--- title: "Thyristor V-I, holding/latching; vertical BJT" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:thyristor-vi-vertical-bjt" source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-vertical-bjt site: BCT Engineering --- # Thyristor V-I, holding/latching; vertical BJT Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2081 Bhadra, 5+3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-vertical-bjt - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain the V-I characteristics of a Thyristor and explain the meaning of Holding current and Latching current. Why we use vertical arrangement in power BJT? ## Short answer Thyristor V-I shows reverse blocking, forward blocking until breakover (reduced by gate current), and low-voltage forward conduction. Latching current is the minimum anode current to stay ON after gate removal; holding current is the lower ON-state minimum. Power BJTs use a vertical structure for large current area, high voltage via thick drift layer, and better heat removal. ## Answer A **thyristor (SCR)** is a four-layer P-N-P-N, three-junction ($J_1, J_2, J_3$) semiconductor switch with three terminals: anode (A), cathode (K) and gate (G). It can be turned ON by a small gate pulse when forward biased, but once ON the gate loses control; it turns OFF only when its anode current falls below the holding current. #### V-I characteristic ```text IA ^ | |<- ON state (VT = 1-2 V) | | IL ---|--| IH ---|--+. | `. Ig2 > Ig1 > Ig=0 | `-.___ ___ -VBR | `----' | -----+-------+---------------------+--> VAK | | forward blocking VBO | | (leakage current) | reverse blocking | (leakage current) v reverse avalanche ``` 1. **Reverse blocking mode:** cathode is positive with respect to anode. Junctions $J_1$ and $J_3$ are reverse biased and $J_2$ is forward biased, so only a small reverse leakage current flows. If the reverse voltage reaches the **reverse breakdown voltage** $V_{BR}$, avalanche breakdown occurs at $J_1$ and $J_3$; the current rises sharply and the device is usually destroyed. 2. **Forward blocking mode:** anode is positive and gate is open. $J_1$ and $J_3$ are forward biased but $J_2$ is reverse biased, so only a small forward leakage current flows. The SCR is OFF and blocks the forward voltage. 3. **Forward conduction (ON) mode:** if the forward voltage is raised to the **forward breakover voltage** $V_{BO}$ (with $I_g = 0$), junction $J_2$ breaks down by avalanche and the SCR switches suddenly to the ON state (the negative-resistance jump). The voltage across it drops to about 1 to 2 V and the current is limited only by the load. With gate current, $J_2$ breaks down at a lower voltage: the larger $I_g$, the smaller the breakover voltage ($I_{g2} > I_{g1}$). In practice the SCR is always turned ON by a gate pulse at a voltage well below $V_{BO}$. #### Holding and latching current - **Latching current ($I_L$):** the minimum anode current that must flow through the SCR immediately after it is triggered, while the gate pulse is still present, so that it stays ON after the gate pulse is removed. If the anode current has not reached $I_L$ when the gate pulse ends, the SCR turns OFF again. - **Holding current ($I_H$):** the minimum anode current that must keep flowing to hold an already conducting SCR in the ON state (gate open). If the anode current falls below $I_H$, the SCR turns OFF and returns to forward blocking. - $I_L$ is greater than $I_H$, usually $I_L \approx 2$ to $3\,I_H$. Example: for an SCR with $I_L = 40$ mA and $I_H = 15$ mA, the gate pulse must last until the anode current reaches 40 mA, but once ON it stays ON until the current drops below 15 mA. #### Why a vertical structure is used in power BJTs ```text E B E ===[n+]=====[p]=====[n+]=== ---------- p base ---------- ------- n- drift region ---- (thick, lightly doped) ------- n+ substrate ------- | C (bottom) ``` 1. **Large current area:** current flows vertically through the whole chip area, so the cross-section is large and the current density and on-state resistance are low. 2. **High voltage:** the thick, lightly doped $n^-$ drift layer sits between base and collector and supports a high blocking voltage without making the chip large. 3. **Better heat removal:** the collector is the whole bottom of the chip, mounted on the heat sink. 4. **Interdigitated** emitter and base fingers on top reduce current crowding. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When forward voltage reaches VBO with Ig = 0, what happens? A. J2 breaks down by avalanche and the SCR switches ON, dropping to about 1-2 V B. J1 breaks down and the SCR is destroyed C. The SCR stays OFF but leaks more D. J3 becomes reverse biased Answer: A. Avalanche breakdown of J2 triggers the sudden negative-resistance jump to the ON state; current is then limited by the load. (Key point: Forward conduction) 2. Which relation between latching and holding currents is correct? A. IL < IH B. IL = IH always C. IL > IH, usually 2 to 3 times D. IL is unrelated to IH Answer: C. Latching current applies just after turn-on and is larger than the holding current needed to stay ON. (Key point: Holding vs latching) 3. In a vertical power BJT, which layer supports the high blocking voltage? A. Heavily doped n+ emitter B. Thick, lightly doped n- drift region C. Thin p base D. n+ substrate Answer: B. The thick lightly doped n- drift layer between base and collector sustains high voltage without enlarging the chip. (Key point: Vertical structure and voltage) 4. Why does vertical current flow help a power BJT carry large current? A. The current uses the whole chip area, giving a large cross-section and low on-state resistance B. It forces current into a narrow channel C. It removes the need for a heat sink D. It makes the base current zero Answer: A. Vertical flow through the whole chip area lowers current density and resistance, and the collector on the bottom also helps heat removal. (Key point: Vertical structure and current) ## Related in Power Electronics - [Thyristor V-I; latching vs holding current](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-latching-vs-holding) — Power Electronics, chapter 1 - [Thyristor V-I, latching/holding, gate signal circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-latching-gate-circuit) — Power Electronics, chapter 1 - [Latching and holding current](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Alatching-holding-current) — Power Electronics, chapter 1 - [BJT switching characteristics and base drive](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abjt-switching-gate-signal) — Power Electronics, chapter 1 ## Related in other subjects - [Why vertical shaft arrangement is preferred](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apowerhouse-vertical-shaft-turbine-generator) — Hydropower Engineering, chapter 8