Heterojunction Bipolar Transistor Recent advances in Heterojunction Bipolar Transistor Reshma Krishnan Madathil Centre for Materials Science and Nanotechnology (SMN) Department of Chemistry University of Oslo reshma.madathil@smn.uio.no FYS 9310 and 4310 Materials Science of Semiconductors

Bipolar Junction Transistors The transistor is a three-layer semiconductor device consisting of either two n- and one p- type layers of material or two p- and one n- type layers of material. The former is called an npn transistor, while the latter is called a pnp transistor So, there are two types of BJT- i) npn transistor ii) pnp transistor

Modes of operation of BJT Active mode: E-B junction is forward biased, B-C junction is reverse-biased Saturation mode: both junctions are forward biased corresponds to small biasing V & large output I – transistor is in a conducting state & acts as a closed (or on) switch Cutoff mode: both junctions are reverse biased corresponds to the open (or off) switch Inverted mode: inverted active mode E-B junction is reverse biased, C-B junction is forward biased Junction polarities and minority carrier distributions of a BJT transistor under four modes of operation.

Advantages of BJT • High voltage handling capability, • High cutoff frequencies of both ft and fmax This is because current flow is vertical, through layers whose thickness is precisely controlled to submicron dimensions by epitaxy or ion-implantation processes. • High current drive capability because the entire emitter area conducts current (as opposed to the thin channel geometry obtained in FET technologies). • High transconductance due to the exponential relationship between input voltage and output current. • High voltage handling capability, because of the ease of establishing a thick collector region to absorb the output voltage without breakdown. • Very uniform threshold voltage for turn-on of the output current. • Very low 1 / f noise, a result of the intrinsic device being relatively well shielded from semiconductor surface and bulk traps.

Requirements for a bipolar device High gain High emitter efficiency High speed Demands and problems for a BJT Demands Problems Heavy emitter doping Band gap shrinkage causing base injection Low base doping Narrow base width High base resistance Solution: Heterojunction Bipolar Transistor Emitter can be heavily doped using a Semiconductor with a band gap larger than the base Base can be heavily doped and be made narrow without increasing base resistance Collector can be chosen from a material to increase breakdown voltage.

Heterojunction Bipolar Transistor (HBT) The basic idea to use a heterostructure in bipolar transistors is almost as old as the bipolar transistor itself. In 1948, W. Shockley described the advantage of a bipolar transistor consisting of a wide bandgap emitter and a narrow bandgap base. It took more than 30 years to materialize Shockley's idea in practical devices The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz.

Difference between the BJT and HBT Advantages of HBT over BJT Use of different semiconductor materials for the emitter-base junction and the base-collector junction, creating a heterojunction. Limited injection of holes from the base into the emitter region, - since the potential barrier in the valence band is higher than in the conduction band. Allows a high doping density to be used in the base - reducing the base resistance while maintaining gain Advantages of HBT over BJT HBTs extend the advantages BJT of their Si predecessors to considerably higher frequencies The use of wide band gap emitters allows HBTs to attain - much higher base doping (up to 1020/cm3) and lower emitter doping (down to 1017/cm3) than that of Si homojunction bipolar transistors Reduced device parasitic and smaller electron transit times. Because of unique emitter/base doping profile and quasi-electric fields possibly implemented by varying material composition

Heterojunction Bipolar Transistor (HBT) Modern BJT Structure Features: Narrow base n+ poly-Si emitter Self-aligned p+ poly-Si base contacts Lightly-doped collector Heavily-doped epitaxial subcollector Shallow trenches and deep trenches filled with SiO2 for electrical isolation

What made the difference? The holes and electrons see a different barrier The holes are not allowed to go back into emitter Drastic reduction in Hole Injection – high emitter doping needn’t be done Reduction in band narrowing effect too. So What’s the use ? High Frequency Device Lower emitter Doping⇒ Smaller junction Capacitance⇒ Higher Speed Electron Mobility for npn GaAs is 5 times that of Si⇒ Shorter Base transit Time Cutoff Frequencies of the order of 40 GHz

n-AlGaAs / p-GaAs / n+GaAs HBT First HBT in the history of BJT n+ collector GaAs AlGaAs B C n-AlGaAs / p-GaAs / n+GaAs HBT First HBT in the history of BJT Possible because of the lattice match between GaAs and AlGaAs Solid-State Electronics 38:9( 1995) 1635-1639.

