Impact of Burn-In Effect and Base Strain on Low Frequency Noise in InGaAsN HBTs Hua Yang 1 , Guofu Niu 1 , Peter J. Zampardi 2 , and Roger Welser 3 , 1 ECE Department, Auburn University, Auburn, AL 36849, USA Tel: (334) 844-1892 / Fax: (334) 844-1888 / Email: yanghua@auburn.edu 2 Skyworks Solutions, Newbury Park, CA 91320 3 Kopin Corporation, Taunton, MA 02780 Abstract — We present the first systematic experimental inves- tigation of low frequency noise in InGaAsN Heterojunction Bipo- lar Transistors (HBTs). The low frequency noise is examined as a function of base current for InGaAsN HBTs featuring different base strain and burn-in effects. I. Introduction GaAs-based heterojunction bipolar transistors (HBTs) are widely used in power amplifiers (PAs) in wireless ap- plications due to superior electronic and material proper- ties. Lower turn-on, offset and knee voltages are sought af- ter for increased power efficiency with the ever-decreasing supply. The incorporation of both indium and nitrogen into the GaAs base reduces the bandgap energy (E gb ), hence the turn-on voltage, while keeping the strain from lattice mis- match minimized [1]. Typical of InGaAsN HBTs, there is a so called "burn-in" effect, namely, initial instability in the current gain of the device. This transient instability is at- tributed to hydrogen related traps in the base layer, which passivate the carbon dopant [2]. A logical question is if the burn-in effect impacts low frequency noise, which is well known to be sensitive to traps. The defects due to strain re- laxation in the base could act as carrier traps, and possibly produce additional low-frequency noise as well. This work presents the first experimental results of low frequency noise in InGaAsN HBTs. We will investigate the impact of burn-in effect and base strain on low frequency noise using InGaAsN HBTs featuring different burn-in ef- fects and base strain levels. Understanding low frequency noise behavior in these HBTs is important as the low- frequency noise is upconverted into phase noise of power amplifiers. II. Device Technology and Reduced Turn-On Voltage Fig. 1 shows the schematic cross section of the InGaAsN HBT used in this work. The InGaAs cap layer of the MOCVD material allows the use of Ti/Au for emitter metal- ization. The thin GaInP layer is depleted and passivates the extrinsic base layer which increases current gain in small area devices, due to the reduction in base current from sur- Fig. 1. Schematic cross section of InGaAsN HBT used in this work. face current recombination. All of the mesa etching for this technology is accomplished via dry-etch and the semicon- ductor surfaces are passivated with silicon nitride. Ti/Au is used for base contact and Ge/Au/Ti/Au is used for collector contact [3]. Fig. 2 shows the Gummel characteristics for a 900 ˚ A GaAs HBT, a low burn-in and a standard burn-in InGaAsN HBT used in this work. A reduction of turn-on voltage is clearly observed. III. Noise Measurement Setup and Noise Source Identification Low frequency noise has been a design constraint in RF applications as it is upconverted into phase noise through nonlinear I-V and C-V relationships inherent in transistors. Of particular interest is the 1/f noise, due to its high value at frequencies near DC. Here, we measure low frequency noise from the collector noise voltage using a common- emitter configuration. The detailed test setup can be found in [4]. The possible 1/f noise sources in a typical bipolar tran- 220 0-7803-9309-0/05/$20.00 ©2005 IEEE IEEE BCTM 13.3