514 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 3, MARCH 2002 A New Model for the Low-Frequency Noise and the Noise Level Variation in Polysilicon Emitter BJTs Martin Sandén, Student Member, IEEE, Ognian Marinov, M. Jamal Deen, Senior Member, IEEE, and Mikael Östling, Senior Member, IEEE Abstract—This work presents a new, physically-based model for the low-frequency noise in high-speed polysilicon emitter bipolar junction transistors (BJTs). Evidence of the low-frequency noise originating mainly from a superposition of generation-recombi- nation (g-r) centers is presented. Measurements of the equivalent input noise spectral density ( ) showed that for BJTs with large emitter areas is proportional to 1/ , as expected. In con- trast, the noise spectrum for BJTs with submicron showed a strong variation from a 1/ -dependence, due to the presence of several g-r centers. However, the average spectrum has a frequency dependence proportional to 1/ for BJTs with large as well as small . The proposed model, based only on superpo- sition of g-r centers, can predict the frequency-, current-, area-, and variation-dependency of with excellent agreement to the measurement results. The SPICE parameter , extracted from is found to be proportional to 1/ with the product cm . The relative variation in the noise level is found to be proportional to , resulting in an absolute variation proportional to . The g-r centers are most likely located next to the thin SiO interfacial layer between the polysil- icon and monosilicon emitter. The areal trap density, responsible for the low-frequency noise within – Hz, is estimated to be cm . From temperature measurement of one clearly observed g-r center, the extracted trap energy level and cap- ture cross-section are 0.31 eV and cm , respectively. Index Terms—Bipolar transistor, interfacial oxide, low-fre- quency noise, noise modeling, polysilicon emitter. I. INTRODUCTION B IPOLAR junction transistors (BJTs) with a polysilicon emitter are suitable and widely used in wireless RF applications due to their high unity current gain bandwidth , high current gain , and excellent low-frequency noise properties. For applications like voltage-controlled oscillators (VCOs) and mixers, the low-frequency noise of the BJT is extremely important, since it can be up-converted to undesired phase noise [1], [2]. As the device geometry is scaled down, Manuscript received September 25, 2001; revised December 10, 2001. This work was supported in part by the Swedish Foundation for Strategic Research (SSF), the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Royal Institute of Technology (KTH), Telefonaktiebolaget LM Er- icsson, and the High-Frequency Bipolar Technology Consortium, organized by SSF. The review of this paper was arranged by Editor J. N. Burghartz. M. Sandén and M. Östling are with the Department of Microelectronics and Information Technology, Royal Institute of Technology (KTH), Stock- holm, Sweden, and also with the Department of Electrical and Computer Engineering, McMaster University, Hamilton, ON, Canada L8S 4K1 (email: sanden@ele.kth.se). O. Marinov and M. J. Deen are with the Department of Electrical and Computer Engineering, McMaster University, Hamilton, ON, Canada L8S 4K1 (e-mail: jamal@mcmaster.ca). Publisher Item Identifier S 0018-9383(02)01562-9. the low-frequency noise of the BJT will increase, resulting in an increased phase noise level in BJT-based circuits. For frequencies below approximately 10 kHz, the equivalent input noise power spectral density ( ) for polysilicon emitter BJTs is usually proportional to with varying between 0.9–1.1 [3]–[10] and is usually referred to as 1/ noise. For the BJT operating in its forward active mode, with at mod- erate or high values, is usually proportional to [3]–[8]. This dependence is typical for any linear resistance, such as the thin SiO layer at the interface between the polysil- icon and monosilicon emitter. For nonlinear resistors, the cur- rent dependence can be different and noise sources located in the base-emitter depletion region do not necessarily have a noise level proportional to at low values of [11]. The low-frequency noise is modeled in SPICE assuming , according to [12] (1) where is the base current and and are SPICE model parameters, representing the magnitude and the current exponent of the 1/ noise, respectively. is usually found to be inversely proportional to the emitter area ( ) [3]–[9] and varies with processing conditions for the polysilicon deposition since it strongly depends on the interfacial quality of the polysilicon/monosilicon emitter [3], [9]. Another noise source in semiconductors is generation-recom- bination (g-r) noise, which exhibits a Lorentzian spectrum. This spectrum cannot be expressed as , since then must be a function of frequency, with varying between 0 and 2. The g-r noise is caused by trapping-detrapping of carriers in traps or crystal dislocations and can contribute significantly to the total low-frequency noise [3], [4], [10], [13]. A single trapping-de- trapping process can be observed as random telegraph signal (RTS) in time domain [14]. For BJTs with several g-r centers, the low-frequency noise can be modeled as a sum of the 1/ noise, given by (1) and a superposition of several Lorentzians [3], [4] (2) where and represent the amplitude and the characteristic time constant of the th g-r center, respectively. Sometimes g-r noise is found to increase in stressed devices, with the trapping- detrapping process taking place at the Si/SiO interface at the emitter periphery [15]–[18]. 0018–9383/02$17.00 © 2002 IEEE