Hot Carrier Degradation of Gain in Bipolar Transistors
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ISIK C. KIZILYALLI AND JEFF D. BUDE AT&T Bell Laboratories, 9333 S. John Young Pkwy., Orlando, FL 32819. AT&T Bell Laboratories, 600 Mountain Ave. Murray Hill, NJ 07974. ABSTRACT In this paper hot carrier related aging of n-p-n bipolar transistors is investigated experimentally and theoretically to bring physical insight into the bipolar transistor h E (common emitter current gain) degradation. Electrical stress experiments are performed on transistors with different base doping profiles at varying temperatures. Detailed process simulations are performed to determine the doping profiles of the base-emitter junction. Monte Carlo transport simulations are then performed at different temperatures and bias conditions to determine the electron and hole distribution functions in the baseemitter junction. AT&T's 0.8 gtm BICMOS technology is used to fabricate the experimental bipolar transistor structures. For this non-self aligned technology we attribute hM degradation to the presence of hot holes and secondary electrons which are generated by hot hole impact ionization. This feed-back due to impact ionization has a dominant effect on the high energy tails of the distribution of both holes and electrons even when the overall current multiplication is low. Simple hot electron energy transport models do not contain the complexity to properly describe ionization feedback and carrier heating, and are therefore inadequate. An exponential dependence of the transistor lifetime on BV EBO is deduced for constant voltage stress (Vstrss < BVEBO) conditions, confirming the importance of secondaries in the process of degradation. 1. INTRODUCTION Reverse bias stress of the emitter-base junction of an n-p-n transistor results in the degradation of the common-emitter current gain (hbi = AIc/AIB) [1-9]. The collector current (Ic) is not affected by the stress, but an increase in the recombination (non-ideal) component of the base current at low VBE is observed as shown in Fig. 1. This effect can be phenomenologically described by increasing the IB2 parameter of the base current (I B) where [10] VBE
IB = IBI e kT
VBE
+ IB2 e nmkT
(1)
and nEL = 2. For BICMOS circuits hE degradation causes the output drive current to decrease, and thus increase the switching time of the circuit. The hFE degradation in bipolar transistors has also the following features [1,2] : (i) Avalanche multiplication in the reverse biased base-emitter junction is not necessary to degrade h F and that h F degradation occurred even when the stress voltage was below the open collector base-emitter breakdown voltage (BVEBo), (ii) increased base doping aggravates the hF degradation, and (iii) degradation is a surface effect located at the emitter-base junction perimeter. Since the base-emitter junctions are heavily doped, the reverse bias junction stress current is mostly due to Zener (band-to-band) tunneling [ 11 ]. Prior work has linked increases in the recombination component of the base current to interface trap generation under the base-oxide by hot electrons [7,8]
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