitivities of a time-resolved (high-frequency) signal and a calibration (low-frequency) signal at various air gaps, we show that the common method used for calibrating time-resolved EOS measurement is valid for both contact and small-air-gap non-contact measurements even though the method cannot be used to calibrate large-air-gap EOS measurements. The most significant result of this thesis is the first electro-optic characterization of ultrafast transistors monolithically integrated with a transmission line/photoconductive switch test fixture. The measured switching time and propagation delay time of a lattice-matched In[sub 0.52⁻] Al[sub 0.48]As/In[sub 0.53]Ga[sub 0.47]As MODFET are 4.2 ps and 3.2 ps, respectively. These are the shortest switching and delay times ever directly measured in a three-terminal electronic device. We demonstrated that the on wafer integration of coplanar transmission line with the MODFET is a significant improvement over previous wire-bonding test fixtures. The parasitic gate inductance of the integrated structure is about an order of magnitude smaller than that of the wire-bonding structure. We studied the effects of different gateaccess structures, semiconductor materials, and bias conditions on the performance of MODFETs. Our measurements of propagation delay times of two MODFET s made of different semiconductor materials directly confirm that a lattice-matched In[sub 0.52⁻] Al[sub 0.48]As/In[sub 0.53]Ga[sub 0.47]As MODFET is faster than a pseudomorphic In[sub 0.20]Ga[sub 0.80]As/Al[sub 0.25]Ga[sub 0.75]As MODFET with similar gate-access layout. We clarified two common misconceptions in the literature regarding the relationships among the delay times, switching time, and current-gain cutoff frequency of a MODFET. The signal propagation delay time τ[sub PROP] observed in a MODFET switching response and the delay time τ[sub d] ≡ 1/2π∫τ defined in small-signal R F measurement cannot be used interchangeably. However, we find that the two delay times τ[sub PROP] and τ[sub d] have similar dependence on drain bias V[sub ds], showing that they are closely related. Further, the 10-90% rise time of the MODFET switching response cannot be directly related to the current-gain cutoff frequency ∫τ as has been suggested. Time-domain simulations of the switching response of MODFET s were performed using a lumped-element model incorporating input and output transmission lines. The results are in excellent agreement with the electro-optic measurements, and we show that omission of the input transmission line leads to large oscillatory artifacts in the response. Finally, equivalent circuit parameters of the MODFETs are extracted from the simulations.