BJT Performance Parameters Common emitter current gain, β : For PNP For NPN Heterojunction Bipolar Transistor (HBT) To improve β , we can increase niB by using a base material that has a smaller band gap energy Note that this allows a large β to be achieved with large NB (even >NE), which is advantageous for increasing Early voltage (VA) reducing base resistance

The Heterojunction Bipolar Transistor Emitter – wide bandgap (AlGaAs) Base – lower bandgap (GaAs) Large bandgap difference (between E-B)  common-emitter current gain can be extremely large Homojunction: no bandgap difference – doping concentration in the E & B must be very high EV increases the valence-band barrier height  reduce injection of holes from B to E can use heavily doped base, maintain a high E efficiency & current gain Schematic cross section of an n-p-n heterojunction bipolar transistor (HBT) structure. (b) Energy band diagram of a HBT operated under active mode. Semicond. Sci. Technol. 18 No 12 (December 2003) 1010-1014

AlGaAs /GaAs /GaAs HBTs fabricated at BELL Labs showed the following: very low values of  =30 Higher values of  were observed in Devices with larger areas. The  increased from 30 t0 about 1800 when the surface of the base region was passivated by chemical treatment to saturate the dangling bonds with sulfur . But the  values were unstable . Several approaches have been used to stabilize the . The most successful one has been chemical treatment with (NH4)2Sx and protect with PECVD silicon nitride

Silicon Germanium HBT (SiGe HBT) Band gap of Si1-xGex depends upon x. Strained layer Si1-xGex without dislocations can be realized with thin layers of base n Si n- Si n+ Si Benefits of SiGe HBT over Si BJT Collector Currents IC is larger for a given VBE IC increase improves  IC increase decreases the emitter charging time. This improves the switching speed. Thin solid films 294 ( 1997) 246-249

Si/SiGe material system high-speed capability – because the base is heavily doped (bandgap difference) high current gain at low Ic - Small trap density at Si surface minimizes the surface recombination current Lower cutoff frequency – because lower mobilities in Si Problem: E efficiency & Ic suffer (caused by EV) To improve: graded-layer & graded-base heterojunction Device structure of an n-p-n Si/SiGe/Si HBT (b) Collector and base current versus VEB for a HBT and bipolar junction transistor (BJT). IEEE Transactions on Electron Devices 52:3(2005)317-324 · 

InP-based material systems Advantages: very low surface recombination Advanced HBTs InP-based material systems Advantages: very low surface recombination Higher electron mobility in InGaAs than in GaAs – superior high-freq performance (in fig., cutoff freq: 254GHz) InP collector region has higher velocity at high fields than GaAs collector InP collector breakdown voltage is higher than GaAs Current gain as a function of operating frequency for an InP-based HBT. Cutoff freq., fT= 254GHz K.Kiziloglu et al. IPRM, 294 (2010)

GaInAs/InP Buried Metal HBT - REDUCTION OF BASE-COLLECTOR CAPACITANCE Buried Tungsten wires of the same area as the emitter metal was used to reduce CBCext SBCT of BM-HBT was estimated to be 22% that of conventional HBT, CBC 30% of conventional HBT fT = 86 GHz, fMAX > 135 GHz of device with an emitter area of 0.5 x 2.5 μm2 fT = 82 GHz, fMAX > 200 GHz of device with an emitter area of 0.3 x 1.5 μm2 Schematic view of fabricated BM-HBT Japanese Journal of Applied Physics, Volume 39, Part 2, Number 6A

Layer structure for the buried metal - HBT

Conclusion HBT’s have tremendous potential for high power applications. Polarization doping gives a promising solution to the p-type conductivity problem. To date, GaAs HBTs with AlGaAs and InGaP emitters are commercially available and used mostly for power amplification in wireless communication systems. Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. Growth technique as well as device design must be carefully chosen.