Doping profile studies for HBT's and HEMT's with improved breakdown and speed characteristics.

Abstract

Step- and delta-doping techniques were applied to AlGaAs/GaAs and InP/InGaAs HBT's, and AlGaAs/GaAs HEMT's in view of examining their DC, breakdown, low-frequency noise, and microwave characteristics. Temperature-dependent measurements confirmed that avalanche impact ionization was the dominant breakdown mechanism in InGaAs collector HBT's. Monte Carlo techniques and 1-D drift-diffusion modeling were used for speed and breakdown simulation respectively. The collector doping profile can be optimized in single HBT's (SHBT's) to improve the breakdown-speed performance. Double HBT's (DHBT's) will outperform all SHBT's in terms of breakdown-speed tradeoffs as long as they are graded or a high collector-emitter bias is applied. A cutoff frequency (f$\sb{T}$) up to 200 GHz was found to be feasible with InP/InGaAs graded DHBT's. N-channel HEMT's with different step-doped profile thicknesses were fabricated and tested at low and high frequencies. I-layer or thin step-doping designs improve the threshold voltage uniformity, device linearity and maximum frequency of oscillation (f$\sb{max}$). Thinner heavily-doped regions show reduced low-frequency noise spectra. A 2-D numerical analysis showed that gate-drain breakdown was responsible for the device breakdown and occurred in the AlGaAs layer under the gate. The incorporation of an undoped region and use of material of low impact ionization coefficients under the gate improve the breakdown characteristics. The electric field dependence of impact ionization coefficients was evaluated based on experimental data and a physical model. A novel self-aligned technology was developed for InP/InGaAs, AlGaAs/GaAs and GaInP/GaAs HBT's. It makes use of RIE to reduce the base-emitter separation to 0.1 $\mu$m, WSi$\sb{x}$ emitter air-bridge contacts and trench mesa isolation to reduce device size, and shallow p-type ohmic contacts to obtain good junction characteristics. However, the CH$\sb4$/H$\sb2$/Ar discharge induces surface damage and contamination which, if not properly removed or minimized, can degrade the current gain or lead to device failure. RIE studies were performed based on conduction through submicron test patterns of InP and analysis of material and device characteristics. InP/InGaAs SHBT's with f$\sb{T}$ and f$\sb{max}$ up to 34.9 GHz and 26.6 GHz respectively have been fabricated.