Fundamentals and Operation of Semiconductor Devices Module 5: Bipolar Junction Transistors Lecture 7: Heterojunction Bip

1. Fundamentals and Operation of Semiconductor Devices Module 5: Bipolar Junction Transistors Lecture 7: Heterojunction Bipolar Transistors (HBT) Sean L. Rommel Microelectronic Engineering Rochester Institute of Technology

2. Bipolar Junction Transistors: HBT III. Field in the Base B. Compositionally Graded Base Region • Grading the composition may also create a drift field in the base region. • One semiconductor will be used for the emitter (Si), and a second will be used for the base (SiGe) Before proceeding further, let us develop some theory regarding the formation of heterojunctions.

3. Bipolar Junction Transistors: HBT Aside: Heterojunctions Definition of Heterojunction: A crystalline structure comprised of two dissimilar materials Each material will have different properties: 1.) Lattice Constant 2.) Bandgap 3.) Mobility

4. Bipolar Junction Transistors: HBT Aside: Heterojunctions Definition of Heterojunction: Depending on the strain resulting from the difference between lattice constants, the material may be classified as: 1.) Lattice Matched Perfect Crystal, not under strain. May be “infinitely” thick. Lattice mismatched crystal; grown past a “critical thickness”; Stress relieved via misfit dislocations. 2.) Relaxed 3.) Pseudomorphic Lattice mismatched crystal, under strain but below the “critical thickness”.

5. Bipolar Junction Transistors: HBT Lattice Matched ahost=aepitaxy Epitaxial layer Host Crystal Little or no strain!

6. Bipolar Junction Transistors: HBT Biaxial Compressive Strain ahost<aepitaxy Epitaxial layer Strain Field Host Crystal

7. Bipolar Junction Transistors: HBT Biaxial Tensile Strain ahost>aepitaxy Epitaxial layer Strain Field Host Crystal

8. Misfit Misfit Bipolar Junction Transistors: HBT Pseudomorphic Relaxed t<critical thickness t>critical thickness

9. Bipolar Junction Transistors: HBT We would like for our BJTs to be pseudomorphic.

10. 5.65 Ǻ 5.43 Ǻ 1.11 eV 0.67 eV 11.8 16 Bipolar Junction Transistors: HBT When we create a compound semiconductor, the material retains properties of each constituent element: Si Ge Lattice Constant Bandgap Dielectric Constant Growing Si1-xGex on Si results in compressive strain!

11. Equation 65 Bipolar Junction Transistors: HBT What happens if we put the narrow gap material (SiGe) in the base? What happens if we grade the Ge composition such that it is highest at the B-C junction? x

12. Bipolar Junction Transistors: HBT • The collector current is strongly affected by the addition of Ge! • Because the intrinsic carrier concentration is a function of position, we must have an electric field in the base!

13. Equation 66 Bipolar Junction Transistors: HBT For a total bandgap narrowing of 100 meV, • Base Electric Field = 104 V/cm Thus, the ratio of the Si to SiGe base transit time is given by Equation 67

14. Bipolar Junction Transistors: HBT The incorporation of SiGe in the base results in a shorter transit time! For DEg=100 meV, the transit time for SiGe is 2.5 times faster!

15. Bipolar Junction Transistors: HBT What happens to the Early Voltage? We recall that Equation 68 Adding SiGe to the base results in: Equation 69

16. Bipolar Junction Transistors: HBT Therefore, Equation 70 For DEg=100 meV, VASiGe is 12 times larger than VASi!

17. Bipolar Junction Transistors: HBT For boxlike doping profiles and low currents, Equation 71 And, Equation 72

18. Bipolar Junction Transistors: HBT Since the base current of a Si transistor is virtually identical to a SiGe transistor, Equation 73 SiGe greatly enhances the gain of a BJT! For DEg=100 meV, bSiGe is 4-6 times larger than bSi!

19. Bipolar Junction Transistors: HBT The total performance enhancement may be seen via the product of Early Voltage and Gain: Equation 74 For DEg=100 meV, bSiGeVASiGe is ~50 times larger than bSiVASi!

20. Bipolar Junction Transistors: HBT Review Questions 1.) What is the difference between lattice matched, pseudomorphic, and relaxed films? 2.) Is SiGe on Si compressive or tensile? 3.)How does SiGe in the base alter device performance?