Advances in Electrical Engineering Systems (AEES)` 196 Vol. 1, No. 4, 2013, ISSN 2167-633X Copyright World Science Publisher, United States www.worldsciencepublisher.org Enhanced Emitter Transit Time for Heterojunction Bipolar Transistors (HBT) 1 Mazharul Huq Chowdhury, 2 Mohammad Saad Alam 1 Nxteer Automotive, Saginaw, MI, USA *2 Auburn Hills, MI, USA Email: saadphd@gmail.com Abstract Simulation results are presented to study the performance of improved emitter transit time using AlGaAs-GaInP composite emitter heterojunction bipolar transistor (HBT). This composite emitter HBT shows significant reduction of emitter- base capacitance C BE and improved high frequency performance. In our simulation we use Medici (2-D simulator) as a simulator. The composite emitter HBT has been compared with the conventional HBT. Results show superior performance of the composite emitter design over conventional one in terms of reduced C BE. The C BE achieved with the composite emitter design was 55% lower compared to conventional designs. However, there are no variations with the collector current which provides enhanced frequency response for a composite emitter design. Keywords emitter transit time, composite emitter heterojunction bipolar transistor, performance, enhanced frequency 1. INTRODUCTION For next generation optical communication, GaInP/GaAs heterojunction bipolar transistors (HBTs) are suitable due to their high reliability, high speed, high power capacity and high reliability. GaInP/GaAs HBTs offer significant advantages over AlGaAs/GaAs devices such as large valance band discontinuity (high electron injection efficiency), high etching selectivity between GaInP/GaAs (increases yield), low surface recombination velocity (lower noise) [1]-[2]. GaInP/GaAs HBTs do not suffer from impurities related to oxygen and thus improve the mean time failure (MIF) [3]. A very good microwave performance of GaInP HBT has been achieved (cut off frequency, f T = 140 GHz and f max = 230 GHz) using various designs such as collector under cut, tunnelling emitter and strained InGaAs base [3]-[6]. However, a common disadvantage in high speed performance of HBTs has been their large base-emitter capacitance (C BE ). This high baseemitter capacitance is caused by limited mobile carrier transport and thus charge accumulation in emitter region [7]. A low base-emitter capacitance has been reported at low collector current density (J C ) by using lightly doped emitter [8]. However, it does not solve the problem at high current density in conventional HBTs. Mobile carrier transport takes place in conventional HBT by diffusion through the base-emitter region. This causes a charge accumulation in the emitter and increased the base-emitter capacitance (C BE ). To reduce this effect a composite emitter design has been reported [9] - [10]. In this work, MEDICI simulation results of a composite emitter design (AlGaAs/GaAs) have been presented which allows significant reduction of C BE and thus improved the high frequency performance. In this design, a graded AlGaAs layer forms an electron launcher at the interface with a GaInP layer. The electron launcher injects the electron towards the emitter with high velocity which lowers the free carrier concentration in base-emitter junction and reduces the C BE. This low carrier concentration is achieved without degradation of transconductance because of the high electron velocity in the launcher region. Therefore, the overall performance is improved compared to the conventional HBT design. 2. Composite emitter HBT design In a composite emitter design a graded layer of AlGaAs is introduced which acts as an electron launcher at the interface with GaInP layer. This launcher layer injects electrons with a very high velocity into a GaInP layer and ensures that electrons have very high velocity before they enter into the base region. This creates a low free carrier concentration in the emitter and hence the transit time (τ E ) decreased. In this composite emitter design the GaInP emitter layer blocks the holes to get back into the emitter and separated
Mazharul Huq Chowdhury, et al., AEES, Vol. 1, No. 4, pp. 196-200, 2013 197 the electron launched region from the base region and by this base-emitter capacitance (CBE) is decreased. Figure.1 shows the structure of a conventional HBT design which is used in this work to compare the performance with the composite structure. Conventional HBT starts from emitter cap of n-type GaInP with concentration of 1 10 19 cm -3 with the thickness of 700 A. Then there is a 2000 A thick layer of emitter which is n-type and concentration of 3 10 17 cm -3. After this there is a blocker layer of 2000 A of n-type GaInP with concentration of 3 10 17 cm -3. This layer provides high frequency performance. After this there is a base layer of p-type GaAs of 600 A thickness and having Figure 2. Structure of composite emitter GaInP/GaAs HBT design By using this composition an electron launcher is made with a height of 0.125 ev. Next to this layer there is a layer of un-doped GaInP with thickness of 100 A. The undoped GaInP layer reduces the spikes created in the conduction band of AlGaAs-GaInP interface. Next to this there is a 400 A thick n-type GaInP layer with a concentration of 5 10 16 cm -3 which acts as an emitter region. After this there is a base region of 500 A of GaAs with a concentration of 6 10 19 cm -3. The last layer is n-type GaAs of 7000 A thick with a concentration of 1.5 10 16 cm - 3. It acts as a collector region. In this design there is also an emitter cap of n-type GaInP of 700 A thick with concentration of 1 10 19 cm -3. Figure 1. Structure of conventional GaInP/GaAs HBT design concentration of 4 10 19 cm -3. And at the end there is a layer of 7000 A thickness of n-type GaAs with concentration of 1.5 10 16 cm -3 and it acts as a collector region. On the other hand, in a composite emitter HBT design (Figure. 2) there is a layer of AlGaAs with a concentration of 5 10 17 cm -3. The aluminum (Al) composition is chosen as 0.22 to prevent inter valley scattering. 3. Medici simulation study of conventional and conventional and composite emitter GaInP/GaAs design In our simulation we have used Medici as a simulator. Medici is a 2D device simulator that can model the electrical, thermal and optical characteristics of semiconductor devices such as MOSFETs, BJTs, HBTs, and power devices, IGBTs, HEMTs, CCDs, photo detectors. It solves Poisson s, Thermal, Current-Continuity and Energy-Balance equations for electron and hole. We consider the following assumptions for our simulation through MEDICI- 1. In 0.49 Ga 0.51 P is chosen as a spacer layer. About 92% of the band gap differences transforms to a positive valence band offset. 2. Current under thermionic field emission is designed as thermionic current multiplied by tunnelling coefficient (HJTEM used as ICCG parameter in Medici). 3. To make ohmic contacts polysilicon is used. It increases gain by reduction in IPE (recombination velocity).
