二极管

1 .Chapter 3 Diodes Functionality and Physical Operation J . Chen

2 . Content 3.1 The Ideal Diode 3.2 Terminal Characteristics of Junction Diodes 3.3 Modeling the Diode Forward Characteristic 3.4 Operation in the Reverse Breakdown Region— Zener Diodes 3.5 Rectifier Circuits （整流电路） 3.6 Limiting and Clamping Circuits （限幅与钳位电路） 3.7 Physical Operation of Diodes 3.8 Special Diode Types 3.9 The SPICE Diode Model and Simulation Examples J. Chen

3 .J . Chen 3.1 Linear and Nonlinear Devices So far, almost all the devices we have learnt are linear Many signal-processing functions, however, are implemented by nonlinear devices Linear amplifier Nonlinear amplifier

4 .J . Chen (a) Diode circuit symbol + v - i Anode Cathode (b) i –v characteristic of the ideal diode ---Reverse bias--- ---Forward bias--- i 0 v (c) Equivalent circuit in the reverse direction v < 0  i =0 i + v - i (d) Equivalent circuit in the forward direction i > 0  v =0 + v - Note: measures should be taken to limit the forward current and the reverse voltage! Diode ( 二极管 ) is the simplest and most fundamental nonlinear circuit element 3.1 Symbol and characteristic

5 .(v) The diode i – v relationship (mA) J. Chen pn junction The most important region is the boundary between n -type and p -type semiconductor, which is called pn junction ( pn 结 ) 3.2 Junction Diodes and its Characteristics

6 .J. Chen (mA) ( nA ) The diode i – v relationship (Note the scale expansion / compression) cut-in voltage 3.2 Junction Diodes and its Characteristics

7 .J . Chen Mathematic Model ： Forward biased Reverse biased 3.3 The Diode Models where saturation current thermal voltage(~25 mv at room temperature) ( In general, n = 1)  

8 .J. Chen Temperature dependence of the diode forward characteristic Voltage drop @-2 mV/  C 3.3 Temperature dependence

9 .J . Chen Circuit Model : usually derived by approximating the curve into piecewise-line According to practical applications, there are different models, such as Simplified diode model The constant-voltage-drop model Small-signal model High-frequency model Zener Diode Model 3.3 The Diode Models

10 .J . Chen 3.3 Ideal Diode Model Forward bias  short circuit Reverse bias  open circuit

11 .J . Chen Equivalent circuit representation . 3.3 Simplified Diode Model Piecewise-linear model of the diode forward characteristic

12 .J . Chen The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation. 3.3 Constant-Voltage-Drop Model

13 .J . Chen 3.3 Small-Signal * Model r d ( n = 2) Incremental resistance *The signal amplitude is sufficiently small such that the excursion at Q along the i -v curve is limited to a short, almost linear segment.

14 .J . Chen Related to the bias 3.3 High-Frequency Model

15 .J . Chen 3.4 Zener Diode Model The diode i – v characteristic with the breakdown region shown in some detail

16 .J . Chen Rectifier circuits Half-wave rectifier Full-wave rectifier The peak rectifier Voltage regulator ( 稳压器 ) Limiter ( 限幅器 ) Logic Circuits 3.5 The Application of Diode Circuits

17 .J . Chen (b) Equivalent circuit 3.5 Half-Wave Rectifier (a) Half-wave rectifier (c) Transfer characteristic (d ) Input and output waveforms

18 .J . Chen 3.5 Parameter determination In selecting diodes for rectifier design, two important parameters must be specified: The current-handling capability The peak inverse voltage (PIV) It is usually prudent, however, to select a diode that has a reverse breakdown voltage at least 50% greater than the expected PIV.

19 .+ + + + + + J. Chen 3.5 Full-Wave Rectifier Transformer with a center-tapped （中心抽头） secondary winding Transfer characteristic Input and output waveforms

20 .J. Chen Circuit 3.5 The Bridge Rectifier Input and output waveforms

21 .J. Chen 3.5 Peak Rectifier The pulsating nature of the output voltage produced by the rectifier circuits makes it unsuitable as a dc supply for electronic circuits. A simple way to reduce the variation of the output voltage is to place a capacitor across the load resistor. The filter capacitor serves to reduce substantially the variations in the rectifier output

22 .J. Chen Voltage and current waveforms in the peak rectifier circuit with , where T is the period.   3.5 Peak Rectifier

23 .3.5 Peak Rectifier J. Chen Block diagram of a dc power supply

24 .J. Chen 3.7 Physical Operation of the diode How can a diode present totally different behavior in forward and backward bias? To answer the question, we need to know: Material, structure and the related features (crystal and semiconductor in particular) New particles to carry charge in addition to electrons New mechanism(s) of conduction in addition to what we have known Techniques to manufacture the devices

25 .3.7 Basic semiconductor concepts Intrinsic Semiconductor ( 本征半导体 ) Doped Semiconductor ( 掺杂半导体 ) Carriers ( 载流子 ) Diffusion ( 扩散运动 ) Drift ( 漂移运动 ) J. Chen

26 .3.7 Elements and material Periodic table J. Chen

27 .3.7 Material and structure Structure is another important factor to determine the physical and chemical characteristics of the material ！ allotrope ( 同素异形体 ), e.g. J. Chen graphite diamond

28 .J. Chen Crystal Regular shape, fixed freezing temperature, fixed boiling point, etc. Why? Regular lattice structure ！ Atoms can not tell from each other: they behave uniquely In noncrystal , however, the atoms of the same element usually play different roles, e.g. Polymer aromatic hydrogen bonds ( 芳香氢键 ) 3.7 Crystal and noncrystal

29 .3.7 Silicon （硅 / 矽） IV element Each atom is bound with four neighbor s via Covalent Bond Its atomic structure is tetrahedron( 四面体，与金刚石相同 ) J. Chen Monocrystalline silicon polycrystalline silicon 单晶硅 多晶硅 Tetrahedron ( 四面体 )