Mazharul Huq Chowdhury, et al., AEES, Vol. 1, No. 4, pp. 196-200, 2013 198 4. In un-doped regions the concentration is about 1 10 2 cm -3 (which is default value in Medici) 5. In Medici eg.model is chosen as 4 as our layer designing is considered to be as compound semiconductor material. Figure 3 shows the energy band diagram of Conventional Emitter and composite emitter of GaInP/GaAs HBT. In composite emitter design, there is a notch created in bas-emitter region which helps electron to tunnel through the emitter region quickly. This quick movement of electron through the emitter region reduces the free carrier concentration in the base-emitter region and improves frequency response by decreasing base-emitter capacitance, C BE. design HBT has been presented in Figure. 4. In a composite design at x= 0.35 µm distance there is a very high electric field of 200Kv/cm as compared to 5Kv/cm electric field present in conventional design. This high electric field helps electron to move quickly through the emitter region before they reach base region. This lowers the free electron concentration in base-emitter junction and thus reduces the C BE. On contrary, there is low electric field in the emitter of the conventional emitter HBT design. Therefore carriers move slowly through the base-emitter junction and accumulate in the base-emitter junction. This increases the electron density in the base-emitter junction and also increases the base-emitter capacitance, C BE. Figure 3. Energy band diagram of Conventional emitter GaInP/GaAs HBT Composite emitter GaInP/GaAs HBT Electric fields are very strong in a composite emitter design compare to the conventional emitter design. The electric field versus distance profile for GaInP/GaAs conventional emitter design HBT and composite emitter Figure 4. Electric field Conventional GaInP/GaAs HBT Composite emitter GaInP/GaAs HBT The drift velocity of compositionally graded AlGaAs design has been presented in Figure. 5. This figure emphasis on enhancement of velocity of composite design which improves frequency characteristics. In the case of conventional emitter design the electron velocity is lower as compare to composite emitter design (Figure. 5). The
Mazharul Huq Chowdhury, et al., AEES, Vol. 1, No. 4, pp. 196-200, 2013 199 high electric field value in composite design speeds up the electron mean velocity to 1.65 10 7 cm/s at x= 0.35 µm distance which is very high as compared to 0.7 10 7 cm/s of conventional design. From the simulations results it is apparent that the that C BE for composite emitter design is very small as compared to conventional emitter HBT because of stronger electric fields in composite emitter design (Figure 4. and 5). The C BE of composite emitter HBT s is lower as compared to conventional emitter HBT design. This leads to enhancement of frequency response of composite emitter design. It has been found that for collector current, Ic=1 µamp/µm composite design gives out collector can be used for the same breakdown voltage and thus reduces the size of the device. On the other hand, narrow bandgap material ensures high carrier mobility through the device and this will further improve the transit time of the device. Figure 6. C BE versus Collector current conventional emitter GaInP/GaAs HBT composite emitter GaInP/GaAs HBT 4. Conclusion Figure 5. Electron mean velocity Conventional emitter GaInP/GaAs HBT Composite emitter GaInP/GaAs HBT 1.2 µf capacitance which is about 55% smaller than that value of 2.7 µf in conventional design (Figure. 6). The proposed model can be improved by using composite collector which is the combination of both wide and narrow bandgap materials. The wide bandgap material provides high breakdown voltage. Therefore, a thinner Simulation results are presented for both conventional and composite emitter AlGaAs/GaInP HBT design. Both the composite and conventional emitter HBT have been studied and analyzed on the basis of MEDICI simulator. The characteristics of composite emitter designs are found very superior as compared to conventional design in terms of low base-emitter capacitance. The base-emitter capacitance, C BE for a composite emitter design is found to be 55% lower compared to conventional emitter design. This lower base-emitter capacitance enhances frequency performance as carriers move faster through the device.